Topic: Something new and interesting for F2/F3 engines!

[HondaMan]

Here's some cast-looking (not forged!) pistons in 836cc with higher-compression crowns for these tough-to-rebuild engines? I might have to get a set for my F2 engine I have awaiting rebuild.

http://www.ebay.com/itm/Honda-CB750-750K-69-78-836cc-65mm-Big-Bore-Pistons-Kit-10-25-1-compression-ratio-/121826184945?hash=item1c5d680ef1:g:GwMAAOSwd0BVqCFx&vxp=mtr

[Retro Rocket]

Here's some cast-looking (not forged!) pistons in 836cc with higher-compression crowns for these tough-to-rebuild engines? I might have to get a set for my F2 engine I have awaiting rebuild.

http://www.ebay.com/itm/Honda-CB750-750K-69-78-836cc-65mm-Big-Bore-Pistons-Kit-10-25-1-compression-ratio-/121826184945?hash=item1c5d680ef1:g:GwMAAOSwd0BVqCFx&vxp=mtr

Is that the wrong link Mark..?  That says "for 750 K 69 through to 78...

[HondaMan]

The thing that makes them "right" for the F2/3 engines is their high compression crowns. In an F2/3 this 10.25 CR rating will drop to about 9.4-9.5 CR instead, but that's still much like OEM compression. That's why I intend to try them out. I fully suspect they will need some "trim" around their edges, if all they did was raise the shoulder +1mm, but if they were smarter than that and raised the dome instead, they should drop right in.

It would be great to have some drop-in pistons for these post-1975 bikes again. They have been gone for several years. If they will work for the F2/3, they will also fit all the others, except possibly the K0/K1 with the hand-finished heads: those might need some additional head trimming if the domes are too high. Still, they would fit.

[HondaMan]

This does raise the question: if I go ahead and get a set, and rebuild this F2 engine I have, does someone want it? I don't have an F2/3 bike to put it in.

[Jerry Rxman Griffin aka MuthaF'er]

I was expecting pistons for a F2/F3 not a -300 style piston or some 12.5 CR. That's kinda a standard CR on aftermarket HP pistons. Any real difference in or are these CruizinImage kits? Same price point.

[seanbarney41]

Is there a reason (other than budget) you don't want to use the forged wiseco's available from CycleX or APE?

[seanbarney41]

...and yes, I would possibly be interested in a Hondaman built F2 although shipping cost would probably end that possibility

[Retro Rocket]

The thing that makes them "right" for the F2/3 engines is their high compression crowns. In an F2/3 this 10.25 CR rating will drop to about 9.4-9.5 CR instead.

That was the only thing I could come up with...

[Old Scrambler]

Mark.........You may have to fly-cut the valve-pockets

Sean........I could haul the motor to Wisconsin on my way back from Bonneville next August

[HondaMan]

Is there a reason (other than budget) you don't want to use the forged wiseco's available from CycleX or APE?

Yes, a very good reason: the forged pistons are not (have never been) the best match for these engines, for a variety of reasons.
The forged pistons have an issue that's kind of hard to explain to the casual rider - when they are new, especially. The pistons expand and contract much more slowly than the cast type: the cast type are material-matched to the cylinders. Thus, with the forged ones, they slap loosely while the engine is warming them up, and when shut off, the cylinders shrink onto them tightly before they cool off. This causes some lube problems with the piston skirts, so the piston-makers tell you to increase the bore clearance to more than 2x the standard allowance. This, in itself, guarantees shorter ring life, and usually shorter piston life as well. (Reasons described below.)

After a fresh rebuild, a typical scenario is: the bike is fired up cold in the AM, then ridden maybe 2-3 miles to a gas station. After fueling (5 -ish minutes turned off), the electric start cannot turn over the engine to start, and the kickstarter feels locked. After another 10 minutes, the pistons cool off and shrink again, it starts up fine, and the perplexed novice rider wonders what happened to his freshly-rebuilt engine. (Then they come back to you, asking WTF?)

Here's what is happening while this is going on: first - understand that the oil film between the piston skirt and the cylinder wall is 93% of the cooling path for the piston (the remainder is the oil spray to the underside of the piston crown, being sprayed there by the rod bearing squirts). This clearance must be small so the oil can make a thin layer of full contact, otherwise the cooling "falls behind". In the forged-piston engines, e.g., Wiseco, this clearance is set at 0.0022"-0.0024" per their spec (CB750-size pistons), while cast pistons are 0.0008"-0.0012" (half as much). As the cylinders warm up, they grow rapidly (light aluminum-zinc alloy), while the denser forged aluminum heats up much slower. Once everything reaches operating temperatures together, all of these pistons have about the same operating clearances (near the minimum size), so the oil can interface the heat to the cylinder walls.

But...during each cold-to-hot cycle, the forged pistons' large clearance does not trap oil well and the pistons tend to scuff, usually much more than the cast pistons. In fact, I have seen 750 pistons with more than 60k miles on them with skirts that look new, while I have never seen forged pistons without scratched skirts, even with as few as a thousand miles on them. This scratch generates grit, which attacks the rings directly above them. It also allows too much oil to slip past the skirt, which puts increased pressure on the oil rings. If you examine forged pistons next to cast pistons, you will often find the return-oil holes (under the oil rings) to be larger, or more numerous, to lower this pressure. Some even have special arrangements, like chamfered holes, to improve on this situation. The oil that does not get scraped off by the oil rings then upsets the 2nd ring as the piston reverses during overlap, causing ring flutter: in real high-RPM engines this can cause ring fracture after a while.

In the above-mentioned scenario, the 'stuck' engine happens like this: the cold engine, ridden to the gas station, heats up the cylinders pretty quickly, while the forged pistons lag behind a little bit. Once the engine is stopped (before all parts have reached full temp), the cylinders cool quickly, the forged pistons not so much. This traps the oil that is on the piston skirts, hydraulically locking the piston(s) in place for a while. This then starts cooling off the pistons, so about 5-10 minutes later the engine can turn over again.

A classic example: read the debut of the CycleX "Super F2" bike's first race. This bike has (had?) forged pistons, big-bore size. The bike was ridden to the Line, then shut off a few minutes until the race Start was signalled. At that point, the engine was locked and the rider could not start it. The pit crew ran up and got it started (I think they used a rear-wheel roll starter?) and he went on to win the race.This is EXACTLY the symptoms. The article about this race blamed the lockup on "too much compression", but that's not what happened: even if it had 7:1 CR, it still would have acted the very same way.

So, after we raced with Powroll and Wiseco forged pistons long enough to learn these things, we quit using them for start-and-run (like LeMans start) races. I have never really understood the need for them in these engines unless either the compression is above 11:1 or a turbo/blower is implemented, anyway? I can understand them in a Ford 427 cubic inch, (dual) SOHC engine making over 850 HP at 8500 RPM, sure (those are 4.25" pistons!), but not in engines this small. For me, they are simply not worth the hassles, IHMO.

[seanbarney41]

very good.  Thanks for typing all that. 

[Don R]

  It's good to read more than an opinion. The how and why is important to understand. Thank you.

[turkey4me]

Is this a good example of what you are describing.  I just pulled these out of a fresh 836 F3 motor. 

[kmb69]

Yes, a very good reason: the forged pistons are not (have never been) the best match for these engines, for a variety of reasons.
The forged pistons have an issue that's kind of hard to explain to the casual rider - when they are new, especially. The pistons expand and contract much more slowly than the cast type: the cast type are material-matched to the cylinders. Thus, with the forged ones, they slap loosely while the engine is warming them up, and when shut off, the cylinders shrink onto them tightly before they cool off. This causes some lube problems with the piston skirts, so the piston-makers tell you to increase the bore clearance to more than 2x the standard allowance. This, in itself, guarantees shorter ring life, and usually shorter piston life as well. (Reasons described below.)

After a fresh rebuild, a typical scenario is: the bike is fired up cold in the AM, then ridden maybe 2-3 miles to a gas station. After fueling (5 -ish minutes turned off), the electric start cannot turn over the engine to start, and the kickstarter feels locked. After another 10 minutes, the pistons cool off and shrink again, it starts up fine, and the perplexed novice rider wonders what happened to his freshly-rebuilt engine. (Then they come back to you, asking WTF?)

Here's what is happening while this is going on: first - understand that the oil film between the piston skirt and the cylinder wall is 93% of the cooling path for the piston (the remainder is the oil spray to the underside of the piston crown, being sprayed there by the rod bearing squirts). This clearance must be small so the oil can make a thin layer of full contact, otherwise the cooling "falls behind". In the forged-piston engines, e.g., Wiseco, this clearance is set at 0.0022"-0.0024" per their spec (CB750-size pistons), while cast pistons are 0.0008"-0.0012" (half as much). As the cylinders warm up, they grow rapidly (light aluminum-zinc alloy), while the denser forged aluminum heats up much slower. Once everything reaches operating temperatures together, all of these pistons have about the same operating clearances (near the minimum size), so the oil can interface the heat to the cylinder walls.

But...during each cold-to-hot cycle, the forged pistons' large clearance does not trap oil well and the pistons tend to scuff, usually much more than the cast pistons. In fact, I have seen 750 pistons with more than 60k miles on them with skirts that look new, while I have never seen forged pistons without scratched skirts, even with as few as a thousand miles on them. This scratch generates grit, which attacks the rings directly above them. It also allows too much oil to slip past the skirt, which puts increased pressure on the oil rings. If you examine forged pistons next to cast pistons, you will often find the return-oil holes (under the oil rings) to be larger, or more numerous, to lower this pressure. Some even have special arrangements, like chamfered holes, to improve on this situation. The oil that does not get scraped off by the oil rings then upsets the 2nd ring as the piston reverses during overlap, causing ring flutter: in real high-RPM engines this can cause ring fracture after a while.

In the above-mentioned scenario, the 'stuck' engine happens like this: the cold engine, ridden to the gas station, heats up the cylinders pretty quickly, while the forged pistons lag behind a little bit. Once the engine is stopped (before all parts have reached full temp), the cylinders cool quickly, the forged pistons not so much. This traps the oil that is on the piston skirts, hydraulically locking the piston(s) in place for a while. This then starts cooling off the pistons, so about 5-10 minutes later the engine can turn over again.

A classic example: read the debut of the CycleX "Super F2" bike's first race. This bike has (had?) forged pistons, big-bore size. The bike was ridden to the Line, then shut off a few minutes until the race Start was signalled. At that point, the engine was locked and the rider could not start it. The pit crew ran up and got it started (I think they used a rear-wheel roll starter?) and he went on to win the race.This is EXACTLY the symptoms. The article about this race blamed the lockup on "too much compression", but that's not what happened: even if it had 7:1 CR, it still would have acted the very same way.

So, after we raced with Powroll and Wiseco forged pistons long enough to learn these things, we quit using them for start-and-run (like LeMans start) races. I have never really understood the need for them in these engines unless either the compression is above 11:1 or a turbo/blower is implemented, anyway? I can understand them in a Ford 427 cubic inch, (dual) SOHC engine making over 850 HP at 8500 RPM, sure (those are 4.25" pistons!), but not in engines this small. For me, they are simply not worth the hassles, IHMO.

Forums have created more experts than actual experience has.

Several misleading "facts" in this dissertation.

[kmb69]

The following is a summary of collected information gleaned from several reputable engine builders:

Forged pistons are usually lighter, stronger and dissipate heat better. Most forged piston brands (Wiseco, as well as others) usually weigh less then OEM cast pistons. Since the material is stronger, they can redesign the whole structure and reduce the amount of material used. They do (almost always) need a little more room in the bore to allow for expansion because the material is denser. It depends on the piston alloy, silicon content and bore size.

As for piston to wall clearance (p2w), not all forgings are low silicon metals. Some forgings have higher silicon content and they do allow you to run a tighter piston to wall clearance. People frequently misread the wear on their pistons. Some of that wear is from their loose p2w. There are a couple things that can be changed in the design of the shelf forged pistons that allow tighter p2w even with the low silicon, less brittle aluminum. The point where high silicon content becomes a factor is when you approach the hypereutectic range, meaning it is "overloaded", if you will, with silicon. These pistons are used in some OEM engines for precisely the reasons mentioned above - they will expand less, meaning closer p2w clearance, they are extremely light, and the modulus of elasticity is not affected by the high silicon content. The downside, however, is with hypereutectic pistons any sign of detonation causes a catastrophic engine failure.

Forged pistons also don't need as much lubrication to run because they can withstand heat a lot better than OEM cast. No matter what alloy or manufacturing process is used, aluminum pistons have a higher thermal conductivity than the cast iron cylinder liners and can dissipate heat faster than the liners.


The following is pirated from Piston Tech:

There are 2 types of ways to make pistons: Casting and Forging.
1. Cast : - Casting means that molten liquid aluminum is poured into a mold that is in the shape of the piston. - they are "WEAKER" than forged pistons and prone to cracking with use of nitrous and boost. For example the Nissan OEM cast pistons typically crack at the number 2 ring land. There 2 types of CAST pistons based on their silicon content.:

1a) Standard Cast (< 12.5% Silicon Content) - Expands uniformly but more than any other type of piston when heated - Needs loosest installed piston to wall clearances , since expands most due to low silicon content when heated. - Noisiest on cold engine start up and has the most blow-by of the 3 piston types in a cold engine because it has the least silicon content and therefore expands the most when the engine warms up from being cold causing it to require larger piston to wall clearances. - requires reinforcement plates or struts near wrist pin area, since alloy expands more as temperatures increase. These struts often crack under high loads like detonation. - requires ring groove insert to withstand ring land pound out and wear.

1b) Hypereutectic Cast (>12.5% Silicon Content, Most are 16-22% Silicon) Hypereutectic is an adjective that refers to the silicon (same material found in sand) content in the piston. Hypereutectic means the piston has MORE silicon added to the mixing with aluminum alloy. Note that this is not SILICONE. Eutectic means 12-12.5% silicon content. Hypereutectic means > 12.5% silicon content. Special melting process combines silicon to "super-saturate" the aluminum alloy. Special molds and casting- cooling processes are needed to get fine dispersing of silicon evenly or uniformly throughout aluminum alloy. There are hypereutectic cast pistons available. These are more resistant to wear and scuffing, expand less with heat, and are stronger (more able to withstand higher cylinder temperatures , pressure, and detonation than standard cast.) More silicon, if uniformly added to aluminum alloy, : - Hardens the aluminum alloy further - Expands less with heat (higher thermal expansion co-efficient) meets criteria for piston design: run tighter clearances. Hypereutectic pistons are stronger and can run narrower ring lands and tighter piston-to-wall clearances, since there is less heat expansion than standard cast. No wrist pin area reinforcement plate is needed due to less rapid expansion with heat. - No iron ring groove insert is needed to protect against ring land pound out. Ring groove is more heat and wear resistant with higher silicon. - But extra hardness also makes the piston MORE BRITTLE (i.e. brittle meaning that it's easier to crack under stress and loading, like high rpms or under boost or detonation) and therefore must be handled more carefully on install. Not all hypereutectic pistons are alike. The size of the silicon granules used and how well the silicon is mixed (distributed) in the aluminum alloy during the casting process affects the quality. If unevenly added and mixed, the silicon clumps and forms hard spots in different parts of the piston which crack more easily under high loads. MOST STOCK OEM PISTONS ARE HIGH SILICON CAST.

2. Forged - the process of making a forged piston involves heating a slug of aluminum alloy that is in the shape of a long cylinder and then pressing it into the die's piston shape under very high pressure, and then machining the surface. The mixture of ingredients in the metal is already in place and there is no adding and mixing like in casting where they added the silicon to the aluminum. - Forging creates a DENSER AND LESS BRITTLE (600% more ductile or less porous) grain STRUCTURE that is stronger than cast piston's crystalline grain structure. Forged pistons can handle nitrous and high boost (temperature resistant and stronger) better than cast pistons. - Forged pistons have high silicon content already mixed in the alloy but have less silicon than OEM hypereutectic cast pistons. For example, an aftermarket forged Wiseco piston has 7% less silicon content than OEM Honda hypereutectic cast pistons. The less porous forged piston conducts heat faster and runs 20% COOLER than cast pistons (even hypereutectic cast) reducing higher (detonation-inducing) temperatures. There is better HEAT TRANSFER AWAY from the piston crown with forging. The heat can be transferred to the cylinder head (when the piston is at the top of the exhaust stroke) and wicked away by the coolant flowing through the cylinder head. Forged pistons have greater thermal expansion than hypereutectic cast pistons and have greater clearances and can be noisier and may have more blow-by in a cold engine compared to hypereutectic cast pistons but it's NOT as noisy as eutectic or hypoeutectic standard cast pistons. Once the engine is warmed up, the clearances will then tighten and there is less noise from piston rock as it travels up and down the bore. In fact, this is another advantage of forged pistons: They seal up and close up the clearances better than when heated. The trick of the racing engine builder's trade is to find the lowest cold engine clearances that they can choose so that when the forged pistons expands at warm-up, the clearances are ultra-tight to get better engine compression and less leaking. The looser installed clearances with forged are not as bad as you would think.

[HondaMan]

Forging creates a DENSER AND LESS BRITTLE (600% more ductile or less porous) grain STRUCTURE that is stronger than cast piston's crystalline grain structure. Forged pistons can handle nitrous and high boost (temperature resistant and stronger) better than cast pistons. - Forged pistons have high silicon content already mixed in the alloy but have less silicon than OEM hypereutectic cast pistons. For example, an aftermarket forged Wiseco piston has 7% less silicon content than OEM Honda hypereutectic cast pistons. The less porous forged piston conducts heat faster and runs 20% COOLER than cast pistons (even hypereutectic cast) reducing higher (detonation-inducing) temperatures. There is better HEAT TRANSFER AWAY from the piston crown with forging. The heat can be transferred to the cylinder head (when the piston is at the top of the exhaust stroke) and wicked away by the coolant flowing through the cylinder head. Forged pistons have greater thermal expansion than hypereutectic cast pistons and have greater clearances and can be noisier and may have more blow-by in a cold engine compared to hypereutectic cast pistons but it's NOT as noisy as eutectic or hypoeutectic standard cast pistons. Once the engine is warmed up, the clearances will then tighten and there is less noise from piston rock as it travels up and down the bore. In fact, this is another advantage of forged pistons: They seal up and close up the clearances better than when heated. The trick of the racing engine builder's trade is to find the lowest cold engine clearances that they can choose so that when the forged pistons expands at warm-up, the clearances are ultra-tight to get better engine compression and less leaking.

This last section helps to explain why we see the sort of (typical) wear shown in the TURKEY4ME post above: in a water-cooled engine (be it bike or car) the thermal control of the head temperatures is far tighter (and lower temps) than those found in the SOHC4 engine. When the heat from the piston crown can't be removed through low-temperature (160-180 degrees F or less) heat-pumping, it must go someplace: that is likely going to be the piston's skirt. This causes skirt growth, helping to close up the larger-than-cast clearances that are specified for the forged pistons. It is the time between the startup and the operating temperature where the troubles start: when the gap is too big, the oil will not lube well, which starts the scuffing action. Once that starts, it  will channel the oil into those scuff lines instead of along the face of the skirt, hence the markings.

Another place where the damage can (and does) start in the example I cited above is: if the engine is "stuck" in the short-run-to-the-gas-station scenario, it is best to leave it alone for a few minutes until the kickstarter can at least be turned, before starting it back up. Otherwise the too-tight clearances at that point will scratch the micro-grooves on the typical forged pistons' skirts, causing them to collapse and become debris. I've seen this in a dozen Wiseco'd 750 engines over the years, and the result was always deep vertical scars in the skirts, which eventually wore the cylinders, too. None of those engines had even 20k miles on them, pointing out how important is is (was) to be mindful of the startup situation.

That's how I came to not favor the forged pistons in these engines.

[MRieck]

 
 I don't think it is fair to compare pistons made in Bedrock (40 year old Wiseco's etc) with modern forged pistons. It is apples to steak. Wiseco had more problems secondary to machining prior to the CNC era than the material used.
To be fair you really have to question how accurate the bores are on scuffed skirts. I have seen some real half a$$ boring jobs and maybe Mark and friends were victims of that exact problem. Just because it "looks " round doesn't mean it is. I have seen plenty of scuffed up skirts with cast pistons as well and that is with the .0012 or so clearance.
 I have never experienced any of these problems with forged pistons.....I guess I am lucky.
 I'd ask Big Jay about his experience with forged pistons as he has probably seen more than anybody on this board. This would include air and liquid cooled.

[kmb69]

.....
Another place where the damage can (and does) start in the example I cited above is: if the engine is "stuck" in the short-run-to-the-gas-station scenario, it is best to leave it alone for a few minutes until the kickstarter can at least be turned, before starting it back up. Otherwise the too-tight clearances at that point will scratch the micro-grooves on the typical forged pistons' skirts, causing them to collapse and become debris. I've seen this in a dozen Wiseco'd 750 engines over the years, and the result was always deep vertical scars in the skirts, which eventually wore the cylinders, too. None of those engines had even 20k miles on them, pointing out how important is is (was) to be mindful of the startup situation.

That's how I came to not favor the forged pistons in these engines.

 I don't think it is fair to compare pistons made in Bedrock (40 year old Wiseco's etc) with modern forged pistons. It is apples to steak. Wiseco had more problems secondary to machining prior to the CNC era than the material used.
To be fair you really have to question how accurate the bores are on scuffed skirts. I have seen some real half a$$ boring jobs and maybe Mark and friends were victims of that exact problem. Just because it "looks " round doesn't mean it is. I have seen plenty of scuffed up skirts with cast pistons as well and that is with the .0012 or so clearance.
 I have never experienced any of this problems with forged pistons.....I guess I am lucky.
 I'd ask Big Jay about his experience with forged pistons as he has probably seen more than anybody on this board. This would include air and liquid cooled.

Mike's points are spot on. And you may be "lucky" but I am pretty sure your success with forged pistons is more related to your engine building prowess and tuning expertise. I too have seen many scuffed cast piston installations usually caused by improper cylinder boring/honing and less than "clean room" assembly practices. Properly prepared engines with forged pistons will and do work just fine. Just because a guy can afford to have a set of forged pistons installed does not make him an expert engine builder and/or tuner. "Most" forged piston installations in these engines include higher compression pistons (more combustion heat) and require appropriate tuning and handling.

[Bill/BentON Racing]

Great read. No doubt about the newer forged pistons being much better than the old ones that first came out. So much depends on round holes, clearances and honing. Hell, you can bore it round, have the right clearance and still f it up all to hell if you don't know honing. I prefer forged pistons,  lighter and stronger way important. But I just race these days. Yoshimura favored cast pistons....but the forged stuff  back then had silicone content all over the place. The advent of CNC etc as Keith stated changed a lot. Let's see, I can cut the valve seats by hand....or I can use the Serdi cutter. Maybe not the best analogy,  but things change, and boy have pistons changed! You check out these new 4 stroke dirt bike race pistons. Need to try four of them!  . Y'all keep it coming, very educational read, thanks, Bill

[MRieck]

Great read. No doubt about the newer forged pistons being much better than the old ones that first came out. So much depends on round holes, clearances and honing. Hell, you can bore it round, have the right clearance and still f it up all to hell if you don't know honing. I prefer forged pistons,  lighter and stronger way important. But I just race these days. Yoshimura favored cast pistons....but the forged stuff  back then had silicone content all over the place. The advent of CNC etc as Keith stated changed a lot. Let's see, I can cut the valve seats by hand....or I can use the Serdi cutter. Maybe not the best analogy,  but things change, and boy have pistons changed! You check out these new 4 stroke dirt bike race pistons. Need to try four of them!  . Y'all keep it coming, very educational read, thanks, Bill

That's because ART made their pistons and all they were doing were cast pieces for motorcycles back then. Yoshimura pistons always had nice pins. 4 stroke MX bike pistons are basically a nail head. Unfortunately they don't last a long time as there is no skirt to stabilize the piston. They are featherweight for sure.

[bwaller]

After five race seasons with low silicone forged pistons they still look good. However new, lighter more modern forged pistons are in place. There has to be a bit of understanding of what's being installed. I wonder how many of those $100 cast piston kits didn't last?

[HondaMan]

After five race seasons with low silicone forged pistons they still look good. However new, lighter more modern forged pistons are in place. There has to be a bit of understanding of what's being installed. I wonder how many of those $100 cast piston kits didn't last?

I think I'll get to find out!
If I have a few $$ left after the Christmas rush, I am going to try a set of the high-compression $100 cast ones in the F2 (maybe the F1?) and see how it works out, just to see.

[Jerry Rxman Griffin aka MuthaF'er]

Mark,

You're talking high compression. I myself consider high compression pistons to be 12.5:1. What numbers are you referring to here? Stock are generally 9:1/9.2:1 ; normal bore kits are ~10.5:1 ; normal high compression  are usually 12.5:1.

Perhaps a little confusion without the numbers?

 

[PeWe]

Really interesting discussion about piston/bore experiences!

I can contribute with photos of 2 pistons. Forged and cast
Forged RC836 that only got about 8000kms. Touring to former Jugoslavia took 5000kms.
Destroyed, but skirts look nice with the brick pattern

Cast Action Fours that replaced the RC pistons, hone bores only and in they went.
I think about 28000kms. 3 tourings going south to Austria, Italy twice, Spain once. About 17000 kms touring.

Edit: The Forged RC pistons are slightly slimmer than the cast Action Fours.

I'll soon open the engine again and I'm curious how my NOS RC cast pistons will look after 5000 kms, half with too lean carbs. Same bores honed again and in they went with 3-piece oil rings from my old Action Fours.

Maybe the bore and pistons will look scuffed? New investments

[HondaMan]

Really interesting discussion about piston/bore experiences!

I can contribute with photos of 2 pistons. Forged and cast
Forged RC836 that only got about 8000kms. Touring to former Jugoslavia took 5000kms.
Destroyed, but skirts look nice with the brick pattern

Cast Action Fours that replaced the RC pistons, hone bores only and in they went.
I think about 28000kms. 3 tourings going south to Austria, Italy twice, Spain once. About 17000 kms touring.

Edit: The Forged RC pistons are slightly slimmer than the cast Action Fours.

I'll soon open the engine again and I'm curious how my NOS RC cast pistons will look after 5000 kms, half with too lean carbs. Same bores honed again and in they went with 3-piece oil rings from my old Action Fours.

Maybe the bore and pistons will look scuffed? New investments

Very cool, PeWe! Looking forward to seeing those RC pistons.