Topic: Need help with ID of RC piston application

[scottly]

If clearance is needed, it would be easier by far to modify the pistons rather than the head. We are talking about a 2 mm wide band around the O.D. of the piston, where there should be plenty of meat. Also, if you modify the chambers, you will lose compression if you replace the pistons with non RC, like PeWe experienced. Notice the stepped dome on my TRW piston.
Piston to valve clearance is a completely different issue.
 

[PeWe]

My project is a really slow one since I have plenty of repairs to do and constantly find more parts to buy. I have to respect family happiness as well Holiday travels before parts... that's tough.
I have a list of parts I have to order today or tomorrow so the project can continue. oil lines, headlight details.....

My goal is to start the engine before winter so I can report the result in the thread(s) I have written about these pistons. I hope I'll get higher compression, closer to my old RC836 my head was made for with opened chambers.

BUT, the bike ran rather well with less compression, AF836 pistons (High compression in a regular head). I used this setup during 3 long tourings in Europe with a good result.
Very smooth and NO knocking engine as the RC could on 5:th gear, accelerate around 100-120km/h depending on where I have bought the gasoline.

Attached photo show the prints after an old 836 fiber head gasket. Fits exact around the chambers.

Attached pdf show piston comparison I did last year. I have marked on the new piston inlet pocket where to grind for the bigger inlet valve to ensure 1mm clearance piston pocket-valve.

[Tews19]

Thanks for the pic PeWe..... If you don't mind posting on here the update when complete that would be much appreciated

[BPellerine]

a couple of pics .bill

[Terry in Australia]

Thanks Bill, am I right in assuming the the deep valve pockets are cast into the crowns of the RC pistons?

If so, that'd be why Big Jay said that "back in the day" it was necessary to modify the chambers, rather than the pistons, removing a 2mm (minimum) wide band of material, 2.5 mm (minimum) deep, might not be a real good idea. Cheers, Terry.

And thanks to PeWe too, I've got a set of NOS Action Fours pistons in a box, I might use them rather than the eBay pistons I just bought. Cheers, Terry.

[Tews19]

Terry are you suggesting these may not be good to use or more of a hassle to make work properly? Maybe I will put back on eBay if that's the case or if a member here is interested I will sell for cheap.

[PeWe]

This seems to be good news for my engine with opened chambers in need of pistons with higher/wider dome. I'll update when engine is started. 'Open chambers' was one option at our local tuner +30 years ago, I paid extra for that when the head was ported.

EDIT: Wider chamber will make more power as long there is a piston filling the area with proper compression?

These pistons might be worthy and own head?

[BPellerine]

this is just my opinion but it looks like the pistons were made with enough material so that the valve relief could be machined into them.I still do not think these pistons could be so far out as 836 was not a new thing at the time.the guy I bought mine from said they were the last of thirty sets that his shop had sold in the day in calif times that by how many dealers there were for rc and you would have a lot of messed up heads!! I can understand pewes forged pistons needing clearance as they are a pretty big domed piston and forged pistons grow.you are the only guy that will have first hand exp that I know of Terry.bill

[BPellerine]

sorry for the mixup this is the pic of the 2 pistons pinned together and a pc of top of piston.bill from top of ring land to top of rc piston is 4mm

[PeWe]

1 more photo I took when I compared 3 different pistons.
My experience of wider chambers and lower compression as I got with AF was OK. The engine had a very nice temperament and maybe need of better con rods reduced....

Setup: 836, Ported head, F2 inlet valves, action fours SS-1 cam (same numbers as WebCam41) lightened alternator.
So, open the chambers for the pistons is not a problem except the cost.

I think it's a good idea to mount it together and clay the pistons using the old gaskets. I clayed the pistons but only the valve pockets when verifying the hotter cam.

Pinging pistons when warm as Terry wrote might be affected by the thickness of gaskets and if the cyl + head are milled.

[Tews19]

Hurry up Pewe! Get the engine done!!!!!!..... Please

[Terry in Australia]

Terry are you suggesting these may not be good to use or more of a hassle to make work properly? Maybe I will put back on eBay if that's the case or if a member here is interested I will sell for cheap.

G'Day Mate, no, I'm not trying to frighten you, but I suspect that the noise emanating from my engine when warm could well be a result of using my RC pistons without having the chambers clearanced. I bought a new set of those cheapo eBay pistons to replace the RC's, but after looking at those Action Fours pistons in PeWe's earlier pics, I might use the NOS set that I've got sitting in a box. Cheers, Terry.

[BPellerine]

pewes head was opened to 65mm to accept the FULL dome rc forged pistons that had a dome that extended to the edge of the piston and did not have the 2mm band machined into the outer edge to make the dome 61 mm.the forged pistons got their high comp with these domes filling the chamber even when the chambers were opened to 65mm.these rc pistons are lower comp WITH A 2MM BAND .the issues that remain might be a shaved head or bigger valves or both.full dome pistons could almost be called a blank that you could mod to taste .took me awhile but the answer was right in front of me in pewes thread.whew!bill

[scottly]

the dome itself is 61 mm across .

I don't think you have a problem, Bill, but now that the question has been raised, clay the pistons, both at the outer diameter and the valve pockets for peace of mind.

[BPellerine]

will be doing as a matter of course during build.bill

[NitroHunter]

BP check your CC'd compression ratio, and use an on-line calculator, and your cam specs, to figure your static cylinder pressure. Cheap and effective. Zero deck is a really beautiful thing. But FYI... I've had 10.5 pistons make 155 psi cranking pressure in a 970 motor with an open chamber ported head and big RC 315 cam, early lift, late close, blows down lots of cylinder pressure, makes great top end, (but ran like $hit under 9K) and an 836 that made 240 psi with '10.5''s, milled small chambers, and an RC 240 cam that flat whipped up wheelies when the wick got twisted around town. Make YOUR combination work. My opinion, wheelies are FUN, use the RC pistons, I wouldn't hesitate to cut a little off the head as it doesn't bring the edges in much (but you really gotta check it with a strip of solder at least once, it's easy) those RC valve pockets give you lots of safety for big cam lifts, .060" is good, I'd  shim the base gasket ONLY WHAT YOU NEED for .035" squish, if that isn't an issue then don't change a thing. Trust me bro, run good gas and big cranking pressure makes a fabulous street machine. Build this motor as a one time, one-off deal; spend a few hours on the precision items and it will give you a life-time of hot-rod builder satisfaction. Hey just my .02. The opinions of others may vary
Good luck.

[BPellerine]

thanks so much nitro I want a good street bike and was thinking a milder grind would be more in line with what I want.this will be a 1 off and probably my last build so I want to have fun,bill

[Terry in Australia]

Well I thought I better add a pic of my RC pistons to this thread, (top pic) they're different to the ones that Tews and Bill posted, but regardless, they showed no signs of any touching, the noises emanating from my engine appear to be the fact that they're as much as .006" "thinner" than my eBay flat top pistons, (bottom pic) so were a pretty sloppy fit in the 65mm bores, and I had a bad case of piston slap, along with a "spare" valve adjusting screw locknut I found under my valve cover when I pulled the top end down.

As mentioned, the eBay pistons are a much tighter fit so my engine should be much quieter from now on, but I'll need to be careful when I break them in. Cheers, Terry.
 
V

[BPellerine]

this rc piston came with buttons for install kit on ebay now

[Sam Green Racing]

BP check your CC'd compression ratio, and use an on-line calculator, and your cam specs, to figure your static cylinder pressure. Cheap and effective. Zero deck is a really beautiful thing. But FYI... I've had 10.5 pistons make 155 psi cranking pressure in a 970 motor with an open chamber ported head and big RC 315 cam, early lift, late close, blows down lots of cylinder pressure, makes great top end, (but ran like $hit under 9K) and an 836 that made 240 psi with '10.5''s, milled small chambers, and an RC 240 cam that flat whipped up wheelies when the wick got twisted around town. Make YOUR combination work. My opinion, wheelies are FUN, use the RC pistons, I wouldn't hesitate to cut a little off the head as it doesn't bring the edges in much (but you really gotta check it with a strip of solder at least once, it's easy) those RC valve pockets give you lots of safety for big cam lifts, .060" is good, I'd  shim the base gasket ONLY WHAT YOU NEED for .035" squish, if that isn't an issue then don't change a thing. Trust me bro, run good gas and big cranking pressure makes a fabulous street machine. Build this motor as a one time, one-off deal; spend a few hours on the precision items and it will give you a life-time of hot-rod builder satisfaction. Hey just my .02. The opinions of others may vary
Good luck.

+1 ,Bill ,I've looked got used 70mm RC's no 836 65mm ones ,mine are in Calendar Girl Bill

Bill not Sam. 

[PeWe]

Well I thought I better add a pic of my RC pistons to this thread, (top pic) they're different to the ones that Tews and Bill posted, but regardless, they showed no signs of any touching, the noises emanating from my engine appear to be the fact that they're as much as .006" "thinner" than my eBay flat top pistons, (bottom pic) so were a pretty sloppy fit in the 65mm bores, and I had a bad case of piston slap, along with a "spare" valve adjusting screw locknut I found under my valve cover when I pulled the top end down.

As mentioned, the eBay pistons are a much tighter fit so my engine should be much quieter from now on, but I'll need to be careful when I break them in. Cheers, Terry.
 
V

Nice photos of RC pistons.
My old RC pistons are also smaller than later pistons. RC seems to have made a correct choice when making the pistons smaller to achieve correct clearance when forged pistons need more clearance bore/piston than cast.
My cylinders could later have cast pistons. If the forged RC should been 65mm from the beginning the bores should be too sloppy for cast pistons to be installed later on, if I have understood the tech stuff correct.

My bike is soon ready for start.
-  Tank on + tighten the gas hose on petcock and fill gasoline that I have waiting beside the bike.
- Maybe I step up the mains before filling the VM29 carbs with smelly fluid. 127.2 to 130.

I get new project every time working with the bike.  My last one is to connect relays for the head light. I cannot accept 55/60W thru the simple handle bar controls. The wires are just soldered direct on a small copper plate. No physical attachment, 5A thru that tine contact point. I have relays, connectors, wiring, crimping tool...

The wiring is otherwise OK, ready for start.

[PeWe]

On ebay now for over double price :-)
I paid $250 US summer 2012 + shipping + 30% tax when its shipped outside EU.....
http://www.ebay.com/itm/RC-Engineering-Pistons-Russ-Collins-Engineering-836-Honda-CB750-Big-Bore-Kit-/111073379622?pt=Motorcycles_Parts_Accessories&hash=item19dc7d4526&vxp=mtr

[Terry in Australia]

Yeah, I alerted the guys here last year to the fact that the seller had "more than 10" sets of RC 836cc pistons and was selling them for cheap, but everyone was saving their money and buying the 130 dollar flat tops, ha ha.

Still, when guys in the US are buying CB750's for only a few hundred dollars, 250 probably seems like a lot of money just for a set of pistons......... Cheers, Terry.    

[PeWe]

Yes, it's not fair.. in US people can buy complete bikes for almost nothing, very cheap parts from the scrap yards as well....

Talking about pistons, RC.

I need to check the plugs on all cylinders when verifying the carb jettings and ignition with my nos cast RC pistons, same as this thread starter has.
The 2013 engine build sounds very nice with a wonderful nervous race sound... a little bit hard. A combination of the new pistons and CycleX CX-7 cam A gas leak stopped longer test.
(Gasoline pipe into carbs 2-3, o-rings need to be replaced and better sealed when these pipes are shortened when I rebuilt the rack for closer mount to fit OEM CB750 carb boots)

I'm sure it will be very difficult to drive the bike without twisting the throttles to the very end. :-)

Below my old RC836 forged pistons. 1984 total meltdown after 5000miles. Had to buy new piston kit, 4 used rods and 2 EX valve guides. The con rods small ends were bluish, same color my 4-1 still has
I have the receipts from my local tuner still in business. Maybe a combination of leaking Mikuni carb boots, ignition and high compression.

[Terry in Australia]

That's interesting mate. Hey I compared the RC rings to the eBay 836cc rings yesterday, the top 2 rings are a perfect fit on my RC pistons, (the end gaps will need to be adjusted, probably) but the oil ring grooves need to be taken out a couple of thou to accommodate the new 3 piece rings. Good to know though, that you can buy new rings for old RC pistons for only $20.00 for a complete set! Cheers, Terry.