Topic: 836cc piston questions [Too Low compression] OK with NOS cast RC pistons

[PeWe]

What to do to get higher compression?

My Action Fours 836 pistons have too low compression (10.5-1 comp ratio) in my K6 -76. I compare with my old RC836 that had very nice compression and much better torque at low rpm.
See attached  photo where the forged RC836 piston is 1.5mm higher and have bigger dome in general compared with the casted AF836.

My RC836 were machined to fit the head and vice versa at the workshop here for soon 30 years ago. I installed the AF836 when I blew the RC836. I noticed lower compression, but the bike worked well but not as wild as before.

I have now a slow project to get my bike on the road again, with more hp.
I need to fix the compression to be able to use a hotter cam as RC315 instead of my mild Action Fours SS-1 if I have understood the technical part correct.

- Should I go for new pistons with higher compression ratio?
   The only one I found have 12.5:1 maybe too much....
- Grind the cyl head and cylinder block?

I have ported head with F2 inlet valves (34mm) that might be one reason for lower compression.
- Possible to use F2 836 pistons?
- Other options?

[754]

 Change to K HEAD for higher compression...OR GET 12.5 to 1 and trim the dome..

[PeWe]

The head is K. Ported and got F2 inlet valves.  I have thoughts about 12.5:1 pistons and hopefully get lower compression in my engine.

It's a pity that I cannot find 11:1 pistons or similar that will fit the head and combustion chamber adapted for 34mm F2 inlet valve.

How much in general can be trimmed from the dome and still be on the safe side?

[PeWe]

I might have found the answer today in an environment friendly solution 
Pistons JE 836cc 12.5:1  AND fuel with 104 octane RON

My AF 836CC 10,5:1 compr ratio give my engine too low compression. Next step of pistons seems to be
JE 836cc 12.5:1.  Nothing between which would be an easy choice for me and not worry about knockings when using unleded 98 octane RON which is harder to find on the gas stations  here.

EDIT April 9: E85 mixture percentages corrected.
I'll use E85 gasoline which is more common nowadays due to the green environment fascists ideas. This is 85% ethanol/15% gasoline (95oct RON) summertime = 104 octane RON which will help against engine knockings. (75/25 in winter) The other gas we have is 95 or 98oct unleded except for diesel.
Important about octane:  USA, Canada use "AKI" vs Europe, Japan.... use RON 
100 RON Octane x 0.95 = 95 AKI Octane (US measure)

When converting the engine for E85 the jets in the carbs must be increased by 25-40% according to what I have read on Internet.
I have main jets 145 in my OEM Keihin carbs. 180-200 would hopefully work fine. Then adjust the air/fuel screw to give richer mix. Maybe step up the throttle needles one step as well.

The only thing to worry about is mechanical clearance when using RC315 cam (DP315 at Dynoman) and the bigger inlet valve I have 34mm. The head has opened chambers for 836 so the piston should be fine there.

In worst case I have to use 1-2 extra cylinder base gaskets

Only one thing... I have to purchase a number of $XXX US batches of parts + shipping and tax
CycleX conrods, all chains with tensioners, heavy duty cyl studs+ crankcase studs, all valves with new springs, JE 12.5:1 piston kit+DP315 cam, 4-1 yoshi replica, new rims + tires, wires, handlebar, etc, etc.... My Honda need to get som fresh parts since not to be used since 1990. I look forward to have all parts so I can work with it

[scottly]

When converting the engine for E85 the jets in the carbs must be increased by 25-40% according to what I have read on Internet.
I have main jets 145 in my OEM Keihin carbs. 180-200 would hopefully work fine. Then adjust the air/fuel screw to give richer mix. Maybe step up the throttle needles one step as well.

The increase in the jet size is based on the area of the jet, not the diameter. 40% more flow than a 145 would require a 171 jet. Also, there are other problems with e-85, such as being corrosive to aluminum and rubber. Perhaps the combustion chamber could be altered to improve flow (de-shrouding valves), while also reducing the compression ratio to allow pump gasoline?

[PeWe]

Thanks for the correction, jet area which is obvious.

Anyway, the future for my engine is either 95 octane RON or E85 if possible to make it possible to use higher compression pistons.  Our old 98 Octane era is soon over completely in this country.
98 octane here in EU is 93 in US
95 here in EU is 90 in US

If E85 still is a bad idea, I'll continue with my pistons I have which most likely will work with 95 oct RON. This will change the domino effect of the engine upgrade. OEM con rods can continue to work. I do the most important repairs to get the bike running again.

EDIT 9 April:  Link about interesting E85 discussion
http://forums.sohc4.net/index.php?topic=49241.0

[PeWe]

Piston question sorted out by new pistons. Found old kit, new in box on eBay $280US w shipm. (only 150 more than new rings to my old pistons)
New pistons more like my old RC's that were Ok in compression.

Photo attached where the new piston at left is compared with RC836 and Action Fours 836 with low compression.

EDIT: I have written another thread about these new pistons.  http://forums.sohc4.net/index.php?topic=106009.msg1185468#msg1185468

[Big Jay]

The old RC pistons had a dome that went out to the edge of the piston. You had to modify the combustion chamber to get them in. This made the combustion chamber volume larger.

Then when you go back to a piston like an AF that was made for stock combustion chamber, you have low compression.

The off the shelf JE 12.5:1 has the same type dome as the old RCs, ( I think) we don't sell many of those because of the chamber work necessary.

[MRieck]

The old RC pistons had a dome that went out to the edge of the piston. You had to modify the combustion chamber to get them in. This made the combustion chamber volume larger.

Then when you go back to a piston like an AF that was made for stock combustion chamber, you have low compression.

The off the shelf JE 12.5:1 has the same type dome as the old RCs, ( I think) we don't sell many of those because of the chamber work necessary.

Yes..there is certainly a lot of chamber work necessary Jay. A lot of work for such a  (relatively) low CR IMO.

[PeWe]

Yes, the chambers were opened for the old RC. The newer pistons however, look promising.
I  was thinking about the 12.5 kit but it might have been a little bit too much. I remember the last domino effect when tuning the engine  
It started with new pistons due to one cylinder/piston scratched. A set of RC836 just happened to be there on the shelf... then ported head, AF SS-1 cam, bigger inlet valves, valve springs, lightened crank, all bearings, WM29....
I'm thinking everyday about all godies I want to buy for the engine. It's other important priorities too, like family...

[PeWe]

Then when you go back to a piston like an AF that was made for stock combustion chamber, you have low compression.

- Most of aftermarket big bore piston kits are made for stock combustion chambers? I thought adjustment of the chambers to bore size was a must do to get a proper chamber without a shelf as an obstacle.

I'll make a comparison with pistons in the engine. Old RC, previous working Action Fours and the latest noname (RC HP6450, Don't know who made them. Russ Collins replicas/copies but cast?)

Mount all 3 pistons types and compare the domes raising over the cylinder. Maybe I cover something over the domes, make a mold and measure the volume with alcohol. Use same cylinder just for sure.

[PeWe]

I installed all my 3 different pistons in cyl 4 today to compare them. I had no clay to make a mold of their domes and measure more exact instead of guessing.
Photos indicate that the new pistons might give better compression than the old Action Fours. As I thought when I first saw the photoes of them
I have to remove some compression    grind one side of the inlet pocket for valve clearance, ~1mm.

[BLUE71TURBO]

 If your gonna run that E-85 fuel, your gonna have to change any rubber that comes into contact with that fuel.  E-85 has some corrosive ability's
 Alot of muscle car guy's found that out the hard way.  Fuel pumps, carbs and fuel lines all had to be replaced.  Not cheap for them.
     The other draw back's are listed in here......... http://en.wikipedia.org/wiki/E85

[PeWe]

Yes, E85 have some issues. I was thinking about E85 in conjunction to JE 12.5:1 pistons. I found other cheap pistons on eBay that seems to have slighter better compression than my old pistons that needed new rings anyway.

It seems that I could have choosen the 12.5:1 pistons, got lower compression but still enough when my head has opened chambers for my first set of RC pistons, machined for the head. The pistons had domes not completed at delivery, possible to make them as needed.  Later AF-836 were made for stock head and therefore lower compression. This is what I have learned here thanks to earlier reply by Big Jay. I have had thoughts about this since 1985. Answer 2012 

The engine has worked well despite lower compression (similar to stock 736cc), OEM pistons. 3 holidays on bike heading south, 5000 km each tour, daily trips on AutoBahn/Italian highways and the even better French highways in constant 150-210km/h without proböems. Refuel every 150-200km.

I have lowered the compression on my new pistons today, with a Dremel grinding tool. I had to adjust the pockets for inlet valves. 1mm on one side of the pocket to ensure the clearance.

- Piston 1 is 1,5 g heavier than 4. I hope this will be OK. Or will it cause vibrations? Or Dremel time inside?
2,3 has same weight though.

The engine is to be disassembled completely, checked inside. Wash all parts, paint the crankcase. Replace those parts the economy let me to and minimum for durability.

[PeWe]

Update:
I have started the engine and work with the carb tuning. The new pistons make a difference to the better. Pistons seems to be NOS RC  that have been discussed in other threads. Old  NOS 1 pcs oil ring replaced by my old used 3pcs rings when the end gaps were within spec. No oil smoke or smell either, just a little bit rich jetted.

[Old Scrambler]

Its been a while.............but GREAT to hear you have it all together. What fuel.....? Standard Rods...?

[Bill/BentON Racing]

Congrats,tell us more, pics also!! Bill

[PeWe]

Fuel. Shell V-Power +98 octane. (98 in Europe is 93 in US) Pistons best alternative for my head. Old NOC cast RC.
Yes std rods.  It was difficult to see APE complete crank with HD rods or from CycleX when I invested in HD primary chains. The rods the weakest point, right now.

I have photos in various threads. Cam timing CX-7, VM29 carb project. I use CB750 original carb boots.

Latest photos take a few days ago. My neighbours are blessed to hear my bike when I testing jet combos and ignition settings retard or advance around f and idle. VM29 pilot and mains. I have tried some combinations.

The tank is just temporary. I have a better in Freedom green metallic with all decals and striping K4 -74 style.  I'm dreaming about candy sapphire blue possible to buy new from Japan. I have soon brain washed my self enough to accept the investment