R65 Piston Compression Height

[DanMay]

Does anyone have a R65 (post '81) piston that they could measure the compression height for me ?  The compression height is the distance from the wrist pin center line to the top edge of the piston, not including the dome.



Thanks !

[Ed Miller]

You're about 5 months too late for me to measure that, as they're back in the bike.  What will you use that information for?  

[arvo92]

I am about to get the cylinders off in the nearest future and replace the piston rings. I can measure the height if you still need it.

I am with Ed here: whats the crack?

[Tony Smith]

Does anyone have a R65 (post '81) piston that they could measure the compression height for me ?  The compression height is the distance from the wrist pin center line to the top edge of the piston, not including the dome.

Sorry, I meant to do this on the weekend, I have a set of 1979 pistons and a set of 1985 pistons, I'll take a side-by-side photo and give you the measurement from a decent set of vernier calipers in the next day or so.

Thanks !

[DanMay]

I am about to get the cylinders off in the nearest future and replace the piston rings. I can measure the height if you still need it.

I am with Ed here: whats the crack?

Well, since you asked...  I am slowly looking at some engine performance modifications for my R65LS race bike.  I just installed a set of Mikuni 36mm VM carbs, and will have the before and after dyno results later this week.  The next step is a set of high performance pistons.  I talked to Richard Moore at Moorespeed (http://www.moorespeed.co.uk/, and we are investigating if his 83mm R75 race pistons will work with the R65.  Combined with one of his custom ground camshafts, he thinks he can get considerable HP improvement.  So, we need the measurements of the stock piston to see if any cylinder and/or rod modification is necessary.  

[Ed Miller]

A long time ago, when I had the parts program on my computer, I found a function that would search what applications used a particular part.  I was trying to figure out why my pistons used the same old style wrist pin retaining clips as the older bikes, and if I remember right, it turned out that my R65 used the same pistons as the R75.   Does anybody know how to confirm that?

[georgesgiralt]

Ed;
I'm sorry to disappoint you, but the post 81 piston fit only the R65 ( 11 25 1 337 261) according to RealOem.

[Tony Smith]

A long time ago, when I had the parts program on my computer, I found a function that would search what applications used a particular part.  I was trying to figure out why my pistons used the same old style wrist pin retaining clips as the older bikes, and if I remember right, it turned out that my R65 used the same pistons as the R75.   Does anybody know how to confirm that?

Please see my later post with a side by side comparison of 1979 and 1985 r65 Pistons


Tony

[Tony Smith]

please see photos below.

These pictures are a 1979 piston next to a 1985 piston, to show the difference (and the similarities) I have locked the two together using the rings fitted to the 1979 edition.
 
A piece of bar turned a few thou less than the wrist pin diameter goes through both pistons without dragging so i would be prepared to bet money that the only difference is about 1.5mm of additional metal above the top ring on the 1985 piston.
 
I will measure it later but it is a totally unpleasant day here and going out to the shed in driving rain to retrieve the verniers didn't appeal.In any event, the hoto shows the difference fairly clearlu.

[Ed Miller]

Ed;
I'm sorry to disappoint you, but the post 81 piston fit only the R65 ( 11 25 1 337 261) according to RealOem.

I hope I'm not the only one using the older, wire type wrist pin retaining clips....  Thanks, I finally figured out how to check usage on the OEM site.  It's actually much easier than on the ETK program.

Tony, would those differences in pistons be enough to raise the compression ratio?

[Tony Smith]

[quote
Tony, would those differences in pistons be enough to raise the compression ratio?

The compression height difference is approximately 1.5mm, by locking the pistons together at the ring groves and putting a bar through the piston pins I am very confident that the compression height is the only difference..

I would use the 85 pistons in a heat beat and if there was no mechanical interference (which I would check by assembling the cylinder and head onto an engine block without a head gasket after painting the piston top with Prussian Blue) the remaining problem would simply be whether or not the compression ratio was greater than desirable (depending on what fuel you are using in your breed of racing). If in fact the compression ratio proved to be too high I would relieve it by shimming the base of the cylinder, I believe suitable shims are still available from Luftmeister to name one supplier.