Thanks for the help, I have a piston/cyclinder question now.

[10drum]

Greetings again, I’m in this 1971 R75/5 to my waist, and really do appreciate all of the good advice I’ve received.

I have pulled the cylinders, cleaned the pistons, checked the piston clearance, and ring gap.

The top of the pistons are stamped with 81.985, the piston clearance is a loose .006”, and I don’t have a thick enough feeler gauge to measure the ring gap. My plug gap tool show the gap to be .080” +or- .010.”

I have a Haynes manual. The specs list:

-81.985 is the largest piston listed.
-.0047” is the maximum piston/cylinder clearance.
-.0177” is maximum ring gap.

.060” over, is the largest bore I’ve done on the British bikes, but they do sell pistons to .120” over.
What I’m seeing (in my small my british bike world), is I need to bore the cylinders.

When I brought the bike home, I charged the battery, got it to start, and rode it around the block. It ran out strong, the idle was flakey, and oil was running out of the push rods tubes. The compression was just under 145lbs, on both jugs. Am I reading something wrong here? Has it been bored 2 time before? Or is 81.985 just one of the standard pistons?

It ain’t all bad, I live for these kind of problems, I don’t have to make it right, I get to make it right.

Thanks for all the replies, you all are great.

Phil

 

[20774]

This is the '71 R75/5, right? Helps to keep including it in your posts. The nominal bore size is 82mm, so the 81.985 you're seeing is basically a stock number. Note that pistons come in three sizes at the nominal condition so that they can account for variations on the assembly line. First over for the /5 is another 0.5mm.

My Haynes lists maximum piston/cylinder clearance as 0.0047"...you probably dropped a zero there. You should be able to measure the ring end gap by stacking a number of feeler gages together. I have the set which is 25-30 blades sandwiched together where I can slide out whatever I need. End gap needs to be measured at multiple locations down the bore...top, middle, bottom. Use the piston to square up the ring in the bore. When I did the top end on my /7, I never got comfortable trying to measure the ovality of the bore...I eventually had a machine shop do it for me.

With 145 psi compression, that sounds like a pretty good number. I'm guessing you didn't do a leak down test which would have provided more information.

I can't really make sense of the criticalness of all of your measurements. But with compression at 145 and you mentioned no oil burning (just leaking), I might be inclined to hone the cylinders and put a fresh set of rings in it. YMMV.

 

[10drum]

20774, first of all, thanks for the reply, and yes, this is on the 71 r75/5. I did drop a number, it is .0047”. I corrected the thread.

With the british bikes I’ve done, the pistons start at standard (STD stamped on piston), then each bore is .020” over, the piston/cylinder clearance is .004”, and the ring gap is set to .010” to .012”. When I saw a ring gap of approximately.080”, I thought wow!, how did I get 145# compression? I sort of thought the 3 piston sizes listed in the Haynes manual were all standard bore. I really think I’ll just need to hone the cylinders and install new rings.

Thanks again for the reply.

 

[SCBuckeye]

"I might be inclined to hone the cylinders and put a fresh set of rings in it. YMMV."

Best advice from above!!

I had similar issues with my R100 and all I did was to purchase new standard size rings, and lightly deglaze (not heavy honing) to get new rings to seat. What an improvement!

 

[10drum]

SCBuckeye, thanks for the reply, I noticed your Will Rogers signature quote, I’m a native to Oklahoma and have been a Will Rogers fan for many years.

 

[10drum]

I was reading the “examining the cylinder barrels” section in Haynes manual, and it gave a method of determining piston/cylinder clearance. I followed that method and my cylinders are within specs. I should be able to hone the cylinders and install new rings. I guess when all else fails read the manual. Thanks for the replies.

 

[na cl k9]

I don’t have a thick enough feeler gauge to measure the ring gap.

stack a few feeler gauge leaves together and add up the total...

 

[10drum]

Na, that’s what I did, I was just shocked at first to see such a large gap. The book list end gap is .0118” to .0177”. I had to stack .063” on the right and .055” on the left, in the wear area, and .056” and .050” respectively, at the bottom of the cylinder. I am assuming the “rings” are worn from an approximate .015” gap to .055”???

I’m thinking I’ll hone and re-ring.

I am taking the parts to an experienced BMW mechanic tomorrow, I hope he’s more professional than the first few “experienced” British bike mechanics I met ten years ago. Thier experience was BS in, “that’s good enough”, and “these old bikes are suppose to leak”.

Thanks for the replies.