R80/7 Piston, rings and bore measurement

[elburro]

Hello,
i write from sunny London, this is my first post on this forum.
I have an r80/7 (1980).
I bought it last year with the intention of customizing it.
I think i have achieved what i'd planned to but, after taking the bike for a test ride i noticed that a tiny bit of oil was comingthrough the finned nut on both the exhausts. (both rings are installed properly) and the max speed i could reach was 25-30 mph (45kph).
Pressure test shows around 5 Bar on each cylinder.
Take all apart and check for piston ring end gaps, they are over the Clymers tollerance. Left cylinder is 1.3 and right 0.80.
Both pistons are marked 84.775 the left one has a 16 and the other H12 mark on them too.
(what are the last two number for?)
I am no mechanic or engineer, just want to see if you can help with this issue.
I am waiting for a telescopic bore gauge to arrive on thursday,
I guess rings replacement will be the best way forward but what size?

Thanks
ElBurro

[boocephus]

Funny you didn't mention if it was smoking.

[Kurt in S.A.]

5 bar compression? That's not even 75 psi...how did you do the measurement? Engine needs to be warm and generally the carbs need to be off the bike otherwise you restrict air supply to the intake.

Oil out the finned nuts? Valve guides??

Good luck on getting accurate bore measurements...I don't really have the touch I guess. I recently overhauled my R100/7 and couldn't get consistent bore measurements using a similar telescoping bore gage. I took it to an engine machine shop and they did it for me. In the end, my bores were too far out of tolerance, so I bought all new...Siebenrock. My bike had 100K miles, though.

If you ring measurements are that far out, I suspect that the bores are really worn and the piston might be rocking inside the cylinder.

Kurt in S.A.

[Ken in Oklahoma]

Hello,
. . . I think i have achieved what i'd planned to but, after taking the bike for a test ride i noticed that a tiny bit of oil was comingthrough the finned nut on both the exhausts. (both rings are installed properly) and the max speed i could reach was 25-30 mph (45kph). . .

Poor top end performance is a classic symptom of torn diaphragms. I would take a look at the carb diaphragms for any holes or tears. It's easy enough to do by unscrewing the 4 screws on top of each carb. (Be careful though. The screws are often "frozen" with corrosion. Heat and penetrating oil would be my first remedy.)

My R80/7 parts bike has the flat top on the carbs (as opposed to the "usual" peak) If yours have the flat tops I believe that replacing the diaphragms also requires replacing the slides. In other words, more money. You will easily see if the diaphragms are OK. And if not, well I suppose that's part of the cost of enjoying these old motorcycles. And the good news is that new diaphragms will likely last for a few more decades.


Ken

[elburro]

Thanks for the quick replies.
Diaphragm already changed on both sides (after test ride)
With engine warm and no carbs on, i have managed to get a better reading, still only 7 bar (100psi).
I have added a bit of oil through the spark plug hole and test read16bar (230psi)
I think the rings were not mounted with the gap pointing up (top),
Could this make things worse?

Thanks
elBurro

[boocephus]

The rings just need to be staggered so the gaps line up. I don't think they stay where you line them up when you assemble the bike.

Are you sure that the tiny bit of oil you see is not anti-sieze? The exhaust gets so hot, I can't imagine liquid oil sticking around for long. The finned nuts do get stuck on and anti-sieze is recommended. Maybe someone just oiled them up hoping they wouldn't stick.

My bike smoked pretty noticably before I did a top end on it. It performed fine, really. i just hated to have the BMW that smoked. There are so many other things that would seem to have more impact to a top speed of 30 mph. Ignition or a mis-assembled carb. If you put it in gear and bump the motor with the back wheel, do you feel resistance?

[Duane Ausherman]

using a bore gauge is the hard,slow and inaccurate way to do the job. I have a website (W6REC.COM) with a lot of information that may be of some use, especially on this issue of rings. It doesn't matter where you put them, they move around. We always put them slightly staggered, but all up for easy assembly. Don't use a ring compressor, you don't need it. To use one risks damage to the piston skirt. The cylinder has a built in ring compressor.

Their are two ways to test how you are losing compression. The leak down test is the most accurate, but on my site is the riding method with a friend following behind you at night.

[elburro]

Actually, i noticed the oil leak as it was burning on the exhaust.
Then i saw more oil on the links.
Duane, i came across your website, it's great and full of information.
I am happy i find this forum.
Unfortunately I have bought the gauge already.
I would like to carry on a leak test but i guess that will be expensive to do in London.

I will have a good read through Duane site to see if I can get more information about the rings.
Thanks
Elburro

[Duane Ausherman]

Just go to http://w6rec.com/duane/bmw/piston/ringtool.htm and see how to very accurately measure the cylinder wear. This won't tell you some of the arcane types of wear, but is overall enough for our purposes.

[elburro]

Here are the measurements taken with the telescopic gauge.
Both cylinders are 84.8mm.
The pistons are marked 84.775.
What ring I need to get the rings end gap to spec?

Regards
Elburro