I have a friend . . .

[Red_Hen]

He bought a non-op R75/5 thinking it would just need a few things to get going.  

I know how the story goes  

Anyway, he discovered he had no compression on the port cylinder.  He asked if I'd help remove the starboard exhaust header stuck inside the cylinder head - with some PB blaster and coaxing, we got it out.  When we removed the cylinder, the piston had rust all over it.  We pulled off the cylinder and it looked good, just some rust.

He took it to the machinist I used on my R65.  He was told the right cylinder needs to be slightly bored - when I heard that, I suggested he find a used cylinder in good condition.  But, he says he spoke to one of the mechanics at Bob's BMW who says cylinder can be be bored @ .005 (not sure if decimal values correct) and by using different rings on that cylinder it will be fine.

To my way of thinking, both cylinders should be the same, and both pistons should wear the same rings.

Just seems wrong.  Anyone care to chime in?

[nhmaf]

Well, technically it could run and all, but it would run smoother if both sides had the same mass moving back and forth.   They do balance pistons and connecting rods for a reason on race bikes.  Depends on how much money your friend has available for the project at this point, I guess.

It sounds like the mechanic thinks the rust situation is not sufficiently bad to warrant going to the first oversize piston?    I hope so.

[Red_Hen]

My friend came to his senses - he's going to do it right.  He's talking about boring both cylinders & buying oversized pistons

Don't see why he can't just buy a used cylinder and use the existing pistons - wouldn't that be more cost effective?

[montmil]

My friend came to his senses - he's going to do it right.  He's talking about boring both cylinders & buying oversized pistons

Don't see why he can't just buy a used cylinder and use the existing pistons - wouldn't that be more cost effective?

A reasonable question, Ken. And a tough one, too.

Buying a used cylinder gets you just that -a used cylinder. You'd need to ask yourself:

1) How many miles on the used cylinder?
2) Is the bore up to spec? Has it, too, been bored?
3) Is the bore tapered due to infrequent oil service and a dirty or missing air filter?
4) Will the less expensive used cylinder end up needing a first over bore?

I don't think there's a solid right or wrong answer here. You make your choice and you pays your money. I'd certainly let your friend make the final decision since he's picking up the tab. Quien sabe?

I think I hear iron butterflies flapping in someone's belly.  

[wilcom]

We pulled off the cylinder and it looked good, just some rust.

There was NO compression... like in "zero"? Did he put some oil in the cylinder and take another test?

If the cylinder looked good, no scores or gouges, just rust,  I would hone it and run it. Clean up the piston and see what you got and this is all "por nada"

If you still had NO compression or low compression I would dig deeper b4 I spent money on new pistons and fresher barrels.

Just my frugal take on it......... nothing spent everything to gain

[tvrla]

Here's the picture as I see it -

The rust on the piston is due to the rings - so it should be fine once cleaned up with new rings.

The cylinder? How bad IS the pitting? Where in the bore? Pix?

If the pitting is minor enough that .005 would fix, I'd be tempted to run as-is - depending on where in the bore it is. Hone both cylinders and in stall new rings.

I'd probably source rings from Deves or Hastings.

[Red_Hen]

I'll pass along your suggestions - thanks.

It was the owner of the machine shop who suggested he bore to clean things up - will follow up with what action taken - thanks again.