Mark -
Welcome aboard ! I don't approve of a "leak down test", but it sounds as if the engine is already apart, so I'll save my breath.

► Any BMW dealer can provide standard rings. Try Max BMW..... http://www.maxbmw.com/

► Not much description and no photos of this "rust". Hone the cylinders with a fine (400+) grit bead hone. If you can catch a finger nail on the ledge, then the cylinders need boring. If not, then re-ring. You may see a slight increase in oil consumption with the rust ring at the bottom of the bore, but that's not for sure.

► If the cylinders need boring you can buy a used set on Ebay or from parts listed in the For Sale portion of the forum much cheaper than you can bore and purchase new pistons.

► You'll want to also be sure and replace the 4 black rubber seals at the bottom of the push rod tubes no matter what. Those need to be pliable or they'll leak badly.

► You'll also want to look at the condition of the valve seats while the heads are off. You'll be looking for signs of valve recession. Probably won't find any, but you need to know that. Then check the valve clearance regularly.

Hope this helps.

Thanks for your help. I attached a picture of the rust in my first post. The bike had oil leaks from the tube seals and rear main seal. There is no ridge at the top of the cylinders so just hone and re-ring will do the job. ( I hope) Rings are ordered this morning.
Cheers, and glad to be here.

Thanks for your help. I attached a picture of the rust in my first post.

Mark -
My bad on the photo. Apologies. However, it would be better to have a photo after the hone. This because a digital camera highlights every detail to the extent that things look worse than they are. That is to say, it's hard to tell rust staining from actual rust pitting. It's only rust pitting you need worry about, because the pitting can pass oil (leading to oil burning) and, when excessive, damage the new rings.

The bike had oil leaks from the tube seals and rear main seal.

Be aware of 2 things... 1) Before you remove the flywheel the crank must be chocked to the rear or the thrust washers will fall out. Read about this important step in your manual. 2) You also want to replace the oil pump o-ring at the same time. That's under the flat plate with 4 screws and often accounts for as much oil leak as the main seal. Fitting any oil seal is much easier when done with a product simply called "P80".

There is no ridge at the top of the cylinders so just hone and re-ring will do the job. ( I hope.) Rings are ordered this morning.

I wasn't referring to a ridge at the top of the cylinder. I was referring to a ridge left by the "rust ring" created by the rusting piston rings. Exact course of action depends upon... 1) how deep the rust pitting into the cylinder wall is, and 2) how far away from TDC the rings were left to sit.

Not to sound insensitive to your predicament, but "hope" has very little to do with a solution that's going to serve you well. You need an expert hone job followed by a soundly reasoned assessment of the depth and location of the pitting.

If there is any doubt, due to the low cost of used R75 cylinders, cylinder replacement may remain your best avenue.

All the best.

Hello,
I finished the honing a couple days ago. The rust didn't clean up fully but I am already at the maximum diameter allowed (3.23in). Since a cylinder swap can be done on the bike I was going to see how it ran. The attached picture is after hone. You mention used cylinders are cheap. Any source other than Ebay?

Thanks for the tip on blocking the crankshaft. I've read that many times and am aware of the danger. The seals are ordered and on the way as are the tools from Cycleworks.

Most of my time has been spent cleaning. I'm amazed at the amount of oily grunge in all areas: even under the starter.
Thanks for the help.

I should add that picture was taken before I put the 30 to 45 degree cross hatch pattern to the bore.

I should add that picture was taken before I put the 30 to 45 degree cross hatch pattern to the bore.

Thanks for adding that. I was looking at the photo asking myself where's the "cross hatch" ??!!

When you previously said the rust was at the "bottom of the bore" I read that as being at the "bottom of the stroke", but those 2 descriptions are not the same. Your most severe rust was at the top of the stroke where it is the MOST important for the rings to make really GOOD contact with the cylinder wall for compression AND oil control. The fact that the rust area didn't get honed indicates it is one or two thou below the level of the bore... and very broad. A scratch, or casting error 1/16" wide in that same area might be passable, but due to the huge width of your rust area, I do believe in my professional opinion that the cylinder is toast.

You have 2 options.... Bore both cylinders to a larger size that cleans up the bore, and purchase new rings and new pistons (probably $350+, not counting gaskets). Or buy good used R75 cylinder(s) and fit your existing pistons with new rings (about half that price, not counting gaskets).

You best place to start searching is on this forum in the AirBazaar pull-down menu simply becasue you're dealing with fellow Club members who want to see you back on the road. Second best place would be the used parts dealers in your Club magazine, like Re-Psycle. Then maybe http://www.ibmwr.org/ or some of the other BMW "breakers" shown on-line. Lastly, Ebay.

Hope this helps.