Bore or Hone Advice

[hudson]

1971 R75/5

Hey gents - the left exhaust has been burning smoke (oil) usually at start up and occasionally at throttle. I finally decided to pull the heads, piston/rings and barrel. After cleaning and inspecting the head over (valve guides, seats, valves, etc.) Most was in order. I ordered a few parts- springs and cam followers (very slight wear), but nothing major overall. Pulling the barrel was a different story. The piston looks and rings (aftermarket) look intact, but there is a scratch I can see and feel with my fingertip in the bore, and it is at the 6 o'clock position- I am pretty sure this is the culprit.

I need some input, in layman's terms please, in how to handle this in a economical way. Piston sizes, rings and sleeves are an area I don't quite know, especially if re-boring is involved, and I don't have the tools to do so anyway. I understand the concept, how they work and install, but not how to fix and replace if work is done on them.

So here are my options and questions:

1) Do I hone or re-bore and put oversize rings and/or pistons? How do I know if just rings or both are needed? How do know what size piston/rings to get after it is machined?

2) I haven't pulled the right side yet- assuming it is in good order, do I have to do both sides regardless? Or just the left one? I was told I should do both.

3) Having both re-bored out the same is almost the same as upgrading to the 1,000 cc. I do prefer to keep it as is, don't really need more HP. I like how it rides and is smooth, which I read would change with this upgrade.

If repair is the way to go, does anyone know a good machine shop somewhere in the Raleigh , NC area? Bombars has moved, and I believe he sourced it out anyway. Boxer2valve is in Hendersonville, NC - 4 hrs away, so if there are any competent shops closer would be most ideal.

Trying to decide the best way to fix this. Your input does not have to technical or overly detailed, just brief comments of piston/rings sizing issues, advantages/disadvantages or anything else that may be useful. Below are some pics I took. I darkened the scratch area so it can be seen more clearly, but it is a photo edit and looks worse than to the naked eyeball viewing it. Thanks guys...

IMG_4464 (Edited).JPG (48.23 KiB) Viewed 2023 times

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[Kurt in S.A.]

My limited knowledge:

1) I'd first consider honing and then see if the scratch becomes less significant. If it doesn't, then the next bore size will be needed. Boring the 750cc engine should go better than trying to bore the 1000cc one...more material to work with. At some point, you should probably have the cylinder measured at three points so you can evaluate overall wear of the cylinder walls. If you only hone, you should probably evaluate the ring lands on the piston to make sure they are within spec.

If you do go for a rebore, it will be a standard increase which I believe is 0.5mm in diameter. The place that would do the boring will likely want to have the new piston and rings in hand to confirm that they all match up when done.

2) I also think doing both at the same time makes sense.

3) I would think cost-wise, boring to the next size should be much less than trying to go to 1000cc. There are kits out there that are bolt-on. Your engine case opening is 97mm...it changed to 99mm for the '76 models.

Kurt in S.A.

[hudson]

Thanks Kurt - I have a 83mm silicon carbide flex cylinder hone. I think the grit is 240,for the end of my drill. I am familiar with how to use. I think I understand how to measure the gaps with a feeler gauge in the rings at different points per snowbum - but not sure what those specs should be though. To measure the ring lands, do you insert a feeler gauge in between the side of the rings and the piston wall of the groove? I don't know what spec that should be either.

The bike has only 31,000 original miles on it. The PO put aftermarket rings on it.

[hudson]

Another pic that was left out.

[Kurt in S.A.]

Wow, that scratch looks pretty bad, at least from the picture. Honing might not do it. Is that the right hone for an iron cylinder? That sounds like something for a Nikasil cylinder, which I assume you don't have.

I've never measured the ring lands, but my guess is you're on the right track. Per my Haynes, the end gaps for a R75/5 are:

top ring -- 0.30-0.45mm
second ring -- 0.30-0.45mm
scraper ring -- 0.25-0.40mm

Kurt in S.A.

[hudson]

Thanks Kurt. I was trying to be optimistic, but....

Well, I guess i just have to bite the bullet and re-bore. If anyone knows of someone who works on airheads for this in my area, let me know. Thanks!

[Kurt in S.A.]

I've heard good things re: Boxer2Valve, so it might be worth the drive. Bombars Beemers is a name I see from time to time...they're in Durham. You could check with them and see if they do it in-house or ship it out. I'd want to make that whoever is used, they have experience with BMW engines. I know that a cylinder is a cylinder, but it's just my concern...

Kurt in S.A.

[hudson]

I believe Bombars moved to NC coast. I will probably take it to Boxer2valve. It's a 4 hr drive, but my brother live about 45 minutes away, so at least I have that going for me! LOL

[Kurt in S.A.]

That does appear to be the case with Bombars...I was using info off an old listing. Nothing wrong with built-in room and board!! Now if Boxer2Valve could do it while-u-wait??

Kurt in S.A.

[melville]

Bore it! I did the same on Ernst, and it's worked out well. 17000+ miles since. The first oversize is .5mm or about .020 over. That scratch looks deep, though, like maybe the first OS wouldn't eliminate it.

Per robtg (retired ace Airhead machinist) go for .0015 skirt clearance on an iron bore Airhead.