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DiabloADV 05-14-2012 07:14 PM

Next bit of advice -- Big Bore or Stock?
I've resolved the next step in my refurbishment of the R90/6 with the bad cylinder. This motor has R90 cylinders, bored out to fit R100 pistons,but with stock R90 heads. I found a nice set of R90 cylinders so I can start over.

So...I could go one of two ways with this. Have the new set of R90 cyl's bored one over, and put new pistons in it. About $750 total cost, sticking with the R90 setup.

Or -- have the R90 cyl's bored to accept the R100 (93.97) pistons that I already have. About $250 total cost. More low-end grunt, which is where I like to ride.

Obviously, I'd like to spend less. But is it a good idea to do that overbore? The machinist is quite experienced with airheads and says he can do it.

I googled the topic and there are some folks who have run 100s with R90 heads. But, the ones I found were cylinder swaps, not big-bore treatments. Can the R90 cylinder be safely bored out that far?

bmwrench 05-14-2012 08:06 PM

1000cc pistons in the early R90 cylinder is risky-the oiling groove in the cylinder spigot makes it very thin in that location. It's been done (I've done it myself), but I was not confident that it was a good idea. You can buy a 92mm piston set from a certain independent shop...

DiabloADV 05-14-2012 08:17 PM


Originally Posted by bmwrench (Post 18688444)
You can buy a 92mm piston set from a certain independent shop...

And that shop would be...?

edit...found it.

gsd4me 05-15-2012 02:24 AM

Go `one` oversize and get the heads twin plugged. When the gearbox has to be done up, fit the 5% higher 5th gear.

carpetburn 05-15-2012 03:51 AM

i run 1000cc nikasil jugs with 75/6 heads[twin plugged but otherwise standard] and has run well for 4 years or so, just replaced exhaust valve guides but heads work fine on bigger jugs.

DiabloADV 05-15-2012 10:49 AM

Higher 5th gear...part number?

My gearbox is getting overhauled in two weeks at a tech day...

gsd4me 05-15-2012 04:02 PM


Originally Posted by DiabloADV (Post 18692424)
Higher 5th gear...part number?

Um....dunno; someone else here will have that info. Do others` think the higher 5th gear will be a good idea on this bike?

DiabloADV 05-15-2012 04:32 PM


Originally Posted by GSD4ME (Post 18694678)
Um....dunno; someone else here will have that info. Do others` think the higher 5th gear will be a good idea on this bike?

It would be a good idea on mine since I have a 37:11 final drive.

sigpe57 05-16-2012 09:53 AM

Keep it original. I had 2 r80gs that had been converted to 1000cc by PO. Never liked it. More noise and vibration. I found out how balanced a 800cc motor is after I bought 2 more STs. It's perfect balance between r65 and r1000. German engineering like Porsche and Mercedes, keep it stock is the best. Go try a r1100 oil head, u will know how much more vibration with just 100 cc more.


DiabloADV 05-16-2012 10:18 AM


Originally Posted by sigpe57 (Post 18700353)
Keep it original.

I hear you, but original is my most expensive option since I already own the 93.97 pistons. They are known good pistons and I'm reluctant to buy used (and more possible trouble).

I have it on good authority that what the bike came with (97mm-spigot R90's bored to 1000) is problematic on a number of levels. The spigot is too thin, mainly. Plus, the cylinders don't line up with the squish band in the R90 heads.

After a visit with the machinist (veteran airhead guy) yesterday, and after picking up two very nice 99mm-spigot R90/6 cylinders with new SS pushrod tubes, the plan is to...

Bore the case to 99mm. Easy and cheap to do.
Bore the new cylinders to match my nice 93.97 dome-top pistons. New rings and pins (which I needed anyway).
Machine the squish band ring on my nice R90 heads to match the new cylinder diameter. Doing a valve job anyway, so this is simple to do while it's apart.
Run it with the compression-reducing cylinder-case gasket.

This shop work will only run me about $300. Total cost of the repair, over what I had to spend on refurb anyway, is about $400. To change piston size would cost a lot more in parts and only save half the shop costs.

A bonus is that the cylinder and head work include having them fine-grade bead blasted, cleaned and returned to me looking like new.

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