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Old 02-27-2015, 09:25 AM   #1
SpiderHelix OP
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Question Newbie with a 90

Hi All:

Picked up a 1976 R90/6 with 17k miles that I am calling "Acid Rain". The name may give you an idea on the bike's condition.

One piston is stuck, stuck. Currently soaking with ATF and acetone blend.

I am almost sure that rusted cylinder and piston is going to be toast and cylinder has a broken fin anyway. Anyone have an R90 cylinder in the hard-to-find 1976 99mm spigot for sale?

My other thought is R100 cylinders and pistons which are easier to find. I would use the R90 heads and carbs. The heads from an R100 are different castings, but use the same valve sizes.

Any real-world experince putting R100 jugs on an r90? I know later ones need the o-ring recesses machined off, but really would like to hear from some one who can tell me how the bike will run.

Thanks!
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Old 02-27-2015, 12:05 PM   #2
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why

90 is a smooth running bike... why change it.
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Old 02-27-2015, 12:35 PM   #3
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He thinks he needs a piston and cylinder because they are stuck so why not use a Nikasil set?

It'll work.

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Old 02-27-2015, 02:22 PM   #4
Kt-88
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I ran exactly that setup for a year. It was wonderful. No real power difference, but still smooth and no mechanical problems. I'd just find the best condition r90 or r100 cylinders and slugs you can. Check Roland Robitialle's stuff- he posts on ibmwr often- and his prices are very reasonable.

There was no complication with the swap, and no re-jetting.
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Old 02-28-2015, 07:45 AM   #5
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Thanks for the replies.
KT88, do you remember what year 1000 cylinders you used?
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Old 02-28-2015, 08:05 AM   #6
Kt-88
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Early. 77 I think. Non-nikasil. They were just what was available.
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Old 02-28-2015, 08:21 AM   #7
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Just bolted on without machine work?

Can't figure out what the ridge is that is supposed to be removed on later Nikasil cylinders according to bwmmotorcycletech dot com.
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Old 02-28-2015, 11:14 AM   #8
reinhardhardegen
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Quote:
Originally Posted by SpiderHelix View Post
Hi All:

Picked up a 1976 R90/6 with 17k miles that I am calling "Acid Rain". The name may give you an idea on the bike's condition.

One piston is stuck, stuck. Currently soaking with ATF and acetone blend.

I am almost sure that rusted cylinder and piston is going to be toast and cylinder has a broken fin anyway. Anyone have an R90 cylinder in the hard-to-find 1976 99mm spigot for sale?

My other thought is R100 cylinders and pistons which are easier to find. I would use the R90 heads and carbs. The heads from an R100 are different castings, but use the same valve sizes.

Any real-world experince putting R100 jugs on an r90? I know later ones need the o-ring recesses machined off, but really would like to hear from some one who can tell me how the bike will run.

Thanks!
my 2 pfennig..

I would reuse the cylinders. Get the rusted piston out. Don't worry about saving it because you can't reuse it (maybe someone else can but that's a different story). Buy two oversize pistons, either OEM or a kit from Motoren-Israel. The depth of the pitting will determin which oversize you need. I would take the jug to the machinist and get his opinion on that.

https://www.motoren-israel.com/index...mbkg0raatpcaa5

look for a machine shop that works on Harley Davidson. Any shop that can bore an old Harley cylinder will be able to bore a BMW cylinder. I used to go to Performance Harley Davidson in Tucson. The last time I had my R90S bored the owner had a Lycoming or Continental jug set up in his lathe when I walked into the shop. Unfortunately the owner died a few years ago and the shop is no longer in business.
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Old 02-28-2015, 04:37 PM   #9
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Plus 1 on parts from Roland....He is in New Hampshire.....
Good quality and reasonable prices.
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Old 02-28-2015, 05:03 PM   #10
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Plus 1 on parts from Roland....He is in New Hampshire.....
Good quality and reasonable prices.
Plus he puts old Cycle mags in the box. I got to read reviews on the NS400 and the Suzuki Gamma in an actual magazine!
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Old 02-28-2015, 06:59 PM   #11
Kt-88
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Plus he puts old Cycle mags in the box. I got to read reviews on the NS400 and the Suzuki Gamma in an actual magazine!

Man, I have so many cool magazines from him. ...more fun than receipts.

No machining, OP. Mine were iron r100 sleeves.
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Old 02-28-2015, 07:17 PM   #12
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Different bikes, I know, but a mate recently put the 1981 nikasel cylinders directly onto his 6/78 case. No machining, slipped straight in.
Both had the ridge, but the younger set was less obvious.




You can clearly see the recess for the o-ring .....


boxerboy81 screwed with this post 02-28-2015 at 07:26 PM
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Old 03-02-2015, 12:06 AM   #13
disston
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The step or ridge that is being referred to is on some cylinders not all. I don't know what year the step appears. I don't think it was the same year that Nikasil started but it could be. There is an extra notch taken out on the block to account for the step. If you use a cylinder with the step on it on a block that's not notched you have to either machine the block or take the step off the cylinder. Usually the step is removed from the cylinder.

It's not easy to find a picture that shows this feature (my bike is the easy, earlier, 97mm setup). The 99mm cylinders are stepped or not. Engines have been assembled and the step not even noticed.

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Old 03-04-2015, 09:39 AM   #14
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Thanks for info and pics! Finally got it apart last night.

The piston is junk, but I think the cylinder will clean up with a bore. It does have a broken fin, though. Add boring, rings and new pistons and it adds up quickly - $760! And I still have a broken fin on top.

Ebay has used R100 top end kits in steel sleeve and Nikasil for about the same money - $400. The steel sleeve will bolt-on for sure, but it sounds like the 1983 Nikasil cylinders MAY have the ridge and need machine work.

Decision time...
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