After getting the BMW running great for the first time in 2 years and having too much fun zipping around town I decided I’d better focus on fixing other long time issues to continue with making the R90 into a solid and reliable side car platform. The pushrod seals and rear main seal had been done by the previous owner shortly before I got it which helped push me over the edge on pulling the trigger on this bike. Unfortunately now 7 or 8 years later the pushrod seals were now starting to weep and were in need of being replaced again. I had already gotten a full set of top end gaskets and oil pan gasket with the carb rebuild parts. I figured it would only take a weekend tops.
What I found when I pulled her apart was shocking and unexpected to say the least. The left cylinder had quite a bit of pitting on the top side of the cylinder and a lot of discoloration on the bottom. The biggest rust spot on the bottom did have a bit of a slight raised swede feel to it and the other spots were smooth with the barrel. The pitting did not feel very deep but could feel it with a finger nail.
Listen to the video, there was no indication that one cylinder was this damaged.
What a way to kill a guys spirit. From the high of finally getting the thing running great and seeing it starting to come together to your dreams dashed on the rocks. Ouch.
I knew one thing, fixing this was not going to be cheap.
The other cylinder looked perfect by the way. Pretty damn good for around 90 to 95,000 miles on it. The right side had always had a bit of oil weeping from the base of the cylinder and I was expecting to find a pulled stud but instead I found it was leaking because this side did not have the O ring at the bottom of the cylinder! Some good news.
At least now I know why I had a ring set in the box of spare parts from the previous owner that was missing the oil ring…
This also finally explains why this thing smoked like crazy if you happened to leave it on the side stand. I know everyone says that they all do that but this was on a whole other level, we are talking Uncle Buck car type smokeage. After I smoked out a tech day and sent people running for air even fellow airheads said mine was particularly bad. If left on the center stand, however, it never smoked, again check out the video up top. Unfortunately you cant always park on the center stand.
I had always assumed that it needed valve guides done, probably still needs those too, and I’d been trying to avoid that expense as long as I could by never parking on the side stand when possible.
So how do we fix this mess? I’ve put it out to the Airhead Community and I’ve come up with a few options…
- New R90 cylinders and rings – $$$$ not an option
- Used R90 cylinders with pistons – I’ve seen sets on http://marketplace.ibmwr.org/ and Ebay from $300-$500 of unknown mileage so a bit of a gamble. Also I have a 1976 and only a set from another ’76 will fit.
- Used R100 cylinders with pistons – I had incorrectly assumed that any R100 cylinders would be a direct swap for my ’76. I learned from Ted Porter that “you could look for a good used set of BMW 1981-on Nikasil cylinders and pistons which will require a minor modification to the cylinder base to be used on your 76-on block. There is a small step of metal inboard of the base oring groove that is designed to fit into a chamfer in the 81-on block which your 76 model does not have. We just remove this step for $70.00/pair and the cylinder fits the 76-80 block just fine. Keep in mind the 81-81 Nikasil pistons were low compression at 8.2:1, the 1988-on Nikasil pistons were 8.5:1 so they’re a little more desirable.”
- Bore to next size with new pistons – A local shop has quoted me 50 bucks a side to bore them and their machinist was pretty confident that one size up would take care of the pitting, [Motobins has first and second over pistons for £154 ea] [ Motoren-Isreal has Wossner pistons for 349 Euros a set ]
- Have cylinders restored, bored and nikasil coated and reuse existing pistons with new rings for nikasil – http://www.powersealusa.com/repair-process/ I heard about this process also from Ted Porter who carries the rings “at $76.55 each, two sets required” and got a quote from PowerSealUSA “From the looks of the photos we should be able to fix them by honing to accommodate the plating thickness then plate/hone to size. Price to plate a steel/cast iron liner is $225 per cylinder.”
- Siebenrock kit – from Motoren-Isreal – 817 Euros with shipping to the US. From Motobins it is more but comes with a gasket set.
I think the first 3 are not an option, at the going rates for questionable used parts I can get new pistons and a rebore. I’ll still keep an eye out for a really good deal on a post ’88 R100 set though.
Wossner pistons and a rebore look like my cheapest option for new parts. The downside here is I really want nikasil cylinders that will last forever. A few guys that have gone this route that replied to my forum threads have been very happy with the results though.
Having my existing cylinders nikasil plated is a very intriguing option that I was not previously aware of and roughly the same price as the others.
I really think the Siebenrock kit with the horse power boost is my best option with this bike being built up for a sidecar tug in the future. Unfortunately $930 is a big expense that I was not expecting, the good news is with the dollar surging this option is $270 cheaper than it was just a few years ago if you buy from Europe.
UPDATE: Several other unexpected expenses that also added up to $1000 have pretty much killed this restore project for the time being so I’m going to go with plan G: hone the cylinders myself, re-ring and call it good for the time being. It’s run decently with plenty of pull all this time in this condition so I don’t think it will hurt to leave it the way it is for another 6 months until tax return season rolls around and I can revisit this then, along with getting heads done.