R90/6 cylinder rebore

[yappo]

can these be bored?i brought a set in to be looked at,and after miking they found them tapered with one about 3 thousands off.is there any precautions that must be taken?must they be heated first?i remember reading that once but i dont know if it applies to this situation.well, anything you can lay on me will be appreaciated.regards to all,brian.

[barryg]

Yes, they can be bored. There are 1st and 2nd overbores. No do not heat. Boring will be done at room temp. Good luck with your project, and find a competent machinist for said job. Leo Goff at Memphis Motorwerks is one I would recommend.

[20774]

Bore-Tech has gotten a lot of discussion on the Yahoo /2 Forum.

http://www.bore-tech.com/

They provide a permanent surface enhancement to keep your cylinder/pistons at their peak condition. Something like the Nikasil, I'm sure.

[yappo]

i'm on the phone with them now.

[crazydrummerdude]

can these be bored?i brought a set in to be looked at,and after miking they found them tapered with one about 3 thousands off.

Is 3 thousandths off the diameter enough to warrant a rebore? :ear

My R90's aren't tapered, but are a consistent 2 thousandths over.

[lostboy]

I wouldn't rebore them if the motors oil consumption was acceptable. BMW usually bored the cylinders straight and round, but clearance was rarely in spec; they are frequently "loose". It's amazing, though, how much smoother and generally sharper they run when the clearance is correct.

[rbryson]

No one has mentioned how performance is affected. As a kid all of us rode Honda CL50's. A friend has his bored and it was so much quicker. I would like to hear from anyone who was had their R90/6 bored. Could they discern a difference? Do the carbs need to adjusted? How about timing? :wave

[barryg]

I use to work in a automotive/motorcycle machine shop. Bored a lot of cylinders and did alot of head work. We did work on standard street application and performance work. For street aplication, most of the work is getting the heads and cylinders back to factory specs, so the customer will get good long term use out of their vehicle. I had customers send me their heads and cylinders with over 100,000 miles on them. They had been meticulous in maintaining their bikes. I reworked the heads and cylinders. Sometimes the cylinders were in tolorence so all they needed was honed and crosshatched. Pistons got a new set of rings. Sometimes the cylinders were worn out and out of spec, so they got bored to 1st or 2nd over as needed and a new set of pistons and rings. When they got their parts back and installed, They couldn't believe how well they ran. They had bought the bikes new and they performed well. Over time the performance had waned so slowly, they never felt it. With the cylinders and head back to spec. their bikes/cars are running at optimum, their happy. Almost like having a new bike. On this type of machine work all your adjustments carbs/timing etc. will be as before. I'm looking at my '74 R90 engine right now in the den. It's on an engine stand. It's like a work of art. Did the heads , dual plugged them. Bored the cylinders out to fit new first over pistons. Need to get it back in the bike.