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k100, new rings?

rwik81

New member
anyone have any knowledge of replacing the rings to increase compression value? Is it worth the trouble? How much could it increase?
thanks.
 
anyone have any knowledge of replacing the rings to increase compression value? Is it worth the trouble? How much could it increase?
thanks.

Well, first, I would need more information, like what year K100, how many miles, do you have a confirmed low compression problem, etc.? The classic K-bikes are famous for rings and pistons going 250,000 to 500,000 miles, so yours is an odd question. Thanks!
 
right

it is all true,
but riders don't care about compression, they just....................ride
but after 500,000 will rings help? by how much?
 
For brick engines; at 500,000 miles it may be time for the engine to be rebuilt. Well, maybe somewhere between 500,000 and 1,000,000. Just sayin. :)
 
...do you have a confirmed low compression problem, etc.?....

Yes, you'd want to do a leak down / compression test before you did any major work on it.

Is it worth the trouble? ....rwik81

If the compression is, in fact, low, it may be more cost effective to replace it with a later, low mileage, unit.
 
Last edited:
anyone have any knowledge of replacing the rings to increase compression value? Is it worth the trouble? How much could it increase?
thanks.

More information is needed:

1. What year and model, and how many miles on the motor.

2. What is the current compression reading for the motor?

3. What symptoms is the motor exhibiting, that you feel indicate a need for service?

If you furnish this info you’re more likely to get useful and accurate responses to your questions. As pointed out, brick-K bikes normally go 200k+ miles before needing attention to valves, which are likely to need attention long before rings would. And if the bike needs valve work it is probably less expensive to buy a used motor, or even a used k-bike, for less money than going completely through the cylinder head. The last used brick-k engines I saw marketed sold for around $500 and that was a long time ago.

A noted west-coast long distance rider had a K100RS in for a valve job at 275k miles, after it began to exhibit difficulty in starting. While the head was being redone, the rods, crank, and pistons were removed for inspection. Everything measured within BMW’s published tolerances and no service required—the cross-hatching on the Nikasil cylinders could still be seen. The engine was reassembled with new bearing shells—just because they had been taken apart—and put back into service with a rebuilt cylinder head, likely good for another 275k.

It’s common to see brick-k bikes go that sort of distance, needing only valve clearance checks every 36k or so, and customary replacement of the rear main seal and clutch o-ring around 65k-75k miles, an interval that can be greatly extended by replacing the OEM buna-n o-ring with a Viton o-ring.

Gas in the top? Oil in the bottom? It’s a brick-k: GO RIDE! :thumb

Best,
DeVern
 
The simple answer is: Assuming the valves are sound, new rings could bring compression back up to OEM factory specification. No more, and probably not a lot less.
 
Given the value of the old K100s these days, I'd have a hard time spending any kind of money on rebuilding an engine in one. Of course given their reputation for longevity, I also have a hard time believing yours really needs the work. If you've managed to put in the several hundred thousand miles that it typically takes to show some engine wear, it may well be time to just pick up another one.

Hopefully you're not being mislead by some blue exhaust smoke at start-up?
 
hall effect sensor

yep, your right, my neighbor sold me some old ones 2 fer a bit.

by the end of this anyone thinkin of duplicatin the way and really just wanna ryd will just go out and buy something that works

but here is start to rebuild, hope pic uploads otherwise will have to try again.
 

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Rings

If you’re just looking for more power rings won’t accomplish that. Buy a bike with a bigger engine. Low compression is another matter and is more likely worn/out of adjustment valves.
 
thats an idea!

I took it apart,
per my manual
measured piston ring gap
old ring new ring
top 0.017" 0.014
middle 0.016 0.014
oil 0.019 0.013

spec. 0.005"

looks like new rings wont do alot of good
 

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I took it apart,
per my manual
measured piston ring gap
old ring new ring
top 0.017" 0.014
middle 0.016 0.014
oil 0.019 0.013

spec. 0.005"

looks like new rings wont do alot of good



As expected. And, just think that the last person to look inside your cylinders was some dude at the factory in Germany a very long time ago. :laugh
 
k100 oil vent hose

hey everyone, happy trails
I made this awhile back and have several extra I can give away.
..............
if noone wants one then more for me, if lots want some(which I don't thik will be case, I'll have to figure out)

send email to bmw@dslextreme.com with your address...........
I supppose first (maybe only ) response will get it.
take care during the covid crisis.
 

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Those bores look pretty good with cross hatching still visible. Don't know how the rest of the bike is, but seeing as you've come this far it would certainly be worth putting new rings in, if the rest of the bike justified it. Without knowing the compression numbers and mileage on the engine, hard to tell what is the best for your situation.
 
bore and piston gap

I tried the new ring thing. stock new rings from factory are out of tolerance, see above.

this week, piston bore and piston side gap
@66.89mm, its a grade A which can be between 66.966 & 67.005mm
unfortunately,
piston clearance is 0.013" max allowed 0.0006-0.0015"

apparently the piston wore,
it might be surmised that little extra gap could cause a "knocking" as the rpm increases leading to more vibration at higher speeds?
 

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Your piston gap is within specs. If you had any piston wear on the bore you would see it right away. If the pistons themselves look good with no rubbing marks then everything is fine. The only way for the pistons to wear is if they contact the bore. That could happen if the rings where worn down to thin. So as I stated previously, if the rest of the engine is good, then install new rings. The rings are the wear item not the piston.
 
main bearing runnout

the saga continues
spec says 0.008"- 0.0022
measured 0.007", just barely ok
 

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development of pistons

Just figured new pistons may in order,
how things have changed in a short time
 

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