Compression readings on new rebuild

[DonW]

Hi all,
I have about 100 miles now on the rebuilt engine (rings, valves, guides) in my '92 R100GS. Hot compression test (after a long ride, both plugs out, carbs on, throttle wide open) on both sides is about 120. My R80ST (with a '92 R100GS engine and about 20K miles) gets 150 on both sides, identical conditions (and essentially identical motors).

Yeah yeah, I know, you're suppose to remove the carbs, but both bikes are set up the same. Even double checked combustion chamber volumes (all are about 65cc) on both bikes thinking maybe the ST has high compression pistons.

Thoughts? Maybe the rings aren't seated enough yet? The GS has a valeo starter that spins the motor about 1/2 the speed of the Bosch on the ST- could that make a diff?

thanks,

Don

[Garnet]

Do both engines have the same compresion ratio? If they are both US engines they both shoud be 8.5/1, but there where 9.5 engines availabe elswhere in the world.

Does the engine in the R80ST have a bit of carbon bulid up?

120 is about right for an 8.5 engine.

[DonW]

Hi, The top end of the engine on my ST came from a US '92 GS, the bottom end is the original '83 R80ST. And the '92 GS is a stock US model.

I wondered about the 9.5 issue, so measure the volume of both compression chambers- nearly identical. And, I put the 1000cc jugs on the ST less than 4000 miles ago, and cleaned all the carbon.

???

Don

I

[Duane Ausherman]

By 100 miles the valve really should be seated. Did you check them by hand lapping? Could they be way out due to new guides done by an idiot?

Unless you have forgotten, or assumed something, you are right to be asking. I would wait another couple of hundred miles before I took it down. Do you have a leakdown tester?

[DonW]

Only one guide need replacement, and it was done by an experienced BMW machinist. I hand lapped all the valves and they seated perfectly...

[Duane Ausherman]

Don, that is what I expected to hear. I would put a few hundred miles on it before considering any tear down.

Do you have a leak down type tester?

[StephenB]

I assume you have measured piston and cylinder clearance and all rings are installed with the correct end gap. The rings were made of cast iron so that they can wear on the harder Nikasil coating and bed in. You installed them completely dry and use the heavy handed engine run in: 1min 3000-4000rpm on dry rings, shutdown,cooldown, 34-40mls run-in ride with heavy throttle action (accelerate-decelerate-accelerate-etc) and finally an oil change to check for particles. Using that method, I arrived at 138PSI with carbs removed on both sides of a cold 8.4 1000cc engine from a R100CS that actually has some small holes in the coating.

This is a good method to get rings seated well, I used it on all my current and past bikes.

Maybe you want to ride another 1000mls und measure again. I would not be worried if the the reading is 120PSI on both. At least it is equal. Just watch it.

Stephen

[DonW]

Thanks Stephen,

Yes, a modified version of that break in- end gaps were good, started it up, let it warm a bit, and then rode the hell out of it for 30 miles. Next day, same thing.

Interesting that you mention jugs with small holes in the coating. I have a few jugs like that and thought they were scrap. Only a few spots... think they are OK to run?

The bike runs great- I'll just continue to ride the heck out of it and see what happens.

Don

[Frog]

Don:

I have a leak down tester if you want to give that a try.

[DonW]

Thanks Blaise, I might. I keep meaning to buy one, but... There is one at Harbor freight on sale now for $35, anybody use this one?

Don