R51/3 – Cylinder pressure after engine rebore:

[Allan.Atherton]

I don't know the CR of your bike, but 60 psi is very low. Did you measure with throttle wide open on hot engine after 5 kicks? Here are some other people's thoughts on the subject:

Compression = (Compression Ratio minus one) to the 1.2 power x Atmospheric Pressure
For CR = 7.5
PSI = (cr-1)^1.2 * 1 atm
= (7.5-1)^1.2 * 14.7
= 9.45 * 14.7
= 139

Alonzo Walker:
Below is the approximate relationship between, compression ratio and compression pressure at cranking speeds:
Ratio Pressure
6.5 110
7.0 120
7.5 130
8.0 140

Duane Ausherman:
For the R60, here are the numbers:
125-140 about as good as it gets, very few get over 135lbs.
110-125 still will tune well.
75-100 hard to start and tune.
40-75 may have to be push started and will not have power on the freeway to exceed 60.
Test engine hot with throttle open. We standardized at 6 kicks for our shop. The way the compression raises is very important. With an old weak engine it will take 10 kicks to reach maximum. When running, the engine only has 1 chance. The first kick should be nearly 1/2 of the total. An old engine will not get nearly to the top end or 1/2 on the first.

Marco Hyman:
On my R69S I get to about 175-185 PSI in 4-5 kicks on both sides. I believe that to be an "in the proper range" (154 - 191 PSI) number for the 9.5:1 compression ratio for that bike.

[Micha]

Any ideas for reasons, why I receive such a low pressure?

[Allan.Atherton]

Maybe you did not do the compression test correctly, or the motor needs the valves lapped, or the cylinders honed and re-ringed. Those would be the simple problems with easy solutions. There could be worse problems.

[Micha]

Well, I fitted new liners + new STD KS pistons. Can you please explain how the pressure test should be made? I attache the pressure hose to the spark plug thread and give a few kicks.
Cheers – Michael.

[schrader7032]

Michael -

Allan can add more but you might want to read this:

http://w6rec.com/duane/bmw/compression/index.htm

What about performing a leakdown test? It's a test where you put pressurized air into the spark plug hole and monitor the rate at which the air escapes. Some systems will compute a percentage for you...there are home-built systems that are documented on the Internet. A leakage of say 0-5% is considered pretty good, while over 10% is probably not so good. At least with this test, you can hear where the air is getting by. If the air comes out the intake ports (through the carbs), it's the intake valves; exhaust port (through the muffler), exhaust valves; and if you hear air coming out the oil filler hole, it's past the rings. This test will give you a better idea of where the problem is.

Like with the compression test, the engine should be somewhat warm. Also, each cylinder must be at TDC on the compression stroke for the side being tested. You have work quickly so you can get to the other side before it cools too much. There's also the issue with keeping the engine at TDC. When I did my tests the first few times, once I pressurized the cylinder, the piston would immediately go to BDC. I had to rig a way of keeping the piston stationary.

Kurt in S.A.

[Allan.Atherton]

... Can you please explain how the pressure test should be made? I attache the pressure hose to the spark plug thread and give a few kicks... I posted that information for you today, a few messages earlier. Your answer shows that you did not do it correctly.

[comet]

If you are only getting 60 PSI when checking compression there is more wrong than your method. Try squirting a little oil in the cylinder first and see if the pressure comes up. If it does your rings probably are leaking. If not, check your valves. Be sure your valves are not set to tight....Comet

[EuroIron]

what are your ring gaps?

[VBMWMO]

I fitted new liners and pistons. The heads where fitted as is, after they where tested for leakage in the workshop and past ok. Before I measured 40 psi on the pressure gauge and now I get 60 psi. Is this enough? It seems a bit weak to me.
Thanks – Michael.