What should compression be after rebuild & break in?

Did Evinrude provide a standard spec or recommended compression rating for a motor? I just had my rebuilt with a re-sleeved cylinder and all new pistons and rings. I have about 4-5hrs on the break-in and I'm getting 110psi on 3 cylinders and 115-116 on the re-sleeved cylinder.

Is that low? I was expecting 125-130. If it was rebuilt correctly, the pressure should be higher than this correct? This seems low to me. I'm going to have a hard time convencing the mechanic its low unless and asking him to re-build the motor under warranty unless I have some standard to go on.

Should I be concerned about these readings? I know it's a bit early and the break-in is not complete yet but I can't understand how it could jump up or increase in compression later. I tested the compression after I got the motor back and the readings were 110 on 3 cylinders and 125 on the resleeved. It's now dropped to 115-116. That's why I'm concerned.

Re: What should compression be after rebuild & break in?

Assembly lube ?

Re: What should compression be after rebuild & break in?

Lyle29464 said:

Assembly lube ?

Click to expand...

If you're asking if mechanic used lube during assembly yes. He even sent me a video showing how smooth everything operated when re-assembling.

Re: What should compression be after rebuild & break in?

Ok I was thinking it might not have been run before you did your tests. The lube would give higher readings.

Re: What should compression be after rebuild & break in?

I rebuilt my 1979 85 hp Johnson. .030 over factory pistons. Ported the intake and exhaust. Original heads. It runs almost exactly 125 on all 4 cyls. Does yours have factory heads, or did someone install the later low compression bathtub heads? Maybe the Wiseco pistons have a different piston crown configuration which could account for some variance. Possible the piston pin height is slightly different than the factory piston. Did the other 3 cyls get rehoned and new rings on the pistons?

Re: What should compression be after rebuild & break in?

Good points about the Wiseco pistons. No, its the factory heads and yes they were re-honed to factory spec. Mech said piston gaps were per spec and in 25yrs, he's never had a Wiseco piston fail or had to go back and rebuild a motor. He fared me to blow this one. .

I ran the motor per the break in procedure over 2.5hrs first trip out and let it have cooling periods. Water was 34-35 degrees so no chance of overheat anyway.

I tested compression after I got back home the next day so motor was cold. It was 125 in the new cylinder and 110 on the other 3 and now its 115 on the new cylinder and 110 on the other 3. That's why I am concerned.

I'm going to take the inspection plate off and have a look. To check the rings and for carbon build up. My carb is running too rich after it was rebuilt also. Spark plugs are black every trip out. I clean them and reinstall. I thought it might be part of the rings settling but now I now different.

Re: What should compression be after rebuild & break in?

Gauge readings vary greatly with conditions and technique. Your readings all look good to me.

A lot of boaters are paranoid about compression readings without cause.

Re: What should compression be after rebuild & break in?

Thank you JB. Coming from you, I feel better. I AM definately paranoid and my mechanic told me the exact same thing. The readings could vary during break in and could eventually increase a little on all 4 but only time will tell. It depends on how long it takes the rings and the cylinders to "marry" and hone in with each other.

110-115 just seems a tiny bit on the low side for a new rebuilt motor. If the other 3 cylinders were only rehoned, compression probably will hold at what it was previously and not increase at all even after break-in but prevent further damage. Having a new sleeve in the other bore however should have factory compression in my mind. It wasn't bored .30 over or anything. They are all 4 at factory bore.