engine compression on rebuilt engine

[cwb60]

I know there are several good boat mechanics on here, so I have a question for any who will reply.

Recently had a rebuilt power head installed on a Johnson 150 fast strike.  It seems to run good, but when checking compression it has 2 cyl @ 105, 2 @85, 1 @95 and 1@90. 

Is this anything to worry about?  I would have thouoght they would be within 10% of each other.

The engine has been run about 3 hours since power head installed.

Thanks

cwb60

[bilgerat]

I'm a diesel mechanic, no experience with outboards, but was it broken in per the manufacturers (or rebuilders) recommendations ?

Proper break in is critical for piston rings to seat properly. 

[Way2slow]

I would be concerned ie that was a motor I built, however, there are a couple of things going on.

When you say "rebuilt", that a mighty broad statement.  It's very common for someone to do nothing but slap a set of rings in a motor that really needs to be bored and call it a rebuilt or even a remanufactured powerhead.  This will cause inconsistant compression readings.  If this was the case, three hours is not a lot of run time to fully wear in set of rings (I didn't say seat because it takes way more than a seating).  I use a 10 hour break-in  but there are different strokes for different folks.  It's possilbe the numbers may get better after a couple of more hours.  That's also a big maybe.

The first hour - two hours of break-in are critical an hopefully it was done properly.

I'm not really up on the Eagle motors (60 degree FastStrikes) but they may do the same thing as the 90 degree loopers.  On the 90 degree motors, the bottom chambers in the heads for number five and six are approx eight CC larger so they can reduce the compression.  Those two cylinders are always going to read lower.

Like I said though, depends on who did it and what they did.  On one of mine, after three hours, I expect the compression on the top four to be within a pound or two and the bottom two will be lower but the same.

A leak down test is always better than a compression test anyway.  Compression or leak down test, I expect to be within 5%, at 10% I consider it's time to do something about it.

[cwb60]

Engine has not been run hard.  A little over  half throttle mostly. Breakin instructions were to run it half to a little over half until it had 10 hrs on it.  Bring it back after 10 hrs and he would retorque heads and check it out.

We checked compression after 3 hrs just out of curosity.  It was running good.  Bottom 2 cyl are the ones that has the lower compression readings.

I asked if he replaced all pistons and he checked his computer and said yes, it shows I bought 6 for this engine.  I assumed that meant he also replaced all sleeves also, but that could be a wrong assumption on my part.

This is a carb engine, so I guess that means it is a 60 degree engine. 

Thanks

[Way2slow]

All 150's from 1992 on were 60 degree motors.

 

Rebuild or Reman does not replace sleeve unless a sleeve is damaged beyound a rebore. Most motors can be bored  up to .030" over  to bring the cylinder back to straight and round, and to remove all scuffs, scratches and wear surface.  Basically like new, just a little bigger, with a set of oversize pistons to make up the difference of the overbore.

Again, even if it had new pistons installed, does not mean it was bored to bring cylinder walls back to specs.  It can still have a tapper or small wear scratches that bleed compression.  It can cost $200 - $300 to bore a V-6 block outboard block and in most locations, they have to be shipped off to have it done, shipping adds to that cost.  If they are in for max profit, that a pretty big chunk out of it.  Knowing most customers wouldn't know the difference, they will get a few years out of it, and warrenty long gone.

Also don't know what was represented to you and what you paid.  If he said it was "rebuilt and charged you around $1,500, then he probably represented it propery and charged you fairly. Then it's just a matter of how good of a mechanic he is, because with new pistons, rings, gaskets, a fair labor charge, etc, it's hard to rebuild one for less than $1,500 and having installed new pistons.    If the price was closer to $2,500 or more, then it should have be a bored and complelety remanufactor power head.  Now these prices are just for a powerhead, not a whole motor.

[bilgerat]

I would be concerned ie that was a motor I built, however, there are a couple of things going on.

When you say "rebuilt", that a mighty broad statement. It's very common for someone to do nothing but slap a set of rings in a motor that really needs to be bored and call it a rebuilt or even a remanufactured powerhead.

This is the absolute truth. This quote should be memorized by anyone that owns an engine.

It is the same amount of time to break an engine down to replace the piston rings. However, if rings were the only part replaced, it is NOT an overhaul. It's a ring job. Any knowledgeable mechanic will tell you a ring job is a band aid at best and not try to pass it off as an overhaul. This is often done by people looking to sell a boat with a smoking engine that they know will not sell as is. Often new rings will cure smoke issues long enough for the check to clear and then you're on your own. I've been involved with several disputes between boat buyers and sellers when my engine surveys (borescope inspections, compression tests) showed serious issues that contradicted the stated condition of the engine.

You as the customer have the legal right to have any and all used parts returned to you.

The moral of all of this is to educate yourself. It's the only way to protect yourself. If you don't know something, ask. If that annoys the guy doing the work, time to find another mechanic.

[fishn hard]

what oil did you use for break-in and explain in detail your break-in procedure this plays a big role in compression numbers

[marinetech]

way2slow is correct. although a compression test is good, a leakdown test tells all. you should be able to check with your mech. and ask him if pistons are std. size or oversized. it is highly unlikely that it was resleeved more like bored if anything. the only reason for a new sleeve would be a really bad score that could not be bored out. if it was bored I would also ask if it was done with a torque plate. when buying a remaned power head or having any machine work done it is in your best interest to ask alot of questions. I am the mech. supervisor for a large marina and boatyard , we see some really scary work done by people that should never touch an engine.

[mctech]

keep in mind the bottom 2 cyl. will be a little lower compression i dont know why but they are top 4 should be between 105 to 110 psi the bottom 2 90 to 100 psi 

[Way2slow]

They do that because the torque on the crankshaft under full load actually causes and advance of the timing on the two bottom cylinders to the point that it will increase the EGT temps to the point that it can cause piston damage/melting. There have been two methods used to compensate for that. One is, they increase the jet size by 2 digits to richen them up and cool them. The other was to lower the compression, doing the same thing as increasing the jet size but much more fool prof. There is a much less chance someone can think the wrong jets are in the bottom two carbs and cause it to melt the piston.

That is also why the timing should always be set with engine under a full load and above 5,000 rpm.

[fishn hard]

They do that because the torque on the crankshaft under full load actually causes and advance of the timing on the two bottom cylinders to the point that it will increase the EGT temps to the point that it can cause piston damage/melting. There have been two methods used to compensate for that. One is, they increase the jet size by 2 digits to richen them up and cool them. The other was to lower the compression, doing the same thing as increasing the jet size but much more fool prof. There is a much less chance someone can think the wrong jets are in the bottom two carbs and cause it to melt the piston.

That is also why the timing should always be set with engine under a full load and above 5,000 rpm.

WOW  way2slow thats dead on the money did they teach you that i school  ;D ;D ;D

[cwb60]

Will try to answer some of the questions that have been brought up about my original post.

Chevron 2cyl marine oil, mixed 50 to 1, is what was in it and the mech said that is alright.  It will have to have gas when we go back to the water, so what would you guys recommend?  Oil injector was removed when rebuilt power head was installed.

Don't really know much about power head that was put on except, he said he put all new pistons in.  I asked about crankshaft bearings and he said they were replaced.  He also said the old power head went to a marine machine shop, The only one around here that has the equipment for boat engines. 

They did not say they put any new sleeves in it, they said they put all new pistons in it.  The comment about sleeves was mine and came from my lack of knowledge.  I thought it was a sleeved engine.

I have called the guy and he said bring it back next week and he will check some things out .  Maybe he can do a leak down test on it.  I do feel better about it since you guys say the bottom 2 cyl are suppose to have a little lower compression.

[Way2slow]

Nope!  Actually, I've never made a dime working on boats or boat motors.  I'm not and outboard mechanic and never have been.

Like I said, I've never claimed to be an expert or claim to know that much about them.  I just know how to work on mine. 

I'm about the same way with boats and motors as I am with automobiles.  There's nothing I can't do to one, I can build engines, up to any amount of HP I want, including the head porting, I can build and modify the transmission, auto or manual, do the body work, put a dam nice paint job on it and do the upholstery, but I can only do this on one kind of vehicle (or boat).

MY OWN!!!!!

[fishn hard]

Nope! Actually, I've never made a dime working on boats or boat motors. I'm not and outboard mechanic and never have been.

Like I said, I've never claimed to be an expert or claim to know that much about them. I just know how to work on mine.

I'm about the same way with boats and motors as I am with automobiles. There's nothing I can't do to one, I can build engines, up to any amount of HP I want, including the head porting, I can build and modify the transmission, auto or manual, do the body work, put a dam nice paint job on it and do the upholstery, but I can only do this on one kind of vehicle (or boat).

MY OWN!!!!!

Well said i always say what works for my motors may not be the best for other folks and I don't know squat about OMCs but i can mod a pretty strong Merc