OMC. 115 Crossflow odd compression

**Fishbum** · 09-09-2020, 02:22 AM

Guys I bought a 1994 115 crossflow that was told had rebuild
a month before then hit something with lower unit. That’s ok because I have a jet for it
but after installing the jet I decided to ck compression. I get 100 on port side and 120
on the starboard. What the heck? Before I start tearing something up
any idea what this might be? Rebuild only part or it? Head gasket?
did they make heads for these that were different volumes? Thanks

**keefallan** · 09-09-2020, 04:27 AM

I have heard from others that there can be variances from bank to bank in compression readings. 120 is the proper reading for the right side. 100 does seem odd. I have heard of 5-7 different from bank to bank, even though I have never experienced it personally. I think you need some others on here who have experienced this to chime in. I dont know who rebuilt it. But if it was rebuilt, I seriously doubt that it was 1/2 done.........but who knows? If it were me, I would pull that head off and check the bores/pistons. I wouldn't be able to rest until I opened it up to see for myself what is going on. I wouldnt like that 100 reading either. That is low for v4 crossflow.

**powerabout** · 09-09-2020, 08:01 AM

not uncommon
2nd hand so check the heads are a pair

**Fishbum** · 09-09-2020, 08:35 AM

Will do

**LakeFever** · 09-09-2020, 08:36 AM

If it’s both holes the same side to side then it’s almost certainly the head. Not too often two holes drop the identical amount of compression

**Fishbum** · 09-09-2020, 09:15 AM

Originally Posted by LakeFever

If it’s both holes the same side to side then it’s almost certainly the head. Not too often two holes drop the identical amount of compression

Ok. Do you know if they made different heads or if it might be one had been surfaced?

**gdp** · 09-09-2020, 12:17 PM

Originally Posted by Fishbum

Ok. Do you know if they made different heads or if it might be one had been surfaced?

Head gaskets the same thickness?

**keefallan** · 09-09-2020, 02:02 PM

They absolutely made different heads. Your heads should have the exact same numbers on them except for the last digit. They will be off by one. The 120 compression reading on the right side is normal for your motor. That head is stock and uncut. Its the 100 number that is strange.

But Powerabout says its not uncommon. So, it may just be a variance in the motor. The only way to really tell is a few things 1) a leakdown test 2) make sure the heads are a matched pair(at least to bring an understanding to you of what you have) 3)pull off the head and visually investigate it.

I would pull the head off for my own piece of mind(if it were me). I would just have to visually see what is going on in there for a difference of 20 compression points. If it looks ok, then you know its the block.........and there's nothing you can do about it............except buying a tight head for that bank. That would raise it exactly 20 points(give or take) and give you a matching compression reading for the motor.

**LakeFever** · 09-09-2020, 03:20 PM

Originally Posted by Fishbum

Ok. Do you know if they made different heads or if it might be one had been surfaced?

No clue. I owned a 115 crossflow since new and never had to open it up these are TOUGH motors. I have rebuilt a ton of other two stroke junk though and having the two holes on one bank both low and both the same with a motor thats not broke? Heads or thicker head gasket as mentioned possible too. Powerabout says its common, not much more I can add other than look into the low side head or run it and live with it if it seems ok and the plugs look ok and your happy with the performance? All about dollars and time vs performance. Your call whether to open it up and start fixxin it

**keefallan** · 09-10-2020, 03:51 AM

I have always wondered if there were different thickness head gaskets for the crossflows. But no one ever gave me an answer when I asked it years ago. I was hoping to know that brand x was this thick, and brand y was this thick.....etc. And I completely agree with you, crossflows are tough motors. Very few give them the props they deserve for that. A crossflow will outlast...........the energizer bunny of motors....thats bad, I know.

**powerabout** · 09-10-2020, 05:18 AM

Way back when 77 140s with bad idle always had a comp diff left and right.
We would sand or mill a head to make them the same.
Checking squish clearance as we went.
I think it was part deck height and part head causing the issue.

**JoshKeller** · 09-10-2020, 05:53 AM

I can tell you on my personal crossflows, the regular head will give 120psi and the bathtub style heads give 100 - 105.

**LakeFever** · 09-10-2020, 06:13 AM

Originally Posted by powerabout

Way back when 77 140s with bad idle always had a comp diff left and right.
We would sand or mill a head to make them the same.
Checking squish clearance as we went.
I think it was part deck height and part head causing the issue.

this is what to do. Cc the heads, measure the head gaskets, and you could also measure the position tdc to deck on each side and compare. If these are all identical? You have some other area where your losing compression which will surprise me if that is the case. Likely bore/ring fit is looser if the top end checks out but even if the bore is a little larger on that bank and maybe whoever installed the rings didn’t gap them properly

**FMP** · 09-10-2020, 07:51 AM

Tear it down, measure and deck if needed and bore it , stick four new Wisecos in it, degree all the ports remove pistons and light work them equal, measure and equal the heads to 130 , do the cages and reeds, set up the carbs , put new leads and plugs on it and coils and the rest if needed, use only minimum no alcohol 91and good oil and have fun. Because