2.0L 135 fatblock heads on 2.5L 200?

**donmac** · 09-12-2024, 05:38 AM

I recall being told a 40 years ago you can't compression test an omc accurately because the rings need crankcase backpressure on the rings to seal them. you can compare cylinders but the gauge won't give you true operating pressures
true or false?

**LakeFever** · 09-12-2024, 11:37 AM

No clue but I’ve heard many things over the years about compression tests being iffy. Dump a little oil in the cylinders first to sell a crap motor was one etc. From my perspective it’s a simple fast way to verify consistency across all holes regardless of the number and in my last test even though the gauge is fragged it did at least show there was a gain and it wasn’t huge and that was enough for me. New cheapo tester arrived today so take this for what you will this is the small chamber heads today

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**tlwjkw** · 09-12-2024, 12:52 PM

thats pretty consistent with what we usually came up with......... we had pretty accurate gauges... same ones we used on top fuel/alcohol Brad motors.. which wasn't very often..... internal problems with those was usually pretty obvious .....

**LakeFever** · 09-12-2024, 03:02 PM

I'm going to run a controlled test on that tester when I get time to home brew certify it.

**NZ Sidewinder** · 09-12-2024, 03:26 PM

Those omc rings are tapered and I'd say that could be a point.
Simple to put the gauge in, earth the plug and run the motor.
Mercs that have 120psi at cranking speed have 135 at idle, from memory.
Oil build up is a big factor.
I've seen 170psi in a 150psi motor from oil built up in the running cylinder

**skialot2** · 09-12-2024, 03:40 PM

Compression gauges never seem to be accurate. They always seem to read a little low. especially the cheap ones. Probably why all manufactures seem to say equal pressure is more important than the actual pressure.

**NZ Sidewinder** · 09-12-2024, 03:46 PM

I just plug them into a couple of different compressors that have gauges and regulators on them.
If they all match up I'm happier that the compression tester is probably close.

**LakeFever** · 09-12-2024, 04:34 PM

Originally Posted by NZ Sidewinder

I just plug them into a couple of different compressors that have gauges and regulators on them.
If they all match up I'm happier that the compression tester is probably close.

Thats what Im going to do when I have some spare time. Burning up all the time I have running the boat so very little tinker time right now