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

**NZ Sidewinder** · 09-12-2024, 04:39 PM

Time is the enemy

**NZ Sidewinder** · 10-05-2024, 06:04 PM

Here's my results using 2.0L big pattern, small chamber heads cut .040.
I haven't cc them or did and have forgotten the numbers but they are around 31-32cc.
Motor is running 30hr old rings in 30hr old bore with fresh ball hone.
At least .009 clearence.
Exhaust port 1.550 to roof and 1.500 to top of chamfer.
Piston flush with deck and .040 gaskets.
140psi even.
Crank cases are stuffed 24cc theoretical and all things considered 20cc would be minimum actual stuffing.
I've only run it through 6k briefly and it's good.
It's run for 3 gallons.
I'll do a leak down on it shortly
Edit
30 or less races, not 30 hours

**LakeFever** · 10-05-2024, 06:47 PM

Mines been running really well. I was contemplating cutting these down for a 10psi bump but on the stock A8 ecu I think I have found the safe limit as it is. The pistons are half washed and these plugs are not the easiest to read but have been black up until the head swap. I have turned the motor 6500 continually for many miles and it runs like a top. A plug from each bank

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**25two.stroke** · 10-05-2024, 07:17 PM

I'm cursed with building motors that make more compression than they should. No idea how. How did I get 160 out of 2.0L heads on a 0.015" over 2.5? I just built a stock 280 and somehow it is making 175 psi not decked as far as I measured and supposedly stock CC heads (although I did not cc them myself, I had Terry cut them and hes done me well in the past). Time to pull em back off and cc myself. No big deal they are oring...

All my 260s make 165 psi with 31cc heads, so I guess that is closer to normal.

**David - WI** · 10-05-2024, 07:23 PM

Originally Posted by 25two.stroke

I'm cursed with building motors that make more compression than they should. No idea how. How did I get 160 out of 2.0L heads on a 0.015" over 2.5? I just built a stock 280 and somehow it is making 175 psi not decked as far as I measured and supposedly stock CC heads (although I did not cc them myself, I had Terry cut them and hes done me well in the past). Time to pull em back off and cc myself. No big deal they are oring...

All my 260s make 165 psi with 31cc heads, so I guess that is closer to normal.

Check your gauge? I had a Snap-On one that was off by 10psi.

**25two.stroke** · 10-05-2024, 07:38 PM

Yeah we got a drawer of em. Boss won't throw them away when they get clogged, so every time I want to do a test I gotte go through 6 at least to find one that works. The old school pocket-watch style ones from mercury are almost indestructible, we have a few of those and they all work except they don't fit in tight spaces or in the v6 middle cylinders unless you take out the head temp sender brackets. The old style that were made for the bottom cylinder of inline motors.

**NZ Sidewinder** · 10-06-2024, 12:15 AM

Bore clearence might be the differnce.
Big Bores = big ring gap.
Also, large clearences and rocking pistons probably buggar the ring edges.
A leak down on mine will tell the story.
I was expecting 165psi but looking at it now, I cut .014 off the top of each piston in order to get the crown down level with the deck.
This changes more than I first thought.