Combustion chamber size

**Merc Speed** · 09-25-2021, 10:58 AM

I am changing heads on my 225 pro max the original heads combustion chamber measures 2.423 in diameter and the other two sets measure around 2.296. the other sets of heads are milled to 33 cc is this why? are they ok to use?

**Merc Speed** · 09-25-2021, 12:39 PM

MY mistake the smaller chamber heads are 34 cc anyone?

**rock** · 09-25-2021, 12:55 PM

That would be fine if you use good gas.

Rock

**JCSmile** · 09-25-2021, 03:52 PM

Go ahead and check compression with a fresh battery, but 34's and 91 are par for the course for a promax. With 34's and a fairly worn promax my compression is 139-142, with 150 being the limit for 91 octane. Might be worth checking which head gasket thickness you are using, there are 0.75mm, 1mm, and 1.2mm, which will have an effect on compression.

**Merc Speed** · 09-25-2021, 06:38 PM

I will check the gasket thickness compression was a even 130 see what it is with new heads previous owner had too many spark plug threads stripped on factory heads coil in one needs more thanks everyone for the help!

**Jay Smith** · 09-26-2021, 09:16 AM

My standard build on 225 Pro Max consists of a 34cc head utilizing a 1mm head gasket , at sea level will yield 138-142 lbs compression with fresh rings and bores which is safe with 91 octane with no more than 25 degrees timing ..

good luck ,
Jay @ JSRE

**Jay Smith** · 09-26-2021, 09:19 AM

Btw , to repair damaged spark plug threads I use only Time Serts for ALL my thread repairs works great for 20 years

**Merc Speed** · 09-26-2021, 06:42 PM

Thanks for the info I put the 34 cc heads on today the timing is set to 24 degrees the boat ran great! Should I go to 25 degrees? the motor has unknown hours but was rebuilt about 6 years ago.

**Jay Smith** · 09-27-2021, 10:54 AM

Will help bottom and mid range a smig I like it a bit safe at 24 ��

**Jay Smith** · 09-27-2021, 10:57 AM

I’d spend the time to dial indicate ALL holes shoot timing at full advance on ALL holes find your “high” hole and use it as your constant, call and I’ll explained
832 597 2603

**Merc Speed** · 09-27-2021, 06:11 PM

I will do than thanks Jay.

**rgsauger** · 09-27-2021, 10:19 PM

Originally Posted by Jay Smith

I’d spend the time to dial indicate ALL holes shoot timing at full advance on ALL holes find your “high” hole and use it as your constant, call and I’ll explained
832 597 2603

good to see you back in here, Jay