hypforlife32

Does anyone know what pistons came in these engines factory? Looks like they where replaced in the port motor already but using a compression calculator it comes over over a full point off every time with the current pistons that are in it. I’m kind of at a loss here. Also what Head gaskets thickness is factory on these ?

BillK

I did one of those engines about 5 years ago and remember that we could not find an aftermarket piston that matched the oem one. My customer actually managed to find a set of oem Mercruiser ones. They were pretty hateful expensive. What piston is in the one that you are working on ?

My AERA engine builders software has a note that says:
MERCRUISER NO LONGER OFFERS THE OEM PISTONS, KB 9956-HC IS A GOOD FIT BUT IS LIGHTER AND YOU MUST REBALANCE THE CRANKSHAFT.
I am not sure if the KB piston is available or not. https://uempistons.com/series-1249-c...l#!sq%3D9956hc

Pretty sure that the engine had the typical composition head gaskets which should be around .038 thick.

What numbers are you coming up with to determine the compression ?

hypforlife32

The pistons that are currently in the motor are H859CP 030. which is a 12cc piston. that math with a 64cc head and a 039 gasket is 10.3ish to 1 which is too high. the piston you sent me will actually work with a 051 felpro marine gasket. and put me a 9.1:1 which will work fine.

Do you agree? I just want to make sure im thinking about this correctly.

BillK

What is the deck hgt ?

hypforlife32

.005 off factory.

BillK

My Sealed Power catalog says the 859 is 9.55 to 1 with .025 deck hgt and a 64cc head. Have you been having any detonation issues with it now ?

hypforlife32

I honeslty dont know. I bought the bout with one motor that was cracked. it ran but ran poorly. water in the oil etc. what math are you using? would 9.5 be ok to run without tune changes or anything?

BillK

When you say .005 off factory that will probably be around 9.020 I would probably use the Sealed Power H890CP30 which is a 23 cc dish. That would put you a little under 9 to 1 ?

I am not a real big fan of the Keith Black pistons although I have used them. I am doing a 454 Mag MPI that has them in it and chose to keep them in order to keep the rest of the combination the same. Just went one bore size bigger.

If you want to stay closer to 9 to 1 then the KB piston should be ok. Just make sure to follow the instructions carefully as far as the top piston ring gap. And since they are lighter you will certainly have to rebalance the crankshaft. The H859 and the 890 are within a couple of grams of the same weight and would save you having to do a balance job.

BillK

I personally tell my customers to stay around 9 to 1 on any type of pump gas. I am reading the compression ratio right from the Sealed Power catalog. If it was in my shop I would use the H890's and give up a little bit of compression.

hypforlife32

i could do those pistons and shave the head 1 cc/.006 and be golden with those pistons and a .039 gasket right? I will eventually be doing the starboard motor as well. so ik now the combo it takes to do so.

my machine shop says they cannot get them! Man this is a pita lol. That is probably the best solution is those pistons. they are coming up 25cc in the catalog but ill take .010 off the head which will be good to go then.