Mercury V6 | Piston Compression Height

[Checkmerc_AT4]

What is the 'stock' compression height of a Mercury V6 piston?
Wrist pin center - Dome
Did this vary between displacements/ models?

Are aftermarket cast/ forged pistons similar to stock geometry?

[rock]

Not sure what you are asking. Stock configuration the piston is near level with deck. Never seen a stock v6 Mercury piston with a dome. Compression is changed by the thickness of the head gasket or by cutting head.

Rock

[Checkmerc_AT4]

Hi Rock.
I'm asking for the dimension - (to the nearest thousandth) measured from the center of the wrist pin to the top of the piston.

And then I'm asking, given - as you say - that the stock configuration seems to always/ usually be designed parallel with the deck - if all of the Merc V6 pistons used this same dimension and what exception cases there might be?

[mercrazy]

I heard Oring blocks have a taller deck.

[rock]

Surprisingly I have no v6 pistons to measure other than optimax which are significantly different. I do believe all v6 non opti pistons are the same. Sorry I could be no help.

Rock

[KIRCHNER]

i dont know that dimension, but my rebuilt 2,5 has 5 cyl @ 0 deck, 1 down the hole 0.010. someone was off the bubble!!!

[Checkmerc_AT4]

i dont know that dimension, but my rebuilt 2,5 has 5 cyl @ 0 deck, 1 down the hole 0.010. someone was off the bubble!!!
Incidentally - this is precisely what I am trying to avoid. I have alot of junk - I need to start measuring.
I would like the factory spec. dimension to compare to. If there were tolerance differences between models/manufacturers & Cast/Forged - this would be helpful to know upfront.


I do believe all v6 non opti pistons are the same.
If this is true - that is exceptionally helpful information. Thank you.

[skialot2]

All the non opti v6 merc specs are the same. Does not matter if it a 2.0 2.4 or 2.5 They all have the same stroke. 2.65". They use the same crankshaft, connecting rods and bearings, wrist pins. The only difference is piston diameter. Except around 1991 they changed from bottom guided to top guided pistons. And in 2000 they went to O ring heads so the block deck is a little taller to make up for no head gasket. Rumer has it the block is .050 taller

[KIRCHNER]

well i my case1 con rod is shorter,2 piston is shorter,3 crank is out by .010 take your pick ill guess the piston is out. it is strange but true.

[skialot2]

well i my case1 con rod is shorter,2 piston is shorter,3 crank is out by .010 take your pick ill guess the piston is out. it is strange but true.

Ok, well the specs are supposed to be the same. LOL!!! I would more likely think the crank is out of spec. Very common on crankshafts.

[Checkmerc_AT4]

I have seen (and measured) a (few) bent crankshafts - but they usually bow in the middle and it's definitely not more than 2-3 thou or I wouldn't imagine the engine would run well, if at all.

I'm hunting for the builders and professionals here. Every piston is different. Every builder has a piston preference.
There's all sorts of info about port timing and maps - but a key component of this is piston compression height. You can effectively increase all port durations by ~1.75° by decking your piston 0.020" (or ramping the port inlet). This would have a very measurable impact on performance. I have not been able to find any information about the proper/stock/baseline/benchmark/standard/regular/cast piston dimensions.
This is the place - you guys are the guys. :cheers:

[Checkmerc_AT4]

I'm asking for the dimension - (to the nearest thousandth) measured from the center of the wrist pin to the top of the piston.
Anybody know?

[rock]

Went back into shop and found a new .030 Pro Marine piston. From top of piston to top of pin bore is 1.115. Pin measures .770 half of which is .385. That makes center of pin bore to top of piston 1.500. Was measured with a caliper by a shaky old man with poor eye sight. Best I can do. Hope this helps.

Rock

[Checkmerc_AT4]

It does help - thank you!
Maybe we can crowd source this:
Piston Compression Heights:
Pro Marine +0.030" = 1.500 -rock
Cast Mercury 2.4 = 1.488
Cast Mercury 2.4 = 1.489

It's not the easiest thing to measure, admittedly. I'm getting somewhere between 1.485 and 1.490 without setting things in a vice and getting serious. Couple of these wrist pin bores are not round either.