merc 140 (pre 1990) flat top piston options

[CamaroMan]

Hi all - doing a rebuild on a 140 - the bore is identical to a chevy 350 and side by side I can hardly tell the difference - any thoughts on adding some static compression to these?

Also- if the pvc is ok since im considering a 4" chevy piston (same pin height as stock) and balancing the motor - couldnt see any bad side effects? just want a little bump in compression - nothing wild

Im very tempted to weigh a std chevy piston and compare to the older 140 ones - i have read the piston height is different.. im looking thru the piston catalogues to see what I can find-

Either way - found these:

https://www.summitracing.com/parts/slp-h897cp30/applications

Very interesting thread here:
https://www.practicalmachinist.com/vb/general/pretty-o-t-mercruiser-140-engine-questions-226841/

 

[Scott Danforth]

the 3.0 already runs on the ragged edge of detonation (advance the ignition a few degrees, and you will burn holes in all 4 slugs). you would need to add a knock sensor (which was only on the EFI model) to be able to increase the compression.

compression height is different between the standard 350 slugs and the 3.0. raw castings start out the same, however the final machined product is different.

not to mention the combustion chamber simply sucks. the 3.0 was originally a fork truck motor running on natural gas.

for the cost of rebuilding a 3.0, you could buy 5 running motors.

 

[fishrdan]

Not sure what year you're working with, but my 72 looked to be balanced from the factory, lots of drill holes in crank, balancer, flywheel, rods had grind marks. When I rebuilt it I did a garage balance (for lack of a better term) and matched all the rods small/big end, which took "very" little, then matched the pistons to the same weight. One of the pistons was WAY off, so I had to take a bit off that piston's wrist pin, about 1/8", to get the weight to match the others. Not as good as a complete shop balance job, but not $150 either...

I'm pretty sure the wrist pin location is different on 140HP (GM 181) pistons VS 350 pistons, otherwise everyone would just use 350 pistons. When I rebuilt my 140, 4 pistons were more expensive than a full set of 8- 350 pistons, go figure... I wouldn't monkey with the compression for the reasons SD mentioned. But I ported the head port matched the manifold, probably netted 1.5HP at WOT

 

[Bondo]

probably netted 1.5HP at WOT

Ayuh,..... Too bad the Vortec chambers haven't been adapted to that motor,......

Probably bump it up to 150 hp or so,......

 

[fishrdan]

Ayuh,..... Too bad the Vortec chambers haven't been adapted to that motor,......

Probably bump it up to 150 hp or so,......

I thought about a Vortec swap, to a Vortec V6 been looking for a rotter hull with complete drivetrain...

 

[Scott Danforth]

I thought about a Vortec swap, to a Vortec V6 been looking for a rotter hull with complete drivetrain...

plenty of 1996 and later bayliners out there...... most of them are hitting the rotten stage