those #'s are with what thickness head gasket ? I would lean towards the lower comp ones and maybe play with head gaskets ? the merc 525SC were only about 7.5-1 or close...so if you could come up around 8-1 I think that would be ideal...

agreed... piston won't be the cause of a failure... will likely be tune related... J.E. Blower piston specifically built for the application would be a bit more resilient but not necessary at the additional price... my concern would be that if you're on that tight of a budget you may be in over your head when it comes to maintaining it... just sayin.. not bashin

Understood. Budget really isn't the issue (I've said it but it isn't the huge consideration that it is for some... it'll get done when it gets done... I do miss being out on MY boat though). My thinking is more around the idea that I had good parts to work with from the beginning AND that I didn't want to bore the block (which created a un-thought-of big hurdle). Good short block with less needed in that area meant being able to put money elsewhere (ex: I probably have another $800-1000 that still needs to be put into the heads... valves, springs, etc). In addition to the propulsion, the boat needs better instrumentation, and to be put back together from having the transom and stringers redone. Lots to do and spend on (maybe I should have bought another boat but I really love my 242).

I do not have a problem with the GM 3999295 or older TRW 2399 piston (2618 allow vs VMS-75). If I can find another set of the GM pistons, problem solved for the most part.

Joe, still using the stock rods. Nothing was wrong with them and they were well within spec. Hell, the entire rotating assembly was perfect with exception of the one piston that needed to be replaced... it probably could have been cleaned up and reused but I was being a quite the stickler in regard to what I was putting back in the engine (that lead to the others being changed too). Had the issue with the rings (and bearings) not come up, the short block would have been done weeks ago (btw... King really came through on the bearing side).

Regarding compression with the bigger dome (18cc vs 14cc), I get a measurement of 9.2:1 which is based on:

4.251 bore
4.0 stroke
-18cc dome
120cc chamber

Same parameters give me 8.9:1 with the 14cc (stock) domed pistons (there's probable a smidge of loss with the valve relief on both, but probably a negligible amount).

For conversations sake, is that too much compression for the blower at say, 7 lbs but with aluminum heads? The tune is very important... I know that. Just curious on the thinking is all.

a flat top "zero" deck 4.25 bore x 4.0 stroke with a .040 compressed head gasket and 121cc chamber with net you around 8.2 compression in my opinion its a safe place to be... it may not be what "Joe" has but Joe isn't a first rounder, we all gotta start somewhere, and safe for the first time around is in your best interest...

If the blocks havent been decked the pistons are prob down in the hole a bit. Have you measured that?

I wouldnt drive that blower harder than 80% range. Which is about where the 3" top pulley would put you. That should net about 7psi on your build. If you went with a flat top, if you ever went with a bigger blower, like a 8-71 , 6-71, or 420, you can take advantage of it with the lower compression.

I measured the deck height before the block went to the machine shop and it was at .010 (dial indicator to gauge piston at TDC, machinists ruler across the bore at the pin, feeler gauge). A little material was removed during the cleanup of the block to freshen-up the deck surface. I have not measured since it has come back so the .007 is a guestimation. It's probably closer to .009. It definitely was not down in the hole like others I've seen. The ratio numbers I posted are very close.

The pulley I have on the blower now is a 2.70 which is too small. I'd like to find a 3.0... the search is on. I have a 3.65 also, but that pulley probably won't do a lot (though it can really tame it down nicely).

Regarding compression ratio with the 250 (and smaller blowers), there's a lot of talk about running them with more compression in order to crutch them up some. Honestly, I think this starts to cause some confusion when trying to put together an engine with one of these blowers. It would seem that it would be far easier to just go with a larger blower, and setup the engine traditionally. For my boat, this isn't an option... I would not be able to fit a 420/6-17/8-71 under my hatch. Just saying is all.

I know I can get the flat-tops readily, but using a 3cc number with my head and with the piston where it is, I come up with 7.9:1 which I think is too low of a ratio for the 250.

Jegs and summit have those pulleys. 155192 is the 3", 155193 is 3.34" if I recall. The 3.34 made about 5psi on mine, and 155192 about 7psi.

If you end up going low compression, the 192 pulley would be my pick. 9:1, I'd run the 193 pulley.

If the heads are bare, and need assembly, how about a flat top with some milling of the heads? Those race series heads have thick decks, and can be milled a bit more than the pro 1 series. Down to 105cc /.070

.010 in the hole would drop your ratio to 8.1 and that is fine

+1