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Trouble sourcing pistons for 350 mag

RPAYNE

RPAYNE

New member
I am building a new bottom end due to a cracked block on my 96 mercruiser 350 mag. I am having a difficult time finding a piston with an adequate design for a proper quench build.
The engine is carbureted and has pre vortec 14096217 heads, which are the same casting as the 083 iron l98 heads.
64cc D shaped chambers, 194/150 valves

s-l225.jpg

Mercruiser specs this engine as 9.4:1 compression although with a .039 gasket and stock full dish pistons I come up with 9.0:1? Anyone know what the compressed thickness of the mercruiser/sierra marine head gasket is?

I am having an extremely difficult time finding D shaped inverted dome pistons that would mirror these chambers.

The only ones I have been able to find are Keith Black KB193-030


sbc_350_keith_black_hypereutectic_pistons_dish_10.31_4.030_bore_kb193-030.jpg
specs:
12cc dish
1.561 compression height


By my calcs this would give me 9.64:1 with the block cut .025 to give a proper quench right at .038 with the felpro marine hg at .039 compressed - even a few points higher if heads need surfaced.

IS THAT TOO HIGH? I will be re-using the mercruiser roller cam (low hours) and was hoping to come in around 9.4:1 - I don't wan't to have to be picky about gas or worry about detonation and will be more than happy to have the same power it had. Any hp picked up from the superior piston design would be a plus. I could live with sticking to premium fuel (only have 91 reliable here) if I had to but would love to be able to run 89 or even 87.

Other options with these jugs
1. Bump quench to .045 at 9.49:1 compression - not a huge difference in comp for giving up quench
2. Open the chambers up a bit and try to add 1-2cc or to the heads could get sub 9.4:1
3. KB also has an 18cc piston but even dropping quench to .035 + surfacing heads only yields slightly over 9:1

I have also seen mixed opinions of KB pistons. Lots of negative or so-so sentiment out there while others say they are fine and have a bad wrap stemming from their wider ring gap requirements that have not always been followed.

My building uses Speed Pro H423DCP for all his stock + builds and swears by them. They have a wider squish band around them than the stock full dish but to me they seem marginally better.

slp-h423dcp30_gi_ml.jpg
Their specs are
12.3cc dish
1.548 compression height


So numbers would be very similar, required a few more thousands off the block to make quench clearances and compression would only be .03 lower. Speed Pro seem to have a decent reputation though and are cheaper than the KBs.

Is anyone aware of any other options because it seems like there is nothing else currently available which baffles me. This is a common cylinder head chamber shape. Or do you have experience with KB pistons and input on 9.64:1 - 9.7:1 compression and what fuel that would require?

Thanks!
 
In my opinion only and I am not an engine builder but I did sleep at a Holiday in Express once,


A lot has to due with your fuel set up.

If EFI you can easily run a tighter quench as you can more precisely tune it. .028-.032

If using a carb a larger quench would be better so .035- .038 would be better. You still may have to pull some timing out of it especially at base timing for easy starting.

A final compression around 9.5-1 +/- a point or two, should not be a problem, but based on cam and quench you may have to run higher octane with a carb. EFI you may be able to tune it better.

I like the pistons in second picture... Valve reliefs only........

In my motorcycle I am running 10.25-1 with 7cc domed forged pistons with a quench of ,028" using multi-layer .030 head gaskets

a couple years back I removed the heads and the the quench pattern was perfect.

I had to add comp releases as i am running a carb and could not dial in the starting timing very well as the ignition I am using has limited adjustability........
 
It's a carb'd motor which makes me that much more nervous about pushing 9.6:1+ compression.

Maybe I am overthinking trying to mirror the heads so closely. The Speed Pro's are cheaper, coated, less compression (very slight) and definitely a better design than the stock full dish for quench. They just didn't seem 100% ideal.
 
Ask yourself this. When you're out on the water, enjoying the afternoon rubbing suntan lotion on the backs of the Swedish Bikini Tanning Team, will you really care?
The point is, that you probably won't notice any difference in performance whether that's fuel use, top speed, or any other metric you want to use. To quote Bill Murray, "it just doesn't matter"
There is only one way to get meaningful performance gains and that's by stuffing more engine in there. Up your 350 to a 383 stroker. That should get you an extra couple knots at WOT, if you up your prop to the next pitch.
 
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Welcome to the Forum. You guys are a blast. Jack got a bike with a V8? You are more insane than I thought. o2.....your thinking suntan lotion.....bikinis....that stuff gets you into trouble, man....?
More seriously....I use Super 8 motels, Holiday Express is too expensive, naw....really, just park the ol' class C at Flyin' J, or Loves......, cook, relax.....sleep.....don't do Walmart parking lots anymore, the wife's a "shopper".
Even more seriously....those KB's are ugly. I agree with your builder....Speed Pro looks better. Just take a little less deck off the block. 9.7 CR might be a little "tall".
On a 4" bore, for every 5 thousandths change in deck height/gasket thickness, or surface head,.....or even offset rod journals when turning crankshaft to add or reduce stroke, yes we have done that.....you modify combustion area by about 1 cc. So figure it out from there to get your desired CR, (compression ratio).

But don't take MY opinion......never even STAYED at a Holiday Inn Express anyway! Don't like their waffles!
 
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Id use what your builder is familiar with, less chance for error. The speed pros will be better than the stock dished pistons, which everyone bags on but work in thousands of engines... the stick compression was 9.4 so i don't see a tad higher causing an issue. What gas do you run? The two marinas where i gas up only have 91 non ethanol vs the 87 the engine was rated for.
 
timguy said:
Welcome to the Forum. You guys are a blast. Jack got a bike with a V8? You are more insane than I thought. o2.....your thinking suntan lotion.....bikinis....that stuff gets you into trouble, man....?
More seriously....I use Super 8 motels, Holiday Express is too expensive, naw....really, just park the ol' class C at Flyin' J, or Loves......, cook, relax.....sleep.....don't do Walmart parking lots anymore, the wife's a "shopper".
Even more seriously....those KB's are ugly. I agree with your builder....Speed Pro looks better. Just take a little less deck off the block. 9.7 CR might be a little "tall".
On a 4" bore, for every 5 thousandths change in deck height/gasket thickness, or surface head,.....or even offset rod journals when turning crankshaft to add or reduce stroke, yes we have done that.....you modify combustion area by about 1 cc. So figure it out from there to get your desired CR, (compression ratio).

But don't take MY opinion......never even STAYED at a Holiday Inn Express anyway! Don't like their waffles!
Click to expand...

😆🤣🤣

I agree on the KB's like the dish design hate the overall piston.....

Dieter said:
Id use what your builder is familiar with, less chance for error. The speed pros will be better than the stock dished pistons, which everyone bags on but work in thousands of engines... the stick compression was 9.4 so i don't see a tad higher causing an issue. What gas do you run? The two marinas where i gas up only have 91 non ethanol vs the 87 the engine was rated for.
Click to expand...

Hit or miss, at my river place the closest marina is about 30 miles up river so its generally what's available before it goes in the water which is regular 91 (with ethanol). I have ran into at least one marina at another lake that only had 89.

I just want it to be as efficient as it can since I am building it... while not having to worry about fuel too much. I don't care about more hp on the water, less worry about what fuel I can find and using less of it are much higher priorities. Everyone rants about the stockers and proper design/quench so I went down the rabbit hole a bit. 😆
 
If you don't care about more hp just keep it simple no quench build is necessary. We’ve been down that rabbit hole with many threads on this site.....

what you outline will work fine with the fuel you mention.
 
I went from the original 185 HP 283 CID engine on my boat to a 260 HP 350 CID ( 5.7L) and the performance increase was.... modest. Given that.... I think that "tinkering" with a drag engine will show results when runs are timed to the 1/1000 of a second, but out on the water with the wind and the waves and the crew/goodies load... likely not see any difference unless you make a major CID upgrade. Just my $0.02...
 
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