454 Pistons WAY Too Heavy!

[burbfixer]

I am freshening up a gen 6 454 for use on propane. Small cam, stock heads, so 5000-5200 rpm max. I ordered a set of Silvolite 'Mercruiser 454' pistons with a 10 cc dome, intending to weight match them to the stock pistons, then de-glaze, re-ring, and go.
The piston/ring/pin assembly is 69 grams heavier than the stock parts, almost all of which is the pistons themselves. From the research I've done, I can get by with maybe 15 grams or so heavier without causing problems, which still means shaving a whopping 54 grams from the piston, pin, and small end of the rod. My budget is already over - extended on this project, so re-balancing the crank is not an option.
Question: Has anyone had experience shaving this much weight from an assembly, or am I hooped?

[BillK]

Not going to happen. About the most I have ever taken off the small end of stock rods is 10 grams and that was ones with a giant knot of metal on top. Taking even 3 grams of aluminum off a piston is a lot. You could try to find a lighter set of pins but by the time you do that you might as well rebalance the crank but that is probably going to entail adding weight which will add $$$$ Might be easier in the long run to find a piston that will come closer to what you need weight wise.

I would personally never consider 15 grams close enough as far as matching a weigh for balance purposes.

[ProPower engines]

I am guessing we are talking forged pistons?? Like the old LS6 domed piston??
If so Mahle motorsport has a piston that is a drop in merc. replacement as well as the same piston but much lighter. They have a 28cc dome that will get you 11-1 with a 118ish cc chamber 626-640 grams depending on bore size

KB icon also has a piston 648Gr 18cc dome

May be worth a look

[Geoff2]

You think THAT is heavy! I weighed a Chrysler factory 440 piston & pin & it was 1100 gr.

[Abbottracingheads]

If they are forged pistons call Rebco Machine in Kansas and let them lighten to your spec.

[BillK]

What compression ratio are you shooting for ?

[PRH]

Similar situation years ago...... 396 bbc.

Stock replacement Badger cast pistons 60g heavier than OE pistons that came out of the motor.
Balancing by adding weight to crank absolutely not in the budget.

I figured...... this can’t be the only rebuild to use these stock replacement pistons without balancing.
So...... I just put it together.

Ran smooth as glass........ no issues whatsoever.

[Dave Koehler]

You got lucky there PRH. We all did once or maybe twice.
My biggest grimace comes when someone comes in the door with a 454 cast crank street deal carrying Sealed Power forged replacement pistons.
Invariably they have a crank that had lighter pistons but expect no extra work is needed. Turns what should be a basic in and out job into real work.
Those pistons were too heavy in the 70s and haven't gotten any attention since then. They may still be using the original TRW molds.

[burbfixer]

Thanks for the help!
I am shooting for 10:1 or a touch better on this one.
These are hypereutectic, not forged.
I experimented last night with a spare identical connecting rod as the ones in the engine, and they do have a big balance pad on the small end. Shaving it down as much as I'd care to lightened the rod by 30 grams.

[PackardV8]

Remember, when balancing, it's not just the piston/pin/rings/rod small end; that large knob on the bottom of the rod cap will contribute 50% of the weight removed to the bob weight reduction.

[vortecpro]

If they are forged pistons call Rebco Machine in Kansas and let them lighten to your spec.

YES

[Dave Koehler]

Rebco won't be able to do much with a hyper piston. Basically it's a fancy cast piece

[burbfixer]

Appreciate the suggestions.
PHR's experience is the reason I asked about this. I'm curious how long the engine ran that way and what the bearings looked like when it was torn down.

There are claims out there to indicate re-balancing the crank isn't that critical, but matching the component weights is. Even the late Mr Sherman - who obviously knew a little about engines - chimed in on this subject in another thread about a 54g heavier piston in a 427. He simply stated : "Go ahead and run it. It wont hurt a thing."

I wouldn't fault anyone for avoiding involvement in this thread, because I'm basically asking 'how to do it wrong'. Still, it seems there's definitely a margin for error with engine balancing. I'm just trying to understand a little better what that margin might be.

Funny, when I don't have lots of money to spend, I end up learning more about things like this. Lack of finances forces me to test some limits that I wouldn't, normally. It's interesting!

[PackardV8]

It is also well to remember the formula for V8 bob weight is only a general consensus of what is best for most cases.

It's long been a maybe-speed-secret to over-balance or under-balance for specific goals and applications.

So yes, if one cannot, for whatever reason, get the parts to the formula, the engine will run and probably be smooth.

Only slightly off-topic here, but we no longer build without square decking the block. The differing compression and combustion pressures which would be produced by some of the wildly out of square blocks we've seen would probably run rougher than a slightly different balance bobweight.

[Racer18]

If 5200 rpm is the max, send it! You and the engine will never know any different. Now, go to 6200+, you probably should re balance.