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New pistons 73g heavier

General engine tech -- Drag Racing to Circle Track

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Knight rider
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New pistons 73g heavier

Post by Knight rider » Tue Oct 08, 2019 4:27 pm

Hi this is my first post in here, I'm after some advise as on whether I can run with a piston and pin combo what is some 73g heavier without rebalancing the engine.
The engine is. 393 clevo street car that sees the drags sometimes with a max rpm of 6800.

I have spoken to the guy that did the balancing before as he has all the weights and measurements. He said the Bob weight would be 5% different and to get it back would need to add Malloy to the crank. Now I only pulled the engine to do some porting head work and put new rings in at the same time but the stroker kit I used was a procomp one which don't use Cleveland pistons so valve reliefs aren't right so decided to change the pistons as well while I was at it, sounds easy well the rods are H rods but procomp used 6.1250 rods with std Cleveland journals sides so I ended up using 408 pistons and had them machine to the correct comp height.

I really don't want to pull the crank and have it balanced again at a cost of 500-600 bucks. All the pistons are within a 1 or 2g of each other. The only other thing I could do is run lighter pins but that would only save 20g

Now the easy answer is balance it but I'm not sure it will make any difference

Love the site by the way and feel free to tell me I'm being dumb wouldn't be the first time

Borsiin
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Re: New pistons 73g heavier

Post by Borsiin » Tue Oct 08, 2019 4:34 pm

I would not rebalance the crank for that....

As long as the pistons are equal in weight i would leave it stock. It is not like there is an 100% correct balance factor and your pistons are ruining it.

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Re: New pistons 73g heavier

Post by Knight rider » Tue Oct 08, 2019 4:49 pm

Thanks for that, by the way it's 73g per piston and pin if I didn't make that clear cheers

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Re: New pistons 73g heavier

Post by Carnut1 » Tue Oct 08, 2019 5:25 pm

Lighter pins and some grinding for weight reduction on the pistons may be a solution.
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Knight rider
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Re: New pistons 73g heavier

Post by Knight rider » Tue Oct 08, 2019 5:51 pm

Pretty easy to see why there heavier, not sure where the best place to just start grinding on the piston would be to see any worthwhile amount in reduction cheers


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Re: New pistons 73g heavier

Post by ProPower engines » Tue Oct 08, 2019 6:27 pm

I would send them back!!
If you compare the old to a replacement piston you got the middle of the road option as a replacement.
As mentioned you can get lighter pins and maybe a tickle of the piston.OR call the piston manufactures and check out what a comparable weight piston is worth or if they can be had.
If its about cost your screwed regardless but the better lighter piston will be cheaper them more mallory in the crank
if you don't want a shake with a 146Gr difference in the bob weights.

If your not using any power adders like NOS or boost you may be able to get the pin weights down enough if the
replacements you have are heavy.
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Re: New pistons 73g heavier

Post by shoedoos » Tue Oct 08, 2019 6:42 pm

Knight rider, start by dropping a bunch of washers on to your scales until they add up to 73 grams....that will give you an indicator as to how much meat needs removed....you'll soon realise mass hogging of the pistons won't be an option and lighter pins will be the way to even up your weights - that's if you don't exchange the pistons....
Last edited by shoedoos on Tue Oct 08, 2019 6:44 pm, edited 1 time in total.

Knight rider
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Re: New pistons 73g heavier

Post by Knight rider » Tue Oct 08, 2019 6:42 pm

No sending them back I've already had .070" machined off the top as I said the H rods that I have don't suit any OTS piston for a 393 stroker Cleveland so I'm using 408 pistons mqchined to suit

I thought the Bob weight was at 50% so it's one pistons weight not two but I'm not sure

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Re: New pistons 73g heavier

Post by Knight rider » Tue Oct 08, 2019 7:07 pm

The new pins are heavy because there longer I can get tool steel ones which are about 20g lighter, would that be enough ? In my mind I can't see it being a problem many engines get rebuilt and bored to 060 OS without a thought of balancing the engine. But I'd rather here that it's been done before and won't be a issue. If not I'll get it done it's not just the money it changes the whole job I'm doing and the time it takes to get it all done and back to me. Cheers and thanks for the help so far

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Re: New pistons 73g heavier

Post by Kurshuk » Tue Oct 08, 2019 8:15 pm

It takes A LOT Aluminum out of a piston for just a couple of grams. Unless you have solid domes and can removes a considerable amount from either side you are not going to take off much weight. Not sure exactly what your application is but be careful going with a pin with less wall thickness, call piston manufacturer and/or a pin manufacturer and get their opinion.

Remember, the pistons are part of the reciprocating weight so 73 grams would be the weight used for calculating the bob weight (not 146) of that journal (assumes 2 rods per journal). Some balancers like to go with a slight over balance and will use 51% of the reciprocating weight rather than 50% in the calculation.

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Re: New pistons 73g heavier

Post by BillK » Tue Oct 08, 2019 8:34 pm

73 grams is a lot. I would personally have to rebalance it and I am not saying that because I do balancing. You would be lucky to get 5 grams out of a piston so that is not an option. There is a lot of things that will start shaking at 6800 rpm.

Just my 2 cents :)
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Knight rider
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Re: New pistons 73g heavier

Post by Knight rider » Tue Oct 08, 2019 9:10 pm

Guess I'll have think hard about what to do, I can see me folding and getting it done which just sucks really for what I started out to do.

Looking at the crank there are some pretty big drilling in the CW the one at the rear is about 1"x4" deep and the web has also had a chunk cut off it and the front one has three holes about 3/4"x4" deep
If I do get it balanced again should I be filling those holes before sending it off. Cheers

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Re: New pistons 73g heavier

Post by Kurshuk » Tue Oct 08, 2019 9:18 pm

No... Do not fill the holes. Let the shop do whatever needs to be done.

I always ask myself... If I don't do what I know should be done... is it worth risking hundreds or thousands more if it fails?

But that's just me, myself and my wallet arguing out loud when I am by myself. :lol:

The biggest problem is the cost of the heavy metal required to get everything back in balance.

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Re: New pistons 73g heavier

Post by BillK » Tue Oct 08, 2019 9:27 pm

Knight rider wrote:
Tue Oct 08, 2019 9:10 pm


Looking at the crank there are some pretty big drilling in the CW the one at the rear is about 1"x4" deep and the web has also had a chunk cut off it and the front one has three holes about 3/4"x4" deep
Those are some deeeeeep holes ? Is the counterweight actually that big ? 1" x 4" is 400 grams :shock: That crank must have been designed for a pretty heavy bobweight to begin with. Your guy might be able to make a couple of slugs of steel and press them in to the existing holes and weld them. He might have to do it that way if there is not a good spot to add heavy metal. I have done a few like that over the years but dont really like it. Sometimes you dont have a choice. I have never had one come loose but that thought is always in the back of your mind.
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Re: New pistons 73g heavier

Post by Pete1 » Tue Oct 08, 2019 9:35 pm

Get the lighter pins. The rest can be easily taken out of the pistons.You will need a mill for that.
If you can hit the old weight within 5 grams, (10 on bob weight) it will run with no problem.

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