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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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Re: New pistons 73g heavier

Post by Knight rider » Tue Oct 08, 2019 9:43 pm

Yeah that was wrong sorry the holes are 1 1/2" deep don't know where I got 4"

I tried quoting people but didn't work- it's my first day :)

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Re: New pistons 73g heavier

Post by Kurshuk » Tue Oct 08, 2019 10:40 pm

Pete1 wrote:
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.
I don't think you will be able to get another 48 grams of aluminum (73 grams - 20 grams for pins - 48 grams of aluminum) out of those SRP pistons safely and without significantly compromising the strength to get within 5 grams of the original piston weight, and if could be done 5 grams on each piston would be 5 grams on the bob weight if I'm not mistaken.

5 + 5 = 10 grams. 50% of weight for reciprocating is 5 grams.

Then take into account to mill the pistons (approx. 85 an hour labor) and buy the wrist pins (eight H13 pins approx 200 shipped) and your cost is most likely close to the cost of balancing what he already has with minimal risk. I prefer to stay neutral balanced +/- 1% especially as you approach 7000 rpm and above.

Too risky for me.

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Re: New pistons 73g heavier

Post by superpursuit » Tue Oct 08, 2019 10:46 pm

Knight rider wrote:
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
Did you have the valve relief re-cut after you machined .070" from the crown of the piston. If you did how close is the valve relief now to the top ring land? Very possibly too close!! If you did not recut the valve relief then you should do a mock up to check piston to valve clearance before you go any further.

I would not attack the piston to lighten it - Only for weight matching the pistons. Is the 73 grams heavier after you cut the .070" from the crown? Have you checked the crown thickness after machining? These pistons are normally only around .140" thick originally. So you may have as little as .070" left in the centre of the crown.

As to your original question - Yes you should rebalance the rotating assembly. No question! Unless you want to keep repairing this engine. Remember the unbalance weight multiplies by the square of the rpm. This multiplies your unbalance weight massively when you are revving at 6800 rpm.

The drilled holes in the counterweights you mentioned should be able to be filled and welded to bring the crank back to a balanceable weight. A 1" x 1.5" deep hole is 150 grams.

I hope this helps.

Allan.
Last edited by superpursuit on Tue Oct 08, 2019 10:56 pm, edited 1 time in total.

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Re: New pistons 73g heavier

Post by Charliesauto » Tue Oct 08, 2019 10:48 pm

Knight rider wrote:
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
I am a little hesitant to step into this fiasco. 70% of the people advising you here obviously know nothing about balancing so I will help you out.

73 grams is to much difference, you need to rebalance.

The lighter pin would be a good idea if you were closer, but you will not be able to get another 50 grams from the pistons.

If you have holes drilled in the counterweights, it is possible/likely that you can rebalance by tig welding some cold roll slugs in those holes and not have to install tungsten.

If you can rebalance without tungsten, should not cost over $250 U.S. money. If you need tungsten, one small pc per end will do it, total job should be in the $350-$400 range. I would not buy the lighter pins, just apply that money to the balance work.

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Re: New pistons 73g heavier

Post by Pete1 » Tue Oct 08, 2019 10:52 pm

"5 + 5 = 10 grams. 50% of weight for reciprocating is 5 grams."

Roger on that. After 4 martoonies, thingz get off a bit.

For vintage circle track engines, we go for as light a bob weight as we can get. I have taken more than that off of forged pistons many times.
From just looking at the picture, I would say it was quite possible.

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Re: New pistons 73g heavier

Post by Kurshuk » Tue Oct 08, 2019 11:08 pm

Pete1 wrote:
Tue Oct 08, 2019 10:52 pm
"5 + 5 = 10 grams. 50% of weight for reciprocating is 5 grams."

Roger on that. After 4 martoonies, thingz get off a bit.

For vintage circle track engines, we go for as light a bob weight as we can get. I have taken more than that off of forged pistons many times.
From just looking at the picture, I would say it was quite possible.
Been there, Done that!!! :D

Them circle track guys do some crazy stuff...

Keep in mind he has already removed about 0.060 to 0.070 of the piston deck... not much left to remove.

Have a great night!

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Re: New pistons 73g heavier

Post by Knight rider » Tue Oct 08, 2019 11:14 pm

I wasn't looking to even try and take 50g out of the pistons. I cut a piece of 5mm plate alloy I have here till it was 50g- it's alot

Yes the pistons have had .070 off top they are or were dish top, no need to machine the valve relief again and there is still over 300" of crown left, the bottom of the crown is about inline with bottom of the top ring
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Re: New pistons 73g heavier

Post by shoedoos » Tue Oct 08, 2019 11:23 pm

can you do a quick weigh of the two different wrist pins? Remember the rotating assembly was balanced with them. There is a possibility the pins are different weights and that will allow you to get within a margin by buying lighter versions. I take it they are .927 pins? If so, there's a whole bunch of different weights and materials available....along with tapered wall etc etc....

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Re: New pistons 73g heavier

Post by Knight rider » Tue Oct 08, 2019 11:56 pm

Yes I have checked that that's why it's some 73g heavier with the pin the piston is around 50g heavier by itself

Old pin 105g new pin 130g it's longer than the old one .927 pin cheers

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Re: New pistons 73g heavier

Post by ProPower engines » Wed Oct 09, 2019 12:24 am

BillK wrote:
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.
If you do plug the holes then be sure to drill and pin the weight still.Seen too many lower ends have a bad day with a flying chunk of weight.
Had a guy with a 340 mopar get a cheap balance and the shop welded and buffed the crank smooth for appearance.
That weight came out and broke the rear main cap and F###ed the new roller cam and 4 lifter pairs before stopping completely.
The next go around the shop pinned the weight. he was faced with the same issues as you having to ad mallory or fill the drilled parts again.
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Re: New pistons 73g heavier

Post by Knight rider » Wed Oct 09, 2019 2:53 pm

So what would be the lightest pin I could run and where to buy them, I don't have the pins in front of me but from memory they're 2.750 long .927 130g .
If I could reduce them by around 40g I'd probably run it as is cheers

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Re: New pistons 73g heavier

Post by Dave Koehler » Wed Oct 09, 2019 3:36 pm

Oh, for pete's sake. Quit wasting time looking for a way out. Get the darn crank re-balanced.
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Re: New pistons 73g heavier

Post by Knight rider » Wed Oct 09, 2019 3:48 pm

Why ? I thought reducing the over all weight would be a good thing anyway. If balancing and adding Malloy and more weight again is the only option that's what I'll have to do cheers

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Re: New pistons 73g heavier

Post by shoedoos » Wed Oct 09, 2019 5:13 pm

Not sure about the weight on the set below....due diligence on your part will find out.....there's loads of different wall and weights on ebay

https://www.ebay.com/itm/Trend-H927-275 ... 4#viTabs_0

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Re: New pistons 73g heavier

Post by FC-Pilot » Thu Oct 10, 2019 12:28 am

What crank is this? Cast or forged? If this is a budget cast crank I might look into getting another crank as the cost of balance and Mallory might be more than a new crank and a basic balance. If it is a decent forged crank then it will probably more cost effective to just put the Mallory in and go with a fresh balance.

Ah, the joys of building something without a very clear picture of the end from the beginning.

Paul
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