Broken Pistons

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Broken Pistons

Postby Vanmor » Sat Oct 02, 2010 8:46 pm

The ring lands are broken on two of my pistons. I was removing the rings while cleaning and the lands came off in my hands. The tops of the pistons looked fine. I thought if you had detonation problems, the piston crown would be all beat up along with broken ring lands. Only my ring lands are broken.

I hate the double eye brow pistons but I was going to reuse them anyway.

I have some new 0.030 over forged pistons but some of the other forums say you can't use them with stock rods.

I read that if you get a lighter pin for the TRW/Speed Pro pistons, you can use them as a replacement.

There went my budget build, but I'm not out yet.
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Re: Broken Pistons

Postby critter » Sun Oct 03, 2010 8:01 am

What size? If you're going back with stock stuff maybe there are some takeouts someone here could give you.
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Re: Broken Pistons

Postby Vanmor » Mon Oct 04, 2010 3:39 am

That would be nice Chris.

That's my first way out. Looks like I have three damaged pistons after all. The detonation must have been really bad. I'm lucky my cylinders weren't ruined.

I found some numbers on the Internet. The stock cast Pontiac piston in a 400 weighs in at 830 grams. This includes piston, rings, and pin. My Sealed Power pistons weigh in at 589 grams. This has to be without the pin, so I weighed my pin and it came in at 200 grams. The rings weigh 80 grams. So I imagine the total piston weight is 869 grams. So my forged pistons are just 39 grams heavier than the stockers.

HPP had an online article that stated that the total weight of a 400 piston, pin, and rod is 1,724 grams. I'm not sure if that includes rings. They didn't show any in the picture. If that's the case, then a stock rod weighs 894 grams. Add that to my forged pistons and you get 1,763 grams.

I read somewhere that if you don't have more than 50 grams of weight (+ or -), don't bother balancing the rotating assembly. If this is true, then that may be my second way out.

Jim Hand states in his book that stock rods are fine to use as long as you don't exceed 5800 rpm in a 400 or 5500 in a 455.

The Internet is full of 5140 rods. They go for about $238 to $250 and weigh in at 860 grams (+ or - 2 grams). Add my pistons to that number and you get 1729 grams. This is only 5 grams heavier than the stock rod/piston combo ! I think it costs $300 or more to get something balanced around here. If the rods are already sized correctly, this would be a third way out.

Of course this is all on paper. I'll have to locate an accurate scale and make some measurements myself.
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Re: Broken Pistons

Postby Vanmor » Tue Oct 05, 2010 12:33 am

I contacted Crankshaft Specialties in Memphis. The owner Doug states that his book calls for a balance tolerance of plus or minus 5 grams.

I also contacted Marion Performance. The owner Greg explained how he takes one piston/rod to get the "bob weight". That one bob weight is used to balance the rest of the assembly.

I thought each rod/piston combo had to be balanced with each throw ?

Oh, he also stated that balancing is about $150 to $200 depending what he had to do. I think that's cheap insurance. You know the old saying, "Sometimes being cheap is expensive."

Update:

I found three websites that explain engine balancing. Boy was I wrong about measuring parts.

Rods have to be weighed on the big end and little end. The little end includes the piston, pin, rings, and the brass pin bushing (if so equipped). The big end includes rod bearing, rod cap, and rod bolts/nuts. The little end goes up and down while the big end rotates. So they have to be measured separately and the calculations
that go with them are more than just getting a weight.

The numbers I came up with are useful for "matching" the piston and rod combos. You get all the combos close in weight THEN you balance.

Internally balanced crankshafts (and Pontiacs have those) are mostly balanced on the ends.

Now that I understand the process, it's easy to understand that balancing an engine is not only inexpensive but actually necessary.
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Re: Broken Pistons

Postby critter » Tue Oct 05, 2010 5:33 am

And thus the big pads at both ends of the Pontiac rods. That's where material is removed to balance, or so I'm told. When I had mine done all of those rods showed marks where they had been ground down.
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Re: Broken Pistons

Postby Vanmor » Tue Oct 05, 2010 9:00 am

And there is also the area under the pin bosses on the pistons. Like the rods, there is an area that you can whittle down to lighten the pistons a bit.

So you have stock Pontiac rods in the T/A ?
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Re: Broken Pistons

Postby critter » Tue Oct 05, 2010 10:37 am

Yep. Stock Super Duty rod. HEAVY, but forged. :Peace:
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Re: Broken Pistons

Postby 77TA » Wed Oct 06, 2010 2:59 pm

The procedure as I understand it by most shops is they will weigh all the rods and pistons and cut, grind to the lightest weight of the bunch... If say #1 Piston weighs 850 and all the others are 850+ they will see if they can get them all to 850 same deal on the pistons. Then they will weigh 1 set of rings and come up with a bop weight which is determined by a formula and there are different thoughts on this too...

I have some of those double valve relief pistons in a 400 block you can have but they are +.060 rods and all.
I think though I only have 7 # 8 is AWOL.... in my shop somewhere....
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Re: Broken Pistons

Postby Vanmor » Thu Oct 07, 2010 5:58 pm

Thanks for the offer. I hope my cylinders are OK enough to use my forged pistons. I may be talking to you about that.

I know one thing, those are the ugliest pistons ever made ! Makes you wonder who thought that one up.
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Re: Broken Pistons

Postby 455ta » Mon Oct 11, 2010 10:38 am

I have a 30 over 455 with trw forged pistons, stock rods that has been raced since 1993. NOT balanced cant count # of passes on this motor. It has had rod bearings and rings put in it once.
1971 Formula (Trans am clone)
1968 Bird project
Built and Raced not Dreamed

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Re: Broken Pistons

Postby Vanmor » Mon Oct 11, 2010 12:28 pm

Is your 455 a street/strip car ? Or is it a full blown race car ? I'd be interested in your set up and how well it runs. Compression, cam, etc.
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