Piston Balancing

R

Ron/IA

Well-known member
The picture is of one of a set of Model A pistons I am going to attempt to balance. This is the inside of the piston. The lightest piston weighs 628-grams, and the heaviest piston is 640-grams.

My question is: Where would you suggest a place on the piston to remove material from it?
 

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James - Could you expand on why split skirts are no good?

These pistons were in an engine I tore down; it had been running with them for 40+ years. The cylinders are in excellent condition, and there is no ridge and very little taper.

What would you suggest for pistons?
 
Hello Ron: Here's an example of what you intend on doing.
You need to remove from the inside of the wrist pin boss only.
Since your pistons are off quite a bit you'll have to take material off of both sides in the piston.
pretty straight forward,you just need the fixture to secure the piston so you don't damage anything, including yourself.
Good Luck.
www.jandm-machine.com
 

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Ron, If you look at J&M Machine's pictures you will see the pistons being weighed with the pins. The pins can also be lightened to achieve balance. In a low horsepower engine like an A, taking 12g off the pin by slightly enlarging the I.D. would not cause any problems. About 1/64" bore enlargement on an A pin takes off roughly 10g.
 
J & M - Thanks for the pictures. As they say; one is worth a thousand words.

Mike - Very good suggestion reducing the weight of the pin. I had not done it yet, but was going to include their weight too.

James - I noticed the suppliers sell "un-split" pistons. I am still curious; what kind of damage have you seen using split-skirt pistons?

The weight of these pistons may be closer in weight than what I stated. I don't have all the carbon completely cleaned from them all.

However, I am going to set them aside and purchase a new set; since it appears the type I have are no longer used. By the way, just for information, these pistons are Silvolite p/n 1001. They are still available, but I do not know if in the split-skirt version.
 
The reason some piston makers quit putting in expansion slots was cost cutting and nothing else. This is not a guess, or an opinion, I called the manufacturers when the company I used first droped them. That guy told me I could always put them in myself, and I said indeed I could but I wouldn't buy unfinished products.

They all used to have them. That is all we ever used and no trouble.

The slots have a very important job, that is to be able to not to expand in the cylinders, in a cold engine, when first started with so much pressure as to score the cylinders.

But you can still have problems "IF", The rods are out of alignment, as many are. If the piston clearance is not right. If there is not enough piston to head clearance, if the crank is not set right, that being, front to back, wrist pin clearance, ect.

I will not use pistons with out expanson slots, and we are not talking modern engines here.
 
Split skirt pistons are round with a heat expansion slot to prevent lock-up. Newer technology pistons are cam profiled into an ellipse, often with a top-bottom taper, to provide room for expansion without the slot. This eliminated the slot which could, in certain circumstances, resonate and open into a crack.

Measure these newer type pistons ONLY at the specific point recommended by the manufacturer. That is usually 90 degrees to the pin and just below it, but check first!

Some high end and modern OEM pistons have a complicated CAD profile to compensate for internal mass transitions affecting ring land flatness during expansion. Those pistons do not measure the same at any two points.
 
Gentlemen - Very interesting information. I looked at all the vendors selling pistons to the Model A hobby, and all appear to be solid with the exception of one; Mike's A-Fordable. Now, maybe these illustrations in the on-line catalog are old/not correct; I can't say for sure. But, it is making me think.
 
Kohnke Rebabbitting said:
The reason some piston makers quit putting in expansion slots was cost cutting and nothing else. This is not a guess, or an opinion, I called the manufacturers when the company I used first droped them. That guy told me I could always put them in myself, and I said indeed I could but I wouldn't buy unfinished products.

They all used to have them. That is all we ever used and no trouble.

The slots have a very important job, that is to be able to not to expand in the cylinders, in a cold engine, when first started with so much pressure as to score the cylinders.

But you can still have problems "IF", The rods are out of alignment, as many are. If the piston clearance is not right. If there is not enough piston to head clearance, if the crank is not set right, that being, front to back, wrist pin clearance, ect.

I will not use pistons with out expanson slots, and we are not talking modern engines here.
Click to expand...

Thanks for your input ! For you information, I just bought/installed a 4" set of EGGE pistons in my B block. These pistons are just as you describe i.e.- with expansion slot. So, you can source whatever you need/desire from EGGE in Los Angeles, as they will make pistons to whatever specs you want !

Regarding 'piston to head clearance', what is the minimum such clearance that you use ?
 
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I've never had a problem with the pistons with a short slot in the skirt. Its the old original style pistons with a full split skirt all of the way to the bottom ov the skirt.The full split skirt pistons are usually set up with less clearance because the width of the split will allow piston expansion without the danger of the piston expanding and seizing in the block, after warm up . In time the gap closes and the pistons rattle as in piston slap. The full split skirt pistons are weak and are prone to the skirts breaking off and landing in the dipper tray with age and wear. I've seen this happen more than once over the past 54 years.. The newer solid shirt pistons need more clearance because they don't have the expansion split. I ususlly allow near .004 thousants clearance with solid skirt pistons to avoid bind after warm up
 
Tom Wesenberg said:
The minimum clearance I heard mentioned was about .030" and that even seems close to me.
Click to expand...

Hey Tom,
I'm having an 'issue' :D, with a guy concerning the fitment of an aluminum super Winfield head on a B. He's in his 90s so credit for longevity, but not for remembering conversation....about such clearance ! Usually write such important info on build sheet,eh ? NO ! So problems. I'm thinking .040 minimum..what you guys say ?
 
Piston to head clearance runs about .050 up. I don't like to experiment with someones motor.

One time, one of our motors were brought back after he got it in the car and said the rods were knocking, and to say the least he wasn't very happy. So he brought the car and I drove it and they sure were knocking very lightly.

I pulled the pan and checked the rods and they were fine, so I pulled the head and it was an after market head that he had ground and put on himself. The chambers were not machined and were rough castings and so close that when they planed it three pistons then very lightly touched the top and you could see about 1/8th wide and about 3"s long clean places where the rest of the piston was sooty.

I had another head in stock, put it on and problem solved.

The only pistons I have seen that had slots and were not cam ground were the very old ones made in the 30an 40's. I have not seen any those for 30 years or better.

All of our slotted pistons we use are cam ground.

Piston trouble for us has been non existent.
 
Nothing wrong with those pistons. Those are T slot pistons- not split skirt and they are cam ground. Silvolite makes a good piston and I have never had a problem with them. What Herman and Purdy said is pretty much right on the money. Get them clean and dry before you start weighing them.
 
long skirt pistons are ancient technology, kinda like wooden false teeth. get yourself some 283 style pistons.
I am re-working yet another A motor with long skirts that was set up too tight at .0025.
I am with Purdy all the way: your spec should be .004, no matter what the piston mfg tells you. Remember, cyls 3 and 4 run at least 20* hotter, and you need the clearance. Of course, as always, in this motor, done by an alleged expert rebuilder, it was cyls 3 and 4 that took the worst beating. But then he had the wrist pins so loose they were more like flounder out of water flopping around. So what am I to expect
 
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