Piston missmatch?

Licensed to kill

Licensed to kill

Well-known member
I hope I',m not being a nuisance with all these questions. Still working on the engine for my '35 tow truck. It's a 47-48 from what i can gather, bored 40 over that sustained an injury to #7 cylinder in it's past. #7 piston is different that all the others (not from the same set), wrists pin is a bit different visually, combustion chamber for #7 has some minor nicks and marks indicating that something was bouncing around in there at one time and the oil pan has some dents from the inside out and some brazing indicating that it had ruptured in the malay in line with #7 journal. The replacement piston is a little rough so I would LIKE to replace it if I can but it would be fine even if I can't. So, I went to a friends place to see if he might have a 40 over piston. Went through a few pails of parts, found a set of unmarked (assuming standard) that look a bit small (221?), a set of 60 over and after several other mail found a single piston marked 40. YEEHAA. Brought it home, checked the diameter and while it's .005 bigger than the other that where in my engine, that'll work. before getting too excited, I thought to double check the wrist pin location
and.........:eek::mad:....not a match:mad:. So, my question is, why do these two pistons have different deck height?? Piston from my engine on the left, short, lower compression piston that I thought would be a replacementon the right.

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left Ford 3 3/4" stroke

Right Mercury 4" stroke so I will assume that you are looking for a 40 over 3 3/16" bore= 239 cubic inch 59ab piston?
 
GB SISSON said:
The one on the right is for a 4" merc crank. Funny thing, I need one .040 merc piston.....
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I can send you this one. Wont work for me. It ,measures 3.200" (IIRC)
P.S., I'll double check with the guy I got it from to see if it's OK that I send it off rather than give it back if you want it. Let me know.
 
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I will measure the seven I have. Will be with a digital caliper. Does it matter which direction across I measure or top or bottom of piston? The seven I have were from a fresh rebuild that overheated badly and cracked the block on it's first run. The guy ended up installing a small block and he gave me this engine. I used the crank in a rebuild 20 years ago. I recently scored a virgin merc crank here in the islands and had it ground. Hopefully his works out. Thanks!
 
There are also variations between different manufacturers. The length difference for the 1/8" lower piston head can me made with a difference in the piston pin location or by decreasing the distance between the top ring groove and the top of the piston. Some have higher domes and some have different shaped domes and some are forged. The main thing is that they all end up about the same in configuration and weight so there is no ring mismatch or vibration issues due to different weights. Egge, Sealed Power, Ross, and others just to name a few make pistons for flathead V8 engines. Also, forged pistons require more clearance on the cylinder wall plus they are three ring so avoid those unless you use them in all 8 cylinders. I'd recommend keeping them all the same or as close as possible to the same.
 
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That all makes good sense Rotorwrench. How can I tell a forged piston? I could post pictures of what I have. They are normal looking dome with 4 rings. They measure exactly 3.20. I know I have seven, but so far only locating six. I pulled one out a couple weeks ago to compare to some bores. Figures....a day late, a dollar short and now a piston short.
 
Sometimes you can tell by the finish on the inside of the pistons. Cast should be somewhat rougher than a forged piston. The forged piston should be very smooth on the inside.
 
Licensed to kill said:
I hope I',m not being a nuisance with all these questions. Still working on the engine for my '35 tow truck. It's a 47-48 from what i can gather, bored 40 over that sustained an injury to #7 cylinder in it's past. #7 piston is different that all the others (not from the same set), wrists pin is a bit different visually, combustion chamber for #7 has some minor nicks and marks indicating that something was bouncing around in there at one time and the oil pan has some dents from the inside out and some brazing indicating that it had ruptured in the malay in line with #7 journal. The replacement piston is a little rough so I would LIKE to replace it if I can but it would be fine even if I can't. So, I went to a friends place to see if he might have a 40 over piston. Went through a few pails of parts, found a set of unmarked (assuming standard) that look a bit small (221?), a set of 60 over and after several other mail found a single piston marked 40. YEEHAA. Brought it home, checked the diameter and while it's .005 bigger than the other that where in my engine, that'll work. before getting too excited, I thought to double check the wrist pin location
and.........:eek::mad:....not a match:mad:. So, my question is, why do these two pistons have different deck height?? Piston from my engine on the left, short, lower compression piston that I thought would be a replacementon the right.

414098864.jpg
[/IMG]
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Are yours flat top or domed ? I may have a flat top . Never mind , I thought you had a 21 stud . Sorry
 
There may have been other forged pistons back in the day but Ross are the most common now days and they are three ring so they are easy to tell. Egge makes some three ring but they are only available in 1/8 oversize and up and they are cast. Forged pistons are a bit heavier than cast since they are more dense due to compression of the metal grain. You can compare them in your hands and feel the difference if you've been around them much.

Speedway had some hypereutectic pistons available for a while but they were discontinued. I'm pretty sure they were 3-ring too. They are stronger than cast and need no extra clearance like the forged race pistons but they were likely too expensive or they had structural problems. I don't know why they quit making them.
 
Looking back at my post #10 it almost sounds like I was hoping to pound out a new piston from a billet.... Not so. I can understand how a con rod or a crescent wrench can be smashed out red hot between heavy steel dies, but trying to imagine the tooling to forge the shape of a piston with the boss for the wrist pin is too complex to imagine for me. That's all.
 
The forging process only compresses the metal to a basic shape of the dies. Quite a bit of machining is still required to finish it. Since they are a three ring they are a bit simpler to machine, The only part that is not machined is likely the area inside the piston with exception of machining the inner bosses to allow clearance for the connecting rod small end to fit. Here is a link. https://www.rosspistons.com/product-tag/flathead/

A lot of helicopter landing gear parts are made of forged aluminum for strength and a certain amount of elasticity so it will hold it's shape. They don't hold their shape as well as steel but they are certainly lighter than steel. Saving weight is important on flying machines.

Here is another link about piston forging processes. http://blog.wiseco.com/what-is-forging

It mentions hypereutectic silicon forgings but I don't think any are made for flatheads currently.
 
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rotorwrench said:
The forging process only compresses the metal to a basic shape of the dies. Quite a bit of machining is still required to finish it. Since they are a three ring they are a bit simpler to machine, The only part that is not machined is likely the area inside the piston with exception of machining the inner bosses to allow clearance for the connecting rod small end to fit. Here is a link. https://www.rosspistons.com/product-tag/flathead/

A lot of helicopter landing gear parts are made of forged aluminum for strength and a certain amount of elasticity so it will hold it's shape. They don't hold their shape as well as steel but they are certainly lighter than steel. Saving weight is important on flying machines.

Here is another link about piston forging processes. http://blog.wiseco.com/what-is-forging

It mentions hypereutectic silicon forgings but I don't think any are made for flatheads currently.
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Thanks! That was very informative. Now I get it!
 
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