Piston measurements

Bob Johnson

Bob Johnson

Well-known member
I have split skirt pistons which I have measured the diameter at several locations. I have measured the diameter at the top and at the bottom of the piston. I have measured parallel to and at right angle to the wrist pin. For a standard bore of 3.876" what are the 4 measurements for a NEW piston?

Top of piston parallel to wrist pin
Top of piston right angle to wrist pin
Bottom of piston parallel to wrist pin
Bottom of piston right angle to wrist pin


Bob
 
The only real measurement to be concerned with is the top compared to the bottom (assuming you know the bore size). This measurement will tell you if the piston has collapsed. Just do it in a couple of places to see if the piston has collapsed or if it's become egg shaped. It doesn't really matter what it is relative to the wrist pin.
 
Bob,


1. Top of piston parallel to wrist pin
2. Top of piston right angle to wrist pin
3. Bottom of piston parallel to wrist pin
4. Bottom of piston right angle to wrist pin

Number 1 an 3 are wrong, you don't measure pistons there unless you want
to know how much "cam" is in the piston. Give us more info..new/used..cast/forged
...solid skirt/split/T skirt...what does it measure...pistons and the bore?

Dudley
 
Bob, the top of the piston should be 3.871 and the skirt should be measured 90 degrees to the pin and should measure 3.873. This from a new solid skirt (I don't use split skirts) piston.
 
Vic in E-TN said:
James:
Why do you not use split skirt pistons?

Vic
Click to expand...
I have had problems with them locking up the motor and scoring the walls and pistons. Evidently it was a re-occurring problem since the most used pistons are solid skirt now and the only split skirts I find are old stock.
 
I have a block with 60 over pistons. From what James said I should see 3.931 at the top of the piston and 3.933 at the bottom 90 degrees from the pin. For the top of the piston I get 3.902 to 3.906 measured just above the top ring. For the bottom 90 from the pin I get 3.931 to 3.933. These are not new but used pistons.

There is a "cam" effect at the bottom of the skirt. The difference between the measurements in line with the pin and 90 degrees from the pin are all just under 0.019". The tops of the piston just above the top ring are round and do not have a "cam".

Bob
 
Bob,

Measuring above the oil ring is fruitless, the " lands " have a smaller dia. the closer
you get to the top of the piston, to compensate for the heat, the closer you get to
the combustion chamber! Do all your measuring BELOW the oil ring and 90 degrees
to the pin. That should tell you about the condition of the pistons below the pins.

Ring grooves and pin fit are another thing.

Dudley
 
I measured the diameter of the pistons just below the oil ring and at the bottom of the skirt. Both measurements were made perpendicular to the wrist pin. The differences were 0.001", 0.003", 0.006" and 0.007".

Bob
 
Bob,

The new set I have here at work show .001 taper, Mr. Rogers set show .002 taper.
I think it's time to visit your local machine shop so they can see whats up with the
pistons and dial bore the cylinders. This is difficult to diagnose in a thread, better
in person. You have split skirt pistons that range from .001 thru .007, I would
think .001 and .003 pistons have collapsed skirts ! Let me know whats what after
your talk at the machine shop.

Dudley
 
I measured the cylinder bores with an inside micrometer. I took 4 measurements at the top and 4 measurements at the bottom of each cylinder. The 4 measurements were taken at 45 degree increments around the cylinders.

The average for the top of the cylinders was
#1 3.936" largest difference 0.0005"
#2 3.936" largest difference 0.0010"
#3 3.936" largest difference 0.0006"
#4 3.937" largest difference 0.0006"

The average for the bottom of the cylinders was
#1 3.934" largest difference 0.0008"
#2 3.935" largest difference 0.0016"
#3 3.935" largest difference 0.0019"
#4 3.936" largest difference 0.0022"

The average taper was
#1 0.0021"
#2 0.0018"
#3 0.0018"
#4 0.0014"

Bob
 
Bob,

The bores numbers look OK for a run block, need to resolve the piston issue. My be
new pistons. Did you run this engine, did it knock when cold?
 
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