460 Ford Forum banner

Need advice with piston problem.

1K views 6 replies 3 participants last post by  161854 
#1 ·
I'm in the process of assembling my 572 cu inches motor. When trying to install the pistons onto the rods, I can't get the wrist pins to go into the rod. The pin fists into either piston or rod no problem, just not together. The top of the rod is hitting the bottom of the piston. I have measured as best as I can with calipers. The top of the rod, from bushing measures .250. On the piston, from the wrist pin to bottom of piston I'm measuring about .235. So I'm lacking .015 plus whatever the actual clearance should be.

My combination is 4.5 stroke crank, 4.5 bore and 6.8 rod.

Pistons are custom Diamond. 38cc dish for P51 heads, 1.245 compression height

Rods are Scat H beam 6.8

Any advice on what to do. So I have some ideas to talk to my machinist and Diamond pistons about Monday morning.
 
#2 ·
Mill the bottom of the piston crown if they are thick enough ... or remove about half what you need from the rod small end and bottom of the piston crown.

Probably re-balance afterward if it is more than 10 grams.

You are the engine builder so, MAKE it all fit ... whatever you've got to do.

Custom pistons are exactly that ... your order tells the piston company how to make them and nothing is assumed if you leave something off that order, whether they interfere with other components or not is immaterial to them.
 
#3 ·
Thank you for your response Randy. I understand the responsibility about make parts fit building my own engine. Just surprised to have this particular problem. The only thing thats really custom with the pistons is the bore 4.5 bore size. Diamond has them in there catalog for smaller bore sizes for the 4.5 stroke, 6.8 rod combination and 38cc dish for P51 heads. I'd give you part numbers, but Diamonds website is currently down. Pistons were ordered from a knowledgeable vendor in Adney at Performance Crankshaft. Just trying to figure out were the error was made, whether it be a order oversight or something wrong at Diamond. Any more insight you have would be welcome.
 
#4 ·
The error was made in not telling the manufacturer the exact amount of space needed above the pin for connecting rod small end clearance; .230" is usually enough.
This is a pretty common problem with pistons having a deep dish and short compression height.
 
#6 · (Edited)
That depends totally upon the usage.
Sometimes only .100" thick piston crown, (in a smaller area), is enough and other times it takes .220" or so for the piston crown to not oil can some.

EDIT: Once again, you are the engine builder so, it is your end responsibility.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top