Piston to bore clearance in 401??

letchcoreCJ7

With all the talk about pistons I started my 401 build last weekend. The block it bored .030 over to 4.195. I measured my Speed Pro Power forged pistons for the .030 over and the bottom of the skirts measure 4.191 at the very bottom of the skirt. Is this where I should measure cause that only gives me .004 of clearance. The block is bored right on. The pistons are bought specifically for the .030 over 401 so why did the piston makers only give me .004 clearance or will this be ok or am I headed for trouble?? Thanks for any help

dwg86

.004 might be a little tight for a forged piston. They expand more than a cast piston. Pesonally I think I would run .006 clearance. What does speed pro recommend?

jeepsr4ever

Speed pro reccomends 4.195 for the .030 bore. You should be alright running that piston in that bore. Do not let it overheat for the first 2000 miles

letchcoreCJ7

jeepsr4ever wrote
Speed pro reccomends 4.195 for the .030 bore. You should be alright running that piston in that bore. Do not let it overheat for the first 2000 miles

Ok thats what I was hoping. Looks like the machine shop bored and honed it right on 4.195.

letchcoreCJ7

Finally found some specs on the Speed Pro Power Forged pistons. They recomend a bore of 4.195-4.196 and a min bore piston clearance of .0025.

1980_Cj7

Mike, what cam did you go with?

letchcoreCJ7

The only cams that I could find that would work with my 9.8-1 static CR was the Comp cam. The 270 is what I think I am going with. It gives me a 7.8-1 dynamic CR. I am going to call Comp Cams tomorrow cause I am also considering there XE 256H, and the XE 262H as well.

1980_Cj7

A couple other things to watch out for that we ran into: 1. Make sure your piston compression height is correct. The pistons should end up flush with the top of the deck. If they don't you can deck the block. 2. You might want to file/grind the oil holes in your rod bearings, although several people told me you don't really need them. 3. Think about adding a coolant cross over line at the back of the engine. Fuzz was telling me about it. We didn't do it the first time but are going to next time. 4. And last but not least, make sure you/whoever puts your timing gear on right, ha, ha.