Number on piston.
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Junior User
Joined: Dec 2001
Posts: 84
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From: Burnaby BC, Canada
Number on piston.
I've just pulled the heads of my 390 and noticed that one of the pistons has a '30' stamped into the top of it. Does this mean that it is an oversize piston?
Fleet Owner
Joined: Jun 1999
Posts: 24,552
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From: Creedmoor, North Carolina
Number on piston.
Don, That generally means that it is a .030 over standard bore. 
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John
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Post Fiend
Joined: Feb 2001
Posts: 6,624
Likes: 8
From: Missouri
Number on piston.
That all depends. Most 390's can go .060 over. A very few probably cut it close at .030 over. It all depends on core shift while the block was being cast. If you have a 1973 or later block that increases the possibility it is safe to go farther. Some will even go .080 after '73. You still can go to .040 over on your next rebuild if you take care of what you have. Sonic checking is the only way to be positive. I went .060 over on my 1974 block and didn't sonic check and have had no problems.
Elder User
Joined: Aug 2001
Posts: 962
Likes: 2
From: Collierville, TN
Number on piston.
Ford's specs say that .060 over is the limit for 352/360/390/410 FE blocks. There are plenty of builders out there who swear that you can go to .080 over on pretty much any FE block without a problem. As rs points out, though, the only ways to know for sure are sonic checking and that old standby - trial and error. The nice thing is that FE blocks are still pretty cheap, so even if you run into a problem, you should be able to pick up a replacement block for no more than a few hundred bucks.
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