Looking through the summit catalog at the pistons i noticed that most of them are made for a bore of .030", and very few for a standard bore. Does say a 1986 350 come stock with an overbore of .030"?

 

No. That is in reference to how much you have bored-out, over-bore, tapped as some people say.. your cylinders. Boring a cylinder, .030" for instance, means that a machine shop drills out .030" of an inch from the cylinder walls. well, i imagine its .030" from both walls, so .015" from each. some people refer to it as '30 over', '40 over', or whatever. all stock blocks have a 0" overbore, i guess if you want to put it that way. the only time you have an overbore is when you have the engine taken apart (to have it rebuilt) and have a machine shop 'bore it out'. .030" usually gets a 350 block an additional 5cubic inches, making it a 355 obviously.. and some people go as far as boring it .060", making it a 358, i believe. so basically the pistons you see in jegs, summit, or whereever, are aimed at poeple who are rebuilding their engine, because when you rebuild the engine(with new pistons) you must bore it some amount. that answer your question?

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• 83 Z28-- not a pretty site to look at, 200k miles and im sure it was treated like a rental car every day of its sad life(before i got it).. sold, thank the lord
• 74 Z28-- 383/400, green on black. pretty clean. FOR SALE!!!

 

if it came stock it wouldn't be overbored. overbored is when the hole is enlarged form stock. a factory 350 has a 4" bore. just because it isn't in the catalog doesn't mean they don't have it. they only put the top sellers in there so call and ask. anything they have in +.030 they will have in stock sizes too.

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ICON Motorsports
1st & 3rd
MM Black Diamond 538 F&AM

 

And to add to the above. Never overbore just to overbore or to get extra horsepower. The cost of "just overboring for fun" is not cost effective in the short and long run. The only reason to overbore is if there is a ridge at the top of the cylinder wall (any one bore will require all to be re-bored/overbored) which could make removal/installation of the piston diffucult. Mind you, if you overbore your engine "just to be on the safe side" remember this, you now have one less rebuild left on your engine. Example: you over bored .030" this time and next time you opt for .030" more (.060" total), well that is just two rebuilds for the block. That is because the block won't like another .030" overbore (.090" total), which means you have to buy another block now. Which then leads to testing it to see if it is a good canidate for a rebuild (assuming used and/or from a junkyard). Just trying to enlighten people is all.

And mtx28, yes it is the difference in diameter size. IE: 350 = 4.001"* bore, 355 (350+.030" over) = 4.031"*.
*This is the block bore diameter, not the piston diameter. That is slightly smaller and depends on the type of process used to make the piston.

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George P. Lara
1984 Z28
2001 SS Coming Soon
SCCA, SCFB, SC3GFB

[This message has been edited by MRZ28HO (edited June 28, 2001).]