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263 pistons???

Guest Zach 16

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Guest Zach 16

Hello all, I finally tore apart my 263 for a rebuild and was about to reassemble until I realized that my pistons are not factory. There are currently 3 different brands of pistons in it but they are not oversized what would this mean? Why would you replace a piston but not oversize?

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This has possibilties as a romance novel. My plot line would be: Tommy bought this car and used it to impress his main squeeze, Elena as they dated in high school. But pre-ignition knock from using cheap gas burned the pistons and this young money-strapped prior owner had to rebuild. Imagine Tommy facing this issue with Prom coming up in three days. He possibly replaced pistons in the block without pulling the engine, using the cheapest used parts he could find. He likely never measured the bore. Poor Tommy may have had to do it more than once until he figured out the knocking problem.

Paper route money doesn't go far--perhaps he got what he needed at a junk yard where parts had been pulled from several cars. Pethaps he got them from some "Midnight Auto Supply" friends of his, though he'd never admit to it.

Of course, Johnny and Elena got married, honeymooned in the Pokonos in his dad's 68 Buick, had a whole passle of kids, and lived happily ever after. But Elena made him get rid of the "old" Buick to make space for their two more modern cars.

And now you own it.

Well, it coulda happened like that...


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The only reasons for "oversize" pistons is when the bore is worn enough to make the skirt clearances too great to use with the existing pistons. In the realm of OEMs, there were piston sizes of "std", "high-limit", and then the normal .020", .030", .040", .060" oversizes. The "high-limit" would be basically a .005" oversize from stock. Use of anything more than the "high-limit" pistons would also require the cylinder bores to be "bored" oversize to accept the particular "oversize" pistons.

How did you determine there were "different brands" of pistons in the motor? Different casting numbers and other related markings? Different piston crown configurations (i.e., domes, dished, valve cut-outs, notches to indicate "forward")? In many cases, "STD" would be stamped into the top surface of the piston, just as .030 would be in a .030" oversize.

Here's a different scenario. Consider, too, that in the realm of OEM pistons and OEM-replacement pistons, they are all within the same weight range--even the oversize pistons. Now, for the scenario . . . Buick probably had several vendors which would supply "spec" pistons to them, meaning that whomever built the particular pistons, they would all interchange and work "as the same" in a particular engine size. Therefore, when the particular engine went down the line, the brands of pistons were not the same at each station, but were built "the same" so they were installed "as provided" rather than trying to match the brands at each installation station. All that matters is that they were all built to the same design specifications rather than which vendor built them. It would be expected that they'd all be from the same vendor, but that might not always be true. It would be more important to keep the line running rather than stop it just so the same brand of pistons might be in one particular engine, if other "spec" pistons are available for that particular engine.

I tend to concur that if the engine is going to be rebuilt, it most probably will be bored .030" oversize, so what's in it now would not be critical. If there is little or miniscule bore wear and the existing skirt clearances are within specs, then just clean things up, put new OEM-spec rings on it, and put it all back together. It might be necessary to touch the bore for a slight clean-up with a hone, but if it cleans up well and measures well, no need to put a boring bar into use on it.

Just some thoughts,


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