In need of flattop 403 pistons

Old January 19th, 2016, 05:05 AM
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In need of flattop 403 pistons

Im new to the site. Im building a 1977 403. Im looking for some flattop pistons to go into that monster bore which is 4.381 with the stock 6 inch rods. I want to bump up the compression a bit over the 7.9. I will use the stock 4A heads with the 2 inch intake valves and port and polish the heads and intake manifold which is a eldebrock 2711. So anybody help me on the pistons and rings. Had to ask
Thanks
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Old January 19th, 2016, 06:38 AM
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Welcome!
Keith Black KB405 I believe. It's a 6cc flat top. But the std bore is 4.351, not 4.381.
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Old January 19th, 2016, 07:55 AM
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Arias also makes (or at least made) them, not sure availability or part number. The KB's were hypereutectic, the Arias units were forged if I remember correctly.
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Old January 19th, 2016, 09:45 AM
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If you use forged pistons, please ensure your bores have a piston clearance of 0.001" PER INCH OF BORE ==> 4-5 thou for the 403.

I have a 403 which was assembled by a PO with maybe 0.002-3 piston clearance. The pistons expanded and scuffed. Which led to heat issues.

When I re-did it I asked the machine shop, whose guys I trust, to hone to 4-5 thou clearance. They told me that 2-3 was enough and 4-5 would rattle and make noise. They were wrong in this case. 4-5 thou is just right and maybe its my hearing but I hear no piston slap and she RUNS GREAT and DOES NOT OVERHEAT.
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Old January 19th, 2016, 09:57 AM
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Originally Posted by ves007 View Post
Im new to the site. Im building a 1977 403. Im looking for some flattop pistons to go into that monster bore which is 4.381 with the stock 6 inch rods. I want to bump up the compression a bit over the 7.9. I will use the stock 4A heads with the 2 inch intake valves and port and polish the heads and intake manifold which is a eldebrock 2711. So anybody help me on the pistons and rings. Had to ask
Thanks
Why not go with the 2.07 / 1.68-1.71 S/S valves ? With the large bore and planned porting polishing it would give the best results and valves cost same or less than the others.
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Old January 19th, 2016, 02:30 PM
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Originally Posted by GEARMAN69 View Post
Why not go with the 2.07 / 1.68-1.71 S/S valves ? With the large bore and planned porting polishing it would give the best results and valves cost same or less than the others.

I wouldn't do hardly any porting when using that intake. Have you ever seen how small the runners are on that? Waste of time imo.
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Old January 19th, 2016, 02:56 PM
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I was thinking more a performer rpm unless he has hood clearance issues like a 77-79 Trans AM shaker and must use a stock height performer . But... I would still make they heads best for the buck. It's not going to make less power with bigger valves . It doesn't have to be high rpm.

Last edited by GEARMAN69; January 20th, 2016 at 05:02 AM.
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Old January 19th, 2016, 04:07 PM
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Similarly the regular Performer on Pontiacs also has tiny ports , smaller than stock iron quadrajet intakes sadly. Also , smaller than an Olds . Generally it's noted that a 455 can be an under performer with it with stock iron outperforming it in some cases but on a different note they seem ok on smaller CID engines. At the same time the negative results on a big 455 are muddied up when first hand I have seen them run 12.7 or better in the 1/4 with a 455 in a 79 Trans Am with 2.73 gears so go figure and that's with stock 2.11 1.77 valves
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Old January 19th, 2016, 04:45 PM
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Thanks for all the input. The block is being machined as we speak. The block had some bad places in it from the broken rings. OEM the block is 4.351 the machine shop told me they will have to go .30 thou. on the bore. I was told that KB pistons are to heavy for a street motor. Just seeing what is out there. It does not hurt.
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Old January 19th, 2016, 06:11 PM
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Heavier wouldn't be Race would be more to the Street side that doesn't make sense. Race is to be lighter and 403 blocks are for low rpm.compared to a 350 ... shouldn't be an issue .
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Old January 19th, 2016, 06:27 PM
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Yea that is what I heard. I think KBs only come in .24 thou. It will take me a while to build this engine. I just need pistons and rings. Thought that someone on Classic Oldsmobile would have 8 laying in their garage.
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Old January 20th, 2016, 04:21 AM
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Originally Posted by ves007 View Post
I was told that KB pistons are to heavy for a street motor.
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Find a new machine shop. And they come in .024 and .039 over I believe.
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Old January 22nd, 2016, 09:08 PM
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They are lighter than the stock replacements and a much better piston. See if it will clean up with .024" overbore. Those 4A heads are pretty crappy, they need a lot of work on the exhaust side.
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