I have a few sets of pistons and am going to throw together a budget 302, I currently have a really high dollar 302 that I built with Brodix heads etc. I have some pistons, which are standard bore, not stamped with any type of overbore and the guy I got them from said they were indeed for a 302 build, not 283 overbore pistons. The wrist pin height etc is correct, so I have a standard 350 block which is in good usable shape, it measures 4 inch. I measured the pistons I have for the 302 and they come up as 3.982.......I then said, wow that is too small, but what type of clearance should I have? I know a .90 over 283 measures 3.965 which is what had me nervous, well not peeing my pants nervous or a squirel caught in a cage pooping everywhere type of nervous. After I meaured the "302" pistons, I took a set of almost new, 350 blower pistons, standard bore - I measured them and they came up as 3.961 I am curious as to what the rule of thumb is on this? will these 302 pistons measuring 3.982 be too loose? or be fine in a street motor? Thanks in advance for your ideas on this. Regards, Tim
Are you measuring the diameter at the skirts? That's where they need to be measured. Any where else will be a smaller diameter.
Depends what kind of pistons they are. Some of the forged pistons need the bore a bit oversize to get the clearance listed in the specs. So....brand? part number?
Squirel - They are forged pistons, that was my next thought as I do hear they need more room, no name on pistons at all. Model A - I did not know that - measured at the top! thanks
Just to clarify, the piston size is most commonly determined by measuring the SKIRT dimension PERPENDICULAR TO THE PIN BORE. The reason for this is that the ring land diameters are much smaller than the skirt diameter, and also the skirts are generally "cam ground" which means the skirt diameter falls away as you move off perpendicular to the wrist pin. They are also often "barrel faced" and "tapered" meaning the skirts have a small bulge and also widen out at the tip of the skirt. Some piston manufacturers say to measure at 1" below the ring land, but that generally gets you about even with the pin. Forged pistons use much more clearance than cast or cast hypereutectic. A forged piston generally requires between .004 - .009" clearance whereas a cast piston generally needs about 0.0005 - 0.002 ". Good luck.
A 4032 alloy has high silicon content which is less growth, less wear but also more brittle then a low silicon 2618 piston. A 4032 can be run as tight as .003 and 2618 .006 I just pulled apart a destroked 350 4.025x2.900 292ci with 4032 pistons at .012 clearance. But it also had .005 on the rods and mains so it's setup to spin high rpm but with that comes excessive wear.
thanks for the information, sounds like I likely will have the right stuff to build my spare 302, but of course will meaure and confirm. my dad and I always have built motors, but has been awhile...so thanks for all the information, I am sure it will help others out too!
Hi. I would have to bow to Squirrel's superior knowledge (we live in the same town so he keeps me humble) and as my machinist friend constantly tells me, "the clearance is in the piston". As long as you are measuring in the proper location (someone already mentioned the skirt perpendicular to the pin) and you have the proper clearance, you should be fine. Remember that forged pistons with proper clearance will rattle (or slap) a little when cold, but after they warm up they'll quiet down. Hope that helps. Palominas, AZ
The Jalopy Journal
