Piston to bore measurements

zach0 · 01-26-2003, 11:59 AM

I just took apart the old 289 and measured the pistons and bore because I want to freshen up the engine. I measured the pistons and they read 30 over on the top of the piston but they were actually 9 under the zero mark (4 inch mark). the bore of the engine came out to be 30 over. Did the person before me rebuild the engine right or can I go ahead and just re-ring the engine and leave it as is. This is going to be the drive around engine, nothing high performance. The pistons in there now appear to be aluminum or that hyper something or other pistons. I never had any problems with the way the engine ran but when I took it apart several of the top rings were cracked.

hobgoblin351 · 01-26-2003, 04:03 PM

I've built engines but I'll admit I'm no machinist! lol Here's my 2cents anyway. Piston skirts are usually smaller than up at the top. If the bore is 30 over then you get pistons for a 30 over. They actually will measure smaller than the bore. They have to, if they were the same size you'd never get them into the bore. Plus you need room for thermal expansion. Did you mark everything when you tore it down. If you're using the same parts they're all broken in to their respective bores and need to go back the same way. As long as there is no scarring in the bores or on the pistons you should be able to just replace the rings, and bearings while you are at it. If they are worn, depending on how bad you might get away with just honeing them alittle.

01-26-2003, 11:59 AM	#1
zach0 Registered Member Join Date: Apr 2001 Location: Hampton, Virginia Posts: 227	Piston to bore measurements I just took apart the old 289 and measured the pistons and bore because I want to freshen up the engine. I measured the pistons and they read 30 over on the top of the piston but they were actually 9 under the zero mark (4 inch mark). the bore of the engine came out to be 30 over. Did the person before me rebuild the engine right or can I go ahead and just re-ring the engine and leave it as is. This is going to be the drive around engine, nothing high performance. The pistons in there now appear to be aluminum or that hyper something or other pistons. I never had any problems with the way the engine ran but when I took it apart several of the top rings were cracked. __________________ 1980 RX7 with 289 Ford V-8 bored .030 over, Very radical cam, Torker 2 high rise intake, Holley 650 Carb, MSD Nascar 6ALN ignition, MSD coil and distributor, Headers with dual open side exhaust. Over 300 horse. 1966 Mustang still in progress

01-26-2003, 04:03 PM	#2
hobgoblin351 Registered Member Join Date: Sep 2002 Location: NYC Posts: 245	I've built engines but I'll admit I'm no machinist! lol Here's my 2cents anyway. Piston skirts are usually smaller than up at the top. If the bore is 30 over then you get pistons for a 30 over. They actually will measure smaller than the bore. They have to, if they were the same size you'd never get them into the bore. Plus you need room for thermal expansion. Did you mark everything when you tore it down. If you're using the same parts they're all broken in to their respective bores and need to go back the same way. As long as there is no scarring in the bores or on the pistons you should be able to just replace the rings, and bearings while you are at it. If they are worn, depending on how bad you might get away with just honeing them alittle. __________________ 1970 Stang Conv: en:Boss351 (balanced/blueprinted) mech cam(240*@.050/.540 lift) Scrw in studs/comp cams rllr rkr Mldn valves 2.19in 1.71ex ported polished cc'd hrdnd ex Keith Black FT pistons(10.25/1) Weiand Excellorator manifold Holley700 DP mech 2nds MSD 6 box and Dis Headers/2 1/2 inch duels tr: modified FMX re:9-inch/3.89 limited slip

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