The 402 isn't a 396 punched 0.030 over... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

davepl · 12-10-2015, 07:28 PM

Riddle me this:

The bore on a 396 is 4.094
If you punch it 0.030 over, you're at 4.124
The bore on a 402 is 4.125
Where'd the extra thousandth go? Does it matter?

For example, if you buy 0.30-over pistons for a 402, they'd be 4.155, but if you punched the 396 0.060 over, you'd be at 4.154

It's only one thousandth of an inch, but it still bugs me. Does a thousandth of an inch on piston diameter matter?

When I build an engine I shoot for 0.003 to 0.005 piston to wall clearance, so that's probably all that matters, and so I figure that missing 0.001 is within clearances. Safe assumption?

**kwmech** · 12-10-2015, 07:54 PM

Doesn't matter

hamjet · 12-10-2015, 09:07 PM

The 400 sbc in 70, and 71 were 4.125, in 72 the sbc 400 went to 4.126. I don't see where the bb 402 was ever 4.125.. The 70 402 bb and the 71 402 bb was 4.126. the tolerance is so slight it really doesn't matter. Whats .001 amungst friends..

12-10-2015, 07:28 PM	#1
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	The 402 isn't a 396 punched 0.030 over... Riddle me this: The bore on a 396 is 4.094 If you punch it 0.030 over, you're at 4.124 The bore on a 402 is 4.125 Where'd the extra thousandth go? Does it matter? For example, if you buy 0.30-over pistons for a 402, they'd be 4.155, but if you punched the 396 0.060 over, you'd be at 4.154 It's only one thousandth of an inch, but it still bugs me. Does a thousandth of an inch on piston diameter matter? When I build an engine I shoot for 0.003 to 0.005 piston to wall clearance, so that's probably all that matters, and so I figure that missing 0.001 is within clearances. Safe assumption? __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

12-10-2015, 07:54 PM	#2
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,500	Re: The 402 isn't a 396 punched 0.030 over... Doesn't matter

12-10-2015, 09:07 PM	#3
hamjet Registered User Join Date: Apr 2013 Location: South Westerlo, New York Posts: 1,325	Re: The 402 isn't a 396 punched 0.030 over... The 400 sbc in 70, and 71 were 4.125, in 72 the sbc 400 went to 4.126. I don't see where the bb 402 was ever 4.125.. The 70 402 bb and the 71 402 bb was 4.126. the tolerance is so slight it really doesn't matter. Whats .001 amungst friends.. __________________ Thanks, Joe.. 1969 C/10, 348 C.I., 3X2 bbl. V8, 2004r , LWB.



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