402 Clearances, specs info needed. (technical) - The 1947 - Present Chevrolet & GMC Truck Message Board Network

REPO1 · 03-14-2005, 07:37 PM

Hey guys,

I'm rebuilding a 402 for my '71 Camaro, and as these have the same engines, I was hoping some of you may have some info I need.

According to my book it says that the piston to cylinder clearance is .0035" MAX.

Now after measuring what I have, it is sitting at .005" Now do you really think the extra .0015" is going to make a huge difference? This isn't a race engine, and I plan on buying file to fit rings to the gaps will be perfect. And I do plan on the very occasional SMALL shot of NOS. I was thinking that the extra clearance may help in a juiced situation? I don't want to have excess piston slap/noise so I am just being sure. This block is already sitting at .060 over the stock 402 size. (4.157" now)

Any info or thoughts on this?

COBRO · 03-14-2005, 08:11 PM

My info shows .0018 to .0026 for factory pistons - you didnt state what kind of pistons you are going to use - forged or cast - the forged grow more and take more clearance - if your using cast pistons I would take it easy with the juice as they will not last very long. HTH:::::::::CB

cableguy0 · 03-14-2005, 08:22 PM

.015 is the difference between life and death in a motor. they have reccomended clearances for a reason and like he said if your using cast pistons ya better go real easy on the giggle juice or your going to need a lot more than just a rebuild. have it bored and have the pistons fitted to the bore any good machinist will do this

REPO1 · 03-14-2005, 10:06 PM

The pistons are not forged, but again the NOS isn't my worry. Its the piston slap that may occur.

The reason I am worried about this is because nobody makes a piston for bigger than .060 over the stock 402 size. I would have to get custom pistons, and that means lots of cash.

ABHaulR · 03-14-2005, 10:54 PM

talking about the juice, when i built my first 383 i got ready to put it in and the motor in the el camino was still running good and i couldn't see taking a good motor out, so i hooked up a 200hp nos kit and had some fun. stock motor nothing fancy, 10- 10lbs bottles of juice later finally popped a piston skirt. LOL...

it was alot of fun

Dwayne

COBRO · 03-15-2005, 01:14 AM

Well sounds like you are going to have to run what you got - I just went thru a 454 for my 72 and the bores were looser than I would have wanted but it runs fine and no slap that I can tell - I used sealed power pistons in mine and they have a teflon coating on the skirts which helps with the piston slap - GM has had slap problems with alot of their new engines because of bad QC and that was one of their fixes was to use pistons with coated skirts. With file to fit rings you should do OK. Instead of custom pistons,if I had to do it I would have the bores sleeved back to std. size - just something to think about. Let us know how she turns out.::::::::CB

**StingRay** · 03-15-2005, 09:57 AM

When you buy hypereutectic pistons with the coating already on it reduces the piston to bore by one thou. Essentially they are at Zero with the coating. If they are hypers they will not expand at all! A band aid solution is the have the piston skirts knurled. It will take up the clearance but won't last long and is really a poor band aid even at that.

OLDCHEV4X4 · 03-15-2005, 01:30 PM

If your going to use file to fit rings, make sure the cylinder bores have NO taper.

03-14-2005, 07:37 PM	#1
REPO1 Shortbox wanna-be. Join Date: Mar 2002 Location: Fort St.John, British Columbia, Canada Posts: 773	402 Clearances, specs info needed. (technical) Hey guys, I'm rebuilding a 402 for my '71 Camaro, and as these have the same engines, I was hoping some of you may have some info I need. According to my book it says that the piston to cylinder clearance is .0035" MAX. Now after measuring what I have, it is sitting at .005" Now do you really think the extra .0015" is going to make a huge difference? This isn't a race engine, and I plan on buying file to fit rings to the gaps will be perfect. And I do plan on the very occasional SMALL shot of NOS. I was thinking that the extra clearance may help in a juiced situation? I don't want to have excess piston slap/noise so I am just being sure. This block is already sitting at .060 over the stock 402 size. (4.157" now) Any info or thoughts on this? __________________ 1970 Chevy C20 Custom Camper 402BB Turbo 400 trans. (Slowly becoming a '70 shortbox 1/2 ton) '71 Camaro SS 402/T400 resto '97 Z28 (11.41@127MPH)

03-14-2005, 08:22 PM	#3
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,948	.015 is the difference between life and death in a motor. they have reccomended clearances for a reason and like he said if your using cast pistons ya better go real easy on the giggle juice or your going to need a lot more than just a rebuild. have it bored and have the pistons fitted to the bore any good machinist will do this __________________ Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.

03-14-2005, 10:54 PM	#5
ABHaulR Registered User Join Date: Jun 2003 Location: West Valley City, Ut Posts: 311	juice talking about the juice, when i built my first 383 i got ready to put it in and the motor in the el camino was still running good and i couldn't see taking a good motor out, so i hooked up a 200hp nos kit and had some fun. stock motor nothing fancy, 10- 10lbs bottles of juice later finally popped a piston skirt. LOL... it was alot of fun Dwayne __________________ 72' GMC Suburban 10 1/2" of Lift, 40" Tires! Waiting for restoration 67' Chevy, What to do??? And Bad Ass American Bulldogs!

03-15-2005, 09:57 AM	#7
StingRay Senior Member Join Date: Mar 2000 Location: Saskatoon,SK,Canada Posts: 2,487	When you buy hypereutectic pistons with the coating already on it reduces the piston to bore by one thou. Essentially they are at Zero with the coating. If they are hypers they will not expand at all! A band aid solution is the have the piston skirts knurled. It will take up the clearance but won't last long and is really a poor band aid even at that. __________________ Saskatoon, Saskatchewan Canada

03-15-2005, 01:30 PM	#8
OLDCHEV4X4 Registered User Join Date: Apr 2000 Location: Chester County PA, USA Posts: 1,228	If your going to use file to fit rings, make sure the cylinder bores have NO taper. __________________ 72 k/10 short wideside 72 k/5 Blazer 72 K/5 Blazer - obstacle course racer 72 custom/10 8' bed 70 K/5 Blazer 67 C-30 GMC Flat bed 86 GMC K2500 Plow Truck 85 GMC K3500 84 M1008 85 M1008 77 G30 Motor Home 72 Sears 12hp Tractor Glenmoore, Pennsylvania

03-14-2005, 08:11 PM	#2
COBRO Active Member Join Date: Jan 2005 Location: RDM,Oregon Posts: 285	My info shows .0018 to .0026 for factory pistons - you didnt state what kind of pistons you are going to use - forged or cast - the forged grow more and take more clearance - if your using cast pistons I would take it easy with the juice as they will not last very long. HTH:::::::::CB

03-14-2005, 10:06 PM	#4
REPO1 Shortbox wanna-be. Join Date: Mar 2002 Location: Fort St.John, British Columbia, Canada Posts: 773	The pistons are not forged, but again the NOS isn't my worry. Its the piston slap that may occur. The reason I am worried about this is because nobody makes a piston for bigger than .060 over the stock 402 size. I would have to get custom pistons, and that means lots of cash.

03-15-2005, 01:14 AM	#6
COBRO Active Member Join Date: Jan 2005 Location: RDM,Oregon Posts: 285	Well sounds like you are going to have to run what you got - I just went thru a 454 for my 72 and the bores were looser than I would have wanted but it runs fine and no slap that I can tell - I used sealed power pistons in mine and they have a teflon coating on the skirts which helps with the piston slap - GM has had slap problems with alot of their new engines because of bad QC and that was one of their fixes was to use pistons with coated skirts. With file to fit rings you should do OK. Instead of custom pistons,if I had to do it I would have the bores sleeved back to std. size - just something to think about. Let us know how she turns out.::::::::CB

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