Thread: 402 BBC build

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Hello All. New to this forum. Hopefully someone can help me with my 402 rebuild. I am restoring my 1970 GMC sierra grande truck. I am currently going thru the engine. The indication is that is came with a 396 but i understand they marked the 402 as 396 also during this time. I have the engine apart and have done some research and found this is a true 402 block 0.040 over bore to 4.166. I want to get about 400+ hp when done. i think I can stay with my block/heads/crank. I want a good street rod and occasional play at the track but mainly street. Here is what i think I know.

Block:
Engine ID - T0406TBG
Casting - 3969854
2 bolt
This seems to be a 402 block built at the Tonawanda plant April 6th 1970 for a truck. Could pass as original equipment I believe but has been rebuilt at some time.

Heads:
Casting - 3931063
Open chamber
2.06/1.72 valves
From my research I have seen some info that indicates 96.4cc and some 100.9cc. Not sure which is correct.

Crank:
Casting - 3904816
Rod journals at least have been undersized 0.010. Going to mike out this weekend to verify these and mains.

Pistons:
Currently marked with "402P" on one side and "F" on the other. I think they might be sealed power cast pistons but data says a 0.125 dome but I measure 0.10 dome. doesn't look like they have been milled down because the 0.040 markings still have raised edges. Doesn't seem to have a lot of wear but again they will be miked this weekend along with the bore. The rods are stock.

I have done the math and believe with this setup my piston deck is 9.750 which would put the dome deck to 9.850. I am thinking about just going with having it bored to 0.060 anyway which means I would replace the pistons. From what I have read the stock crank/heads/rods are plenty strong for my use.

My main trouble is the geometry between the pistons/cam/push rods/rocker ratio/valve springs etc. I can't find any formula to determine this and I don't have the experience to just know. Do I want to go roller cam? What duration/lift? I have found plenty of info to check these clearances once built but nothing to check before parts are purchased.

Thanks for any help to get me started.

Originally Posted by chevybuilder

Hello All. New to this forum. Hopefully someone can help me with my 402 rebuild. I am restoring my 1970 GMC sierra grande truck. I am currently going thru the engine. The indication is that is came with a 396 but i understand they marked the 402 as 396 also during this time. I have the engine apart and have done some research and found this is a true 402 block 0.040 over bore to 4.166. I want to get about 400+ hp when done. i think I can stay with my block/heads/crank. I want a good street rod and occasional play at the track but mainly street. Here is what i think I know.

Block:
Engine ID - T0406TBG
Casting - 3969854
2 bolt
This seems to be a 402 block built at the Tonawanda plant April 6th 1970 for a truck. Could pass as original equipment I believe but has been rebuilt at some time.

Heads:
Casting - 3931063
Open chamber
2.06/1.72 valves
From my research I have seen some info that indicates 96.4cc and some 100.9cc. Not sure which is correct.

Crank:
Casting - 3904816
Rod journals at least have been undersized 0.010. Going to mike out this weekend to verify these and mains.


My main trouble is the geometry between the pistons/cam/push rods/rocker ratio/valve springs etc. I can't find any formula to determine this and I don't have the experience to just know. Do I want to go roller cam? What duration/lift? I have found plenty of info to check these clearances once built but nothing to check before parts are purchased.

Thanks for any help to get me started.

the F on the pistons means front .. the piston to valve and all others stuff is check for your build. deck height .heads that are milled .were the valve are seating in the head all come in to play .push rod get check at mild lift they will not be same as stock pushrods if you go rollercam. valve springs are used for your cam you are going to use. welcome to engine building 101 i build all bbc for customers with hyd roller or solid roller for race engines . as for block .heads. rods. crank. good size and machine work /prep is what pays off all them parts can see 400+ all day long if all machine work is right and the engine is built right

Thanks Pat for the response.

I checked the pistons, at least a few of them until I found they had 0.005" clearances from the ideal bore. this I think is too much. I also checked the rod bearing roundness until I found one that was 0.0025 out of round. From these results I think I should go ahead with boring the block to 0.060", new pistons, and unfortunately new rods. I haven't check the crank yet but after finding the condition of the rods and inspection on the rod bearings I think the crank should be turned. This means it would be 0.020" undersize now. From what I have read this is ok.

I could pickup a used engine and salvage parts to save $ but I don't want to get into it and find similar results. Thought I better go new.

Anyone have any advise they would like to share about these results?

Originally Posted by chevybuilder

Thanks Pat for the response.

I checked the pistons, at least a few of them until I found they had 0.005" clearances from the ideal bore. this I think is too much. I also checked the rod bearing roundness until I found one that was 0.0025 out of round. From these results I think I should go ahead with boring the block to 0.060", new pistons, and unfortunately new rods. I haven't check the crank yet but after finding the condition of the rods and inspection on the rod bearings I think the crank should be turned. This means it would be 0.020" undersize now. From what I have read this is ok.

I could pickup a used engine and salvage parts to save $ but I don't want to get into it and find similar results. Thought I better go new.

Anyone have any advise they would like to share about these results?

bearings are not round from 12:00 to 6:00 is where they will have the least size 9:00to 3:00 parting line they will open up about .002 more . you need to check housing bores for out of round mains and rods .most all the time mains are ok .rods can go out of round and rod s should be rebuilt with new bolts or new rods like scat I beams .you need to check the crank for shaft size and how much they vary this is all stuff i do every day if cast pistons .005 to wall is to much . for size on rods and mains housing bores i like them all to at small spec no more then .0002 from target size

Thanks again Pat for the help...

That is really good news. I was up late last night looking at prices for rotating assemblies and was set back by the costs. A character flaw with me I guess that when I buy I usually go over board and by top of the line when not necessary just to be sure.

I will get the crank and mains checked and hopefully more good news there.

Not to do with your build,but curious.Does the front of one of the heads have a diamond'ed "Y" stamp on it??.I wondered what happened to engines I ran tested there and if it has that stamp on it,it is one of mine.

I couldn't find that marking. I cleaned off as much grease as I could and haven't had them reconditioned yet. I bought the truck in the late 90's and the guy gave me a lot of receipts with it. He claimed and I have a receipt that the motor was built at City Motor in Dallas. I have a receipt for a short block with the cam specs. Supposedly a guy in Carrollton had the truck before he did.

Thanks for looking.I got to thinking about it,might be a good thing you didn't find my stamp on it........................LOL

over the years i seen many stamps on bbc heads have a set in the shop now with a <> stamp on the inside on the valve cover many number stamps and letter stamps on the ends of the heads

The stamp would be on the pads where the accessories mount to the front of the heads and that I.D'ed which tester ran test the engine as passing the run test.Mine was a diamond with a <"Y"> in the center.

Yea it had some numbers and such stamped for sure but not that sign. Had the name Jose painted on the crank.

Well I have decided on using KB16-60 Hypereurectic pistons. 1.765 compression height .180 dome with the stock 6.135 rods and stock 3.76 crank. This with the COMP CAM 11-209-3 .507/.531 lift and 219/229 duration with 1.72:1 rockers, flat tappet hydraulic lifters and appropriate springs. From many hours calculating I believe I will have about about at least 0.08" intake valve clearance and over 0.1" exhaust valve clearance I hope. Of course I will verify these numbers upon assembly. From what I have calculated I hope to get 440hp @ 4000rpm. This is with a 500-550 cfm carb. I think about 9.84:1 compression.

Anybody have any suggestions/input? Pie in the sky or what?

Here is my spreadsheet from my calculations. It doesn't take into consideration of the intake valve relief in the piston and dome.