[Jerry H.]

Ok all you 421 engine guru's, help me with this one. I am no mechanic, and I need your help. I have heard this discussed here before. I have access to a set of standard 428 pistons. I have been told these will work with for my 421 engine (with the proper overbore). I am running the 76 heads and 068 cam. I have also heard that I will have trouble with the valves hitting the pistons with this configuration. What is correct? What overbore is this also? Thanks in advance for your replies.

[Jerry H.]

Ok all you 421 engine guru's, help me with this one. I am no mechanic, and I need your help. I have heard this discussed here before. I have access to a set of standard 428 pistons. I have been told these will work with for my 421 engine (with the proper overbore). I am running the 76 heads and 068 cam. I have also heard that I will have trouble with the valves hitting the pistons with this configuration. What is correct? What overbore is this also? Thanks in advance for your replies.

[Doug Hopkins]

Jerry, The 428 pistons will fit,but in 1967 Pontiac changed the valve angle in the heads,and in so doing the notch in the 428 pistons are at a different angle than the 421.You probably can get by with the 068 cam because the lift is not very extreme,but I would check piston to valve clearance to be sure.You also could switch to an 092 or 093 head,and that would solve any doubt,or buy 421 pistons from Kanter.I have read some bad chatter on these,but I used them with no problems.

[BVR421]

Are you planning to use ?what year? heads on this motor? just to eliminate any clearance issue at all, I would have the pistons dished (scooped out) in the valve relief area. This may be more expensive but it is also dealing with the compression problem when using pre smog era heads.
The only relatively inexpensive off the shelf piston I know of for 428 with a proper dish for street compression is the KB-Silvolite. Its a hyperutectic cast piston. I havent looked at a pic of those recently to see what the valve relief area looks like, I remember it being just right. Other than that its a custom cut forged piston $$$

[LASJayhawk]

421 pistons will be flat tops, 428's will be dished. so that might be the best way to go. you should be ok on piston to valve clearance under about .450 gross lift,but I'd check it with clay to be sure.

[BVR421]

good point, they are dished to equal a 400 piston so factory could use the same heads on 428/400 BUT it is still at least 10-1 and unless you believe gas will get better not worse, I would shoot for 9-1

[Jerry H.]

Thanks for the replies, Guys.

Bvr421, KB's Silvolite website doesn't show the 428 or 421 pistons anymore. I checked just last week.

So far, from what I gather, if I use my code 76 heads and melling 068 cam, and bore the block .30, the standard 428 pistons will work ok. Is this correct?