427 Block

[Walter R.]

Is there any difference in a 427 block with 425 HP and block with 390 HP?

[Roy S.]

Re: 427 Block

Short answer Yes. I will leave it to the experts to explain.

[Joe L.]

Re: 427 Block

Is there any difference in a 427 block with 425 HP and block with 390 HP?

Walter------

The block CASTINGS are the same. The difference is in the machining of the blocks. The L-36 block is machined for 2 bolt main bearing caps; the L-72 is machined for 4 bolt main bearing caps. In addition, for the L-72 there is machining in the area above the oil filter for oil cooler plumbing. For the L-36 there generally is not such machining. I've heard of some L-36 blocks being found with it, though.

An interesting side note: the bearing caps on 4 bolt main engines were not made of the same material as the block. The block was made of gray cast iron; the main caps were made of ductile/malleable iron, a much stronger material. For the 2 bolt caps, I'm not sure. I don't know if they were gray iron, nodular iron, or ductile/malleable iron.

[Walter R.]

Re: 427 Block

Thanks Joe and Roy,
I should have mentioned that mine has 4 bolt main and I knew that differentiated the 390 from the 425. Bought block about 5 years ago and when I purchased I knew the 4 bolt differentiated the two engines but I had forgot. One other thing is the block I have is standard bore and no markings on the engine pad. I know that could mean the engine was decked or the stamping was sanded off but I don't think so as pad looks extremely consistent. I was told this is probably a service block as they came with no stamping on the pad. Your comment would be appreciated.

[Roy S.]

Re: 427 Block

Post a picture of the engine pad deck surface! Or send it to me as a private message if you do not know how to post or do not want to post a picture.

[Joe L.]

Re: 427 Block

Thanks Joe and Roy,
I should have mentioned that mine has 4 bolt main and I knew that differentiated the 390 from the 425. Bought block about 5 years ago and when I purchased I knew the 4 bolt differentiated the two engines but I had forgot. One other thing is the block I have is standard bore and no markings on the engine pad. I know that could mean the engine was decked or the stamping was sanded off but I don't think so as pad looks extremely consistent. I was told this is probably a service block as they came with no stamping on the pad. Your comment would be appreciated.

Walter------


Yes, SERVICE bare or fitted blocks usually had no numbers stamped on the engine pad.

I have heard of other cases, though, in which 1966 L-36 originally had 4 bolt mains. 1966 Chevelles with L-34 (360 HP) usually had 4 bolt mains. The L-34 is the 396 cid equivalent of the L-36.

[Michael J.]

Re: 427 Block

I have heard of replacement long block assemblies with the plant, assembly date and suffix on the pad.

[Walter R.]

Re: 427 Block

Engine in machine shop. Will post when available. Thanks.

[Justin S.]

Re: 427 Block

Joe,
Wouldn't the valve reliefs be cut on a 425 hp block as well?

[Joe L.]

Re: 427 Block

Joe,
Wouldn't the valve reliefs be cut on a 425 hp block as well?

Justin------

Yes.