Anything more than that and you are pushing your luck.
1972 400 CID engine
Thread Starter
Le Mans Master
Joined: Jul 1999
Posts: 5,005
Likes: 38
From: jackson nj
1972 400 CID engine
I am considering doing a 400 CID engine for my 72 Vette and have a line on a 1972 400 engine block, but I don't know anything about these blocks for overbore. What I'm looking at, is how far can they "safely" be bored out?? As I don't already have a block, and most that I see available, have already been bored, I have concerns on "how big" the cylinders can be taken. I've spent the last 20 years on the LS engines, and have never had a 400 block in any of my cars, with no experience in the way of machine shops in my area to ask. I also have to wonder if a shop that I might be talking to, is just giving me lip service on the maximum bore size just to get my money. I don't have plans for any power adders, so it will be an NA engine, but I am still concerned about the life an overbored 400 will give me, and also, what is the best rod length for one?
Last edited by Redvette_22; Apr 17, 2017 at 11:05 AM.
Le Mans Master
Joined: Apr 2001
Posts: 5,436
Likes: 1,253
From: Sarver Pa
2021 C1 of the Year Finalist - Modified
I am going to build one also and have done em before. I like .030 over .040 is still good and I consider .060 the limit w/sonic check. If you are building a mild torque motor the stock rods are o-k, I will use 5.7 and consider 6.0 the best.
Last edited by Pop Chevy; Apr 17, 2017 at 12:36 PM.
7th Gear
Joined: Apr 2017
Posts: 7
Likes: 0
I am also building a 400, but for my 72 Camaro. There are three basic rod lengths to choose from. Stock 400 long, stock 350 medium, and short rods for high rev race motors. The long rods will put a lot of force on the bores at B.D.C. egg shaping your bores over time. This is what has made a bad rap for the 400. I will use 350 rods and order the pistons to match. The pistons will weigh a little more but have a longer skirt for more stability in the bore. As for the block, I have a 4 bolt main. I hear the two bolt are actually stronger. I wont be racing, just want the fun factor of torque. I did this before in the early ninety's for a 69 El Camino I used to tow with. It worked so great that I am doing it again.
Drifting
Joined: Jun 2012
Posts: 1,990
Likes: 183
From: Port St. Lucie Fl
I am also building a 400, but for my 72 Camaro. There are three basic rod lengths to choose from. Stock 400 long, stock 350 medium, and short rods for high rev race motors. The long rods will put a lot of force on the bores at B.D.C. egg shaping your bores over time. This is what has made a bad rap for the 400. I will use 350 rods and order the pistons to match. The pistons will weigh a little more but have a longer skirt for more stability in the bore. As for the block, I have a 4 bolt main. I hear the two bolt are actually stronger. I wont be racing, just want the fun factor of torque. I did this before in the early ninety's for a 69 El Camino I used to tow with. It worked so great that I am doing it again.
The common builds are with 5.7 or 6in rods. People will argue both ways about which is better. The deciding factory is compression ratio created by chamber of the heads you will use. With my 406 I had no choice but to use 5.7 rods with dished pistons because I didn't have a budget to replace the 58 cc heads.
Racer
Joined: Jan 2013
Posts: 302
Likes: 2
From: Sayville New york
4 bolt main 400 block made prior to oct of 1972 is the strongest .after that date gm went into weight saving mode which affected the 400 block because they took the "meat" from under the mains toward bottom edge of block. later years two bolt mains place inner bolts in the meatier part of the block so they went to two bolt with splayed outside bolts in new main caps. NEVER use 400 rods (5.565 ) thats how the 400 got a bad rap side load will destroy piston skirt,then destroy engine.there is no argument among people in the know only wannabe !!!!!!!all the power is derived from hanging at TDC with 6" rods and the added benefit of reduced sidewall pressure. 400 block prior to 72 .060 later two bolt .030.however best to sonic test for casting shifts.great motor when built with modern aluminum heads and retro-roller cam.in a CNC machine to square block.
Last edited by Captain bob; Apr 17, 2017 at 04:23 PM.
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
4 bolt main 400 block made prior to oct of 1972 is the strongest .after that date gm went into weight saving mode which affected the 400 block because they took the "meat" from under the mains toward bottom edge of block. later years two bolt mains place inner bolts in the meatier part of the block so they went to two bolt with splayed outside bolts in new main caps. NEVER use 400 rods (5.565 ) thats how the 400 got a bad rap side load will destroy piston skirt,then destroy engine.there is no argument among people in the know only wannabe !!!!!!!all the power is derived from hanging at TDC with 6" rods and the added benefit of reduced sidewall pressure. 400 block prior to 72 .060 later two bolt .030.however best to sonic test for casting shifts.great motor when built with modern aluminum heads and retro-roller cam.in a CNC machine to square block.
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Melting Slicks
Joined: May 2012
Posts: 2,192
Likes: 343
From: Allenton Wisconsin
The 434 build in the 69 is .040 on the bore, 1971 chevy block,4-bolt, flat top Mahle pistons, 6" scat rods. This is the 3rd season running with no problems, great torque engine. T
Thread Starter
Le Mans Master
Joined: Jul 1999
Posts: 5,005
Likes: 38
From: jackson nj
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
Bluprint also sells a 400 block just another opiton
Shame GM doesnt sell a better version with a roller cam provision and 1 pc rear seal they would sell a million of them
Shame GM doesnt sell a better version with a roller cam provision and 1 pc rear seal they would sell a million of them
Thread Starter
Le Mans Master
Joined: Jul 1999
Posts: 5,005
Likes: 38
From: jackson nj
Has anybody done a sonic test on their block before attempting any work on the cylinders? If so, what dimensions did you get for the "major" and "minor" side of the cylinders? What is considered "safe" for the wall thickness "after" boring the block?
Instructor
Joined: Mar 2017
Posts: 223
Likes: 15
From: Ijamsville Maryland
I am considering doing a 400 CID engine for my 72 Vette and have a line on a 1972 400 engine block, but I don't know anything about these blocks for overbore. What I'm looking at, is how far can they "safely" be bored out?? As I don't already have a block, and most that I see available, have already been bored, I have concerns on "how big" the cylinders can be taken. I've spent the last 20 years on the LS engines, and have never had a 400 block in any of my cars, with no experience in the way of machine shops in my area to ask. I also have to wonder if a shop that I might be talking to, is just giving me lip service on the maximum bore size just to get my money. I don't have plans for any power adders, so it will be an NA engine, but I am still concerned about the life an overbored 400 will give me, and also, what is the best rod length for one?
Le Mans Master
Joined: Apr 2001
Posts: 5,436
Likes: 1,253
From: Sarver Pa
2021 C1 of the Year Finalist - Modified
I never had a 400 that ran hot and I've had a few of them. I've towed with em, raced with em, put em in a hot rod truck. I surprised a LOT of people with one in a 57 Chevy pickup.
Drifting
Joined: Jun 2012
Posts: 1,990
Likes: 183
From: Port St. Lucie Fl
.030 over is my limit for the street on my 400, the machine here stated tht you can do .040 over may require some block fill in coolant passages around the bottom of the cyl. But I won't try that, they recommended .060 over and block fill only for use in race cars that don't see street use.
Thread Starter
Le Mans Master
Joined: Jul 1999
Posts: 5,005
Likes: 38
From: jackson nj
.030 over is my limit for the street on my 400, the machine here stated tht you can do .040 over may require some block fill in coolant passages around the bottom of the cyl. But I won't try that, they recommended .060 over and block fill only for use in race cars that don't see street use.