how to tell a chevy 400 from a 350???

i made a post last week about the killer deal i was getting on that 400 and m-21 well i want to make sure it is a 400 how can i tell with out tearing it apart??? i heard something about the 400 having 1 extra freeze plug.

Thanks
Bryan

 

TRUE.......most of the time........

If it has three....400

I have owned (hunted for) a few 400's that came with just 2.

Sneaky racers limited to 350 LOVE THEM.......will pass a visual tech check.

.

 

thanks for the info..

 

Quickest way is to look at the harmonic ballancer if it is stock it will have a scallop as it is externally ballanced. 2 of the cast nos end with 511 or 509 their are others but I dont recall. check web site mortec it has lots of chev cast no.

 

Most have the three freeze plugs.
the flex plate/flywheel has a chunk of metal welded on it because the crank is externally balanced. THe dampener is a thick sob with a "chunk" missing on one side towards the engine for the same balance reason.
pull the pan to see if it's two or four bolt main.
If you're "building" a race engine I hear the two bolt main is beefier and some prefer to use it after machining it for four bolt caps but if you're using it as-is in the bottom the four is slightly better.

Go here to check numbers for exactly what you got. http://www.mortec.com/

 

Here's a pic of my block. You can see it doesn't have 3 plugs, but it DOES have the raised boss where the third would be... just another option for you to look at!

 

Some of the info that I collected when building up a 400 for my 64 Malibu:

CHEVROLET SMALL BLOCK INTERCHANGE, 400s have the following casting numbers on the upper left side of the bell housing flange directly behind the drivers side cylinder head. If the motor is still in the car you should still be able to see it (The last three digits being reproduced in the side, as "509" etc.):
3951511 70 74
3951598 70 71
3951529 70 80
3951509 74 80
3973493 71 73
3998997 72 74
3932882 75 76
14010509 79 80
Examples, out of 4 blocks with code 3951511 with dates of Jun and Sep 70, Feb 72, are 4bolt blocks, and the 4th has a date of Aug 72 and is a 2 bolt block. 400s for 70 72 cars were 4bolt only and 73 later cars had 2 bolt only even though both 2 and 4bolt blocks sometimes shared the same casting numbers. Two things for sure, all 400s have steam holes in the deck between the cylinders and all have the larger crank bore for 2.65in. mains.

Regarding the 3 freeze plugs, basically sb400s have 3 plugs on each side. Although there is an exception. Late production 400s have provision for 3 plugs but some only have a boss where the center plug would be. The 350s do not have a center plug or a center boss.

4-bolt mains were used from 70-72. Even light duty trucks from 75-80 used only 2 bolt blocks.

Hope it is some help.

Jim

 

My 400 block also has two freeze plugs per side with the raised boss where the third would go (like the picture that Hot Rod to Hell posted).

I have read that the blocks that have the third freeze plug hole have 4 bolt mains, but I can't confirm that since I have never actually seen a 4-bolt main 400 block.

 

I also have a 400 4 bolt that DOES have 3 plugs, but I do not know if ALL 4 bolts have 3 plugs or not.

 

also you have to use the 400 flexplate with the 400 a 350 one won't work cause of the external balancing, a buddy of mine put a 400 in his camaro years ago, and used the 350 flexplate ,and all the bolts came loose and it sounded like he had blown the motor,he had to take it all apart and re do it with the original one .and if you're gonna run a 4 speed there's some fancy machine work that has to be done.that's why you don't see many 400 sbc with 4speeds.well just my 2 cents!and good luck!

 

What machine work? The only thing I can think of =would be maybe the hole in the end of the crank for the pilot bushing. I've heard Chevrolet put the 400 in some trucks with 4 speeds. I have one that my Dad bought new in a 72 Caprice and was planning to use a Camaro 5 or 6 speed behind it.

 

Really. I agree. How many 350 4 speed cars have been stroked to 383's??? bet a bunch of 'em! That would be the same exact setup as a 400/4 speed!

 

Actually HRTH, Most 383's now use aftermarket cranks that are internally balanced. I just bought an Eagle crank for my new blower motor, and had the choice of internal or external balanced. There weren't that many original 400 cranks available, and some that were, were balanced with heavy metal for internal balancing. But I think flywheels may still be available with external balancing from GM.