ID-ing a 400 sbc blk

[rdobbs]

Looking to buy a 1981 Chev pk. Guy says it has a 400
in it. What is the best way to ID this engine without
looking at numbers. Can it be done by looking at freeze plugs?

[rspears]

One of the Chevy sites says that a 350 SBC will have two freeze plugs per side, but the 400 two bolt main block will also have only two. If it has three freeze plugs per side you're looking at a four bolt main 400. All of the 400's used a different balancer up front that has a bob weight on the back side, while the 350 is smooth front & back.

[NTFDAY]

If it's a stock engine the easiest and best way is to look at the harmonic dampner/balancer, if there is a machined factory groove in the balancer it's a 400, if not it's a 350

[techinspector1]

Some additional info....

'70-'73, block casting number 3951509, 2-bolt mains is possibly the most desirable block to begin with, due to being a heavier casting.

'73-'80, block casting number 330817, 2-bolt mains.

'70-'73, block casting number 3951511, 4-bolt mains.

The 4-bolt main blocks didn't have sufficient material in the webbing for the extra bolts to hold onto and have shown to be weaker than the 2-bolt blocks, although there are some circle-trackers who have used either design up to 800 hp (so they say).

These casting numbers are on the driver's side at the extreme rear of the block, right next to where the bellhousing bolts up and can be seen with the motor in the vehicle by leaning over the driver's side fender. You may need a mechanic's mirror and a strong flashlight, but this is the way to definitively identify the motor. I like to take an ice pick with me to scratch around on the casting number and remove grunge so I can read it. On the passenger's side of the motor, stamped onto the block deck just in front of the end of the cylinder head, is a suffix number that will further identify the date of the block, the factory where the motor was built and what vehicle it was used in from the factory. This suffix is often missed because it is obscured by an alternator or other engine-driven accessory. If no suffix number is visible, the block may have been decked at the machine shop by a previous owner. Regardless of whether there is a suffix number there or not, if you intend to build the motor from scratch, you should have your machine shop measure the block deck height, so you can coordinate the stack of parts you will use and verify the squish.

Be aware that the block has siamezed cylinders and that the cooling passages do not completely encircle the cylinders. Because there is a coolant dead-end at 6 places on each side of the block, the factory drilled steam holes in the block. Any cylinder head and any head gasket that is used on the 400 block must be drilled also, to match with these holes in the block if the motor is to be used at low rpm's, like a street motor. If the rpm's will be high all the time, as in a race motor, then there will be sufficient speed to the coolant to wash away these steam pockets that are formed at the dead ends and steam holes in the heads and gaskets will not be required.
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