400 or 351 - how do I tell?

A friend of mine has a 1973 Ranchero, which he was told is a 351ci - but from the VIN (3A49S302060) it looks like it's 400. How do I tell the difference?

The only way to know for sure is to take number one spark plug out and turn to top dead center . then rotate one half turn insert a wre into the spark plug hole parilel to the bore there is a large enough gap between the 400 with 4 " stroke .Compaired to the 351 with 3.5" stroke that you can tell witch you have

If it is the original engine to the car, it is a 400.
one way to see the difference between a 351C and a 400 or 351M is the thermostat housing sits up about a 1/2 inch from top of the block on 351M/400 and on 351C the thermostat housing sits flush to the top of the block. Also 351M/400 have big block transmissions while 351C bolt up to small block transmissions.
If you are trying to establish between a 351M and 400 there is nothing on the outside of the engine that will establish that. They are the same except for the crank and pistons. This might not be of much help, but the 351M was not in production until the 1975 model year when it replaced the 351C as a H code engine.

I see from the VIN that your friend has a Squire. Does it still have the Squire trim?

Just to the right of the distributor will be a rib in the block casting if it's a 400, nothing if it's a 351.

Originally Posted by danv

Just to the right of the distributor will be a rib in the block casting if it's a 400, nothing if it's a 351.

Again...this assumes the engine is original or has been replaced with the same engine it was built with - thats not guaranted, remember, the car is 30+ years old. A lot of changes may have happened over the life of the vehicle.

The 351C is pretty easy to distiguish from the 351m/400. The intake is different as is the (front) timing cover area and as you mentioned the thermostat.

Similar to the FE family, mix-matching parts on 351m/400 is nothing more than swapping in the correct rotating assembly...the basic blocks are essentially the same and will take either, same with heads, intakes, cranks, rods and pistons. You can go up or down using either block. The only way to tell, for sure, is to check the stroke. There a few shade tree ways to do that, the post above gave a pretty good method.

You can also simpli look at the motor mounts as well. The lazy way.. but works everytime for me in the wrecking yards. The 351 will have mounts like the 289/302 where the bolts are vertical.

On a 400 motor, they go into the block sideways or at an angle. Works for me.

Originally Posted by slik

You can also simpli look at the motor mounts as well. The lazy way.. but works everytime for me in the wrecking yards. The 351 will have mounts like the 289/302 where the bolts are vertical.

On a 400 motor, they go into the block sideways or at an angle. Works for me.

The 351M and the 400 are the same block and use the same mounts. There is no way to externally distinguish a 351m from a 400.

>The 351C is pretty easy to distiguish from the 351m/400. The intake is different as is the (front) timing cover area and as you mentioned the thermostat.

No help - t-stat location is the same on 351C, 351M and 400 with exceptions for deck height. 351C will have a small block bell housing pattern and have a C4 or FMX behind it. It will also sit on regular small block (289 / 302 ) mounts. The t-stat housing will be the same height as the intake manifold seal surface on the block.

Anything else is a 351M or 400. Fin next to dist., motor mounts bolt to the block with three bolts, big block bell, 1" more deck height. As noted, the stroke on a 351M is 3.5" and on a 400 it's 4", so pull a plug and stick a wire in there to get a rough reading.

Bear in mind folks that the early '72, '73, & 74 400 CID Engines were not "Modifieds" but their own unique engine (based largely on the Cleveland design) just as the Cleveland is it's own unique engine. The rib next to the distributor and the different engine mounts are two ways to tell, another way with this one is to take and inspection mirror and locate the partial vin on the block below the backside of the drivers side head. If it matches the year, plant and sequence number it is the original engine to the car and that would be a 400.