Computer/ firing order ?

This doesn’t prove that the block is a HO block. Some trucks evidently use a HO firing order
with a low lift cam. However, it will definitely prove that a block can’t be HO because the firing order is wrong.

Remove the #1 & #3 spark plugs. Put your finger in #1 spark plug hole. Crank the engine over until you feel compression on #1 cylinder. Slowly turn the engine until the TDC mark and the timing pointer line up. Mark TDC on the balancer with chalk or paint. Put your finger in #3 spark plug hole and crank the engine 90 degrees. You should feel pressure trying to blow past your finger. If you do not feel pressure, repeat the process again. If you feel pressure, it is a HO engine.

No pressure the second time, remove spark plug #5. Put your finger in #1 spark plug hole. Crank the engine over until you feel compression on #1 cylinder. Put your finger in #5 spark plug hole and crank the engine 90 degrees. If you feel pressure now, the engine is not a HO model, no matter what it says on the engine.

Using a small carpenter or machinist square to mark the harmonic balancer off into 90 degree sections may be helpful here.

A 15/16 deep socket & breaker bar or ratchet may be used to turn the engine.

The HO firing order is 1-3-7-2-6-5-4-8.
Non HO firing order is 1-5-4-2-6-3-7-8

jrichker said:

This doesn’t prove that the block is a HO block. Some trucks evidently use a HO firing order
with a low lift cam. However, it will definitely prove that a block can’t be HO because the firing order is wrong.

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Doesn't this just prove or disprove that the cam is a HO/351 cam or not? The block doesn't determine firing order.

Those pistons don't care about the cam. If you're getting compression they have to work together so it should work.

Maryland Stang said:

Doesn't this just prove or disprove that the cam is a HO/351 cam or not? The block doesn't determine firing order.

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Absolutely correct. The cam determines the firing order.

However, the reference was to a completely assembled engine block. The idea is to be able to pick an unknown engine from a stack in the junkyard (or already in a car) and tell if it is definitely not an HO engine.