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Big Block Chevy Cam Identification

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While turning the distributor during diagnosis process your customer noticed the other four cylinders would start running and the
original four go dead. Now, since it is a big block I truly understood
a phrase that my brother always used: "There is no substitute for cubic
inches," because it did actually run on either of the four cylinder
sets.

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In your mind you’re thinking "Great, I’m dealing with another guy who
is certainly not the sharpest tool in the work shed," but in the
interest of customer service you entertain the bizarre possibility that
there is something wrong with the engine and not the ignition system.
Finally after what seemed like enough time to grow a beard on the phone
with the customer you agree to take the engine back into your facility
and provide the customer with another one, certainly expecting him to
have the same problem as the original.

A few days and hundreds of tasks later your customer calls and says the
new engine runs great. You wonder how that can be possible and realize
that it is time to get the returned engine on the bench and pull out
the CSI tools and find out what happened. Heads are perfect, cylinders
good, pistons and rings fine, all the 
bearings look great, the timing gear’s in time – there is absolutely
nothing wrong! You see the camshaft lying on the bench and know there’s
nothing wrong there…or is there? You pull out another new camshaft and
lay it next to the one you just removed and scratch your head, because
the two are very different. First thought, this is simple. Somehow a
reverse rotation marine camshaft was incorrectly packaged, right?
Wrong! Finally you just walk away and right it off to a bizarre
anomaly. Then weeks later you read a short blurb about how the firing
order on 8.1L big block engines is different than a normal big block.
The 8.1L FO is 1-8-7-2-6-5-4-3 where a 454/7.4L is 1-8-4-3-6-5-7-2 as
seen in Figure 1.

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Despite the differences, however, an 8.1L camshaft will drop right into
a 454 like it belongs there – but it will only run four cylinders.

So guess who had the elevator that didn’t get to the top floor? It
wasn’t the guy who called up saying that his engine only ran on four
cylinders, that’s for certain. If you’re in the engine business long
enough you’ll have those days when you feel like one of those suckers
that you used to get after a shot at the doctor’s office – I think they
were called Dum-Dums.Figure 1 The camshaft on the left (with the red arrow) is for an 8.1L GM. Make note of the lobe differences in comparison to that of a 454/7.4L on the right.

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