Identifying a 366Hd vs 427Hd

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englertracing
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Identifying a 366Hd vs 427Hd

Post by englertracing »

Any way to tell the 366 and 427 apart?
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Re: Identifying a 366Hd vs 427Hd

Post by mag2555 »

Well I recall the block castings being different due to the far larger Bore size of the 427.

Both motors use the same 3.760" stroke, but the 366 uses a r44T spark plug and the 427 uses a r43T so may be this simple detail will help you determine what your looking at.
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Re: Identifying a 366Hd vs 427Hd

Post by rustbucket79 »

Externally just the casting number on the block. Sub 4” bore on the 366, 4.25” for the 427, so naturally a different casting.
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Re: Identifying a 366Hd vs 427Hd

Post by MadBill »

I believe all 366s are tall deck (10.2" vs 9.8"). I've never looked, but it should be easy to externally ID deck height* If it's a short deck it's not a 366.(*say something like upper water pump bolt to china wall measurement).
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Re: Identifying a 366Hd vs 427Hd

Post by englertracing »

Was trying to determine if the engine installed in a truck was a 366 or the 427.
It's for sure a tall deck. Just not sure which variation
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Re: Identifying a 366Hd vs 427Hd

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Re: Identifying a 366Hd vs 427Hd

Post by mag2555 »

A old GM guy once told me that the balancer and 3 pulley combo on the 427 are cast in certain years.
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Re: Identifying a 366Hd vs 427Hd

Post by SupStk »

Check to see if the crank flange has a dowel pin. It seems to me the 427T had the dowel and 366 did not.
If the engine is together With the transmission, a 366 had a normal truck bell housing whereas the 427T had an adaptor for the dual disc clutch and HD tranny.
Concerning the damper and pulley set-up, 366 had cast one piece units too.

Just memories of an old fart who used to be a GM truck tech.
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Re: Identifying a 366Hd vs 427Hd

Post by motormonkey »

Wanna say the 366 has the peanut heads and the 427 were the Oval heads.
Do you currently have the engine?
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Re: Identifying a 366Hd vs 427Hd

Post by Paul Kane »

Off topic but just in case someone can actually use them: I have these 366 forged dome-top pistons, 0.500” tall dome, they work with the stock stroke crank and the 6.66” rod. Message me if interested :shock:
4DCAC487-3BA1-4122-9EFA-A9ECE9BD8F96.jpeg
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Re: Identifying a 366Hd vs 427Hd

Post by rustbucket79 »

Paul Kane wrote: Thu Aug 12, 2021 9:55 pm Off topic but just in case someone can actually use them: I have these 366 forged dome-top pistons, 0.500” tall dome, they work with the stock stroke crank and the 6.66” rod. Message me if interested :shock:

4DCAC487-3BA1-4122-9EFA-A9ECE9BD8F96.jpeg
Ooh I don’t think I’ve ever seen a piston with a smaller market than those. :shock:
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Re: Identifying a 366Hd vs 427Hd

Post by MadBill »

There must be a story here... :)
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Re: Identifying a 366Hd vs 427Hd

Post by motormonkey »

:shock: I've seen it all now. LOL
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Re: Identifying a 366Hd vs 427Hd

Post by af2 »

motormonkey wrote: Fri Aug 13, 2021 3:34 pm :shock: I've seen it all now. LOL
At least there is only 3 rings on those 800# slugs.....
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Re: Identifying a 366Hd vs 427Hd

Post by HDBD »

BTW the 366 heads are a nice head to modify for a street motor, oval full size port, and small valves so with bigger valves and pocket porting they really shine. The chamber was closed on the ones I salvaged many years ago. Put them on a 94 TBI truck with a mild cam and it woke right up.
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