So I went to replace the plugs on my 1970 Pontiac GTO that has a supposed 73 455 motor between the frame rails. Problem is, the plugs would not fit. I pulled Autolite  #26 plugs out of the holes and attempted to replace them with Champion #58 pugs that are the same diameter and thread as the autolites but the base above the threads on the champions did not fit the tapered holes like the Autolites.

I would like to know the heads that I have on this motor but whomever rebuilt and ported the heads ground off the numbers on the center exhaust ports. Gee thanks.

I'm hoping to post  up some numbers remaining on the heads and hopefully a resident Pontiac guru can help me ID the heads for my personal knowledge and an opportunity to get the right plugs.  Thanks,

drivers side middle ports

drivers side

drivers side

passenger side

360 - are there any markings left on the front of the block, by the water pump, on the passenger's side head?

The A187 should be the casting date which would translate to January 18, 1967 or 1977. If it 'is a 77 then, it should be a 6X head.

Look for the identification code on the raised portion of the head. This is code for the number of cc's the head has. Assuming this is a '77 head for a moment, it would have been a 6X head regardless with 2.11" intake, 1/66" exhaust valves. A "4" would mean 93 cc heads from a 350/400 W72 engine. A "8" would mean a 100 cc heads from a 400 engine.

 

So for now, my suspicion is you have a bastardized motor. A '73 block with '77 heads. So try spark plugs for a '77 Firebird or Trans Am and see if they fit.

Edited January 29, 20188 yr by Frosty

I used Champions to start with in mine.   Switching to AC solved alot of issues.   Good luck.

I punched in a 73 455 and it gave me a plug number that matches the plugs in the engine now. I will go check that raised number and see what we have. Thanks

 

Justa, I had Autolites, bought Champion but am going to pick up AC Delcos this week. Thanks.

 

Edited January 29, 20188 yr by 360Rocket

A '73 455 motor should take an AC R44TS. A '77 Trans Am with a 400 would take an AC R45TSX. One heat range different. However both plug sets should be tapered seats with no crush-able O-ring.

The block number just for conversation sake.

 

 

18 hours ago, 360Rocket said:

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Okay - a YC code block for 1973 is a 455 ci 215/250 HP tied to automatic transmission. It would've had 4X heads and a 8.0:1 compression ratio. It would not have had the unitized distributor.

 

As for heads, you definitely have 6X heads and 100 cc chambers. Pontiac didn't cast the 4 or 8 into the heads in 1967. So we can eliminate it as a '67 head.

Did I mention I was an AC Delco bigot? The nice thing about NOS plugs, they never go bad sitting on the shelf.

Good luck 360!

Edited January 30, 20188 yr by Frosty