# 1966 421 Tripower



## Sirpless (May 10, 2017)

I found a 1966 421 tripower that had been rebuilt in 1987. The engine was never used and stored properly. I took the engine to Boyd's in Norman, OK to have it cleaned and checked. The engine had Howard's aluminum rods and I wanted them replaced with new H-beam rods. They examined the 421 and advised the engine was a 4 bolt main 455 that was bored 30 over and was in great shape. However, the engine block code indicates it is a late 1966 421 block. The factory bore for the 421 is 4.09 while a factory 455 is 4.150. However, this block shows a bore of 4.180 which would be a 455 bored 30 over. Now, the shop is concerned about the overbore being too much. It is my understanding that the 389-455 blocks are the same block with different bores and stroke lengths. The shop has not yet advised me to use another block. Does anyone have a suggestion?


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## BearGFR (Aug 25, 2008)

Yeah, first make sure exactly what you've got. Get all the date codes and casting numbers off the block and research them. If it is in fact a 421 that's been punched out to 4.180, don't panic just yet, but do be prepared to find a facility that can sonic check this specific block to find out exactly how much metal thickness is in all the bores. We're talking mass produced parts here, and there were variances. Could be this one's ok, could be not --- but the only way to know for sure is to have it measured.

Bear


p.s. Even if it IS too thin, if it's something special to you and you want to use it anyway, you could always have it sleeved. Of course there's more cost involved, but it's not impossible.


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## PontiacJim (Dec 29, 2012)

If it is a 455, it will be cast "*455*" on the side of the block, and inside the lifter valley where your lifters go, you will see a "55". Many 455's have the holes for a 4-bolt main cap but only used the 2-bolt main caps. 4-bolt main caps can be added to these.

Pontiac did use the same letter codes on engines throughout the years. This is where casting date codes come in - back at the distributor hole.

I don't understand why the shop would tell you it was a 455 and then be concerned that it was .030" over. If it is indeed a 455, then .030" is not a concern. Is this a reputable engine builder?

If it is a 455, .030" is not a problem. I went .060" over on my 455. Did not have it sonic tested as always seems to be recommended, but my machinist said not to worry - so its his call if it grenades.


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## Sirpless (May 10, 2017)

9782611 is the engine casting number which shows to be a late 1966 421.


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## PontiacJim (Dec 29, 2012)

Sirpless said:


> 9782611 is the engine casting number which shows to be a late 1966 421.



What is the engine code?????

421CI has 2 freeze plugs vs 3 on the '67 and up blocks.

421CI had 4 bolt mains.

Check this out: https://www.jalopyjournal.com/forum/threads/pontiac-v8-tech-389-vs-421-blocks.972245/

It appears the 421 has more material in the block, so .090" may be fine. I would have the walls sonic checked if you are concerned. I suspect with aluminum rods that someone knew what they were doing as that is typically drag race stuff.


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## Pinion head (Jan 3, 2015)

Pay to have the block sonic tested. Boyd's will have sonic test equip. Have cyl bores measured with a dial bore gauge & have each numbered piston miked. The pistons are most likely going to be old school high expansion alloy) forged TRW's, or maybe Arias, not a lot out there 30 years ago in the aftermarket. With an .090 over bore, I'd look at what the sonic test results are. I've built off 428 & 455 blocks that tested as thin as .135-.140 on the thrust, then we used 6.8" rods (less side loading of the piston skirt). On one such block we hard blocked it up to the bottom of the freeze plugs, then lightly bored & finished honed. If doing a partial fill with hardblock, don't wait til after block has been bored.


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## Pinion head (Jan 3, 2015)

PontiacJim said:


> ... I suspect with aluminum rods that someone knew what they were doing as that is typically drag race stuff.


possibly, choices for "good" Pontiac rods were very thin 30 years ago. Many higher performance Pontiac street builds of the time received alum rods from HO or Nunzi, one just had to be careful & warm up the oil before hammering on it.


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