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Old 04-25-2022, 02:36 PM
blackadder blackadder is offline
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Default 455 crank in a 421/428 ?

I have a brand new machined 455 crank and I have a 65 421 & a 68 428 engine. I was contimplating putting the 455 crank in one of them. Anyone have any insights on what all is involved or what ws/maybe an issue with this combo? Was looking to use 7K3 or a set of 64 heads (64 preffered on the engine. Thanks

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Old 04-25-2022, 02:52 PM
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Just use a 455 type piston with the correct compression height and that crank is a bolt in.

Either one could use a standard bore 455 piston or better yet whatever custom bore size that’s most appropriate for where the cylinders clean up.

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Old 04-25-2022, 02:59 PM
sdbob sdbob is offline
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I have a 428 block that cut me long story, yrs ago. I have a 455 crank and 1 set nos SD 455 pistons and rods I keep thinking doing the same thing.

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Old 04-25-2022, 04:39 PM
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The 7K3 heads breath better then the 64 casting !

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Old 04-25-2022, 07:04 PM
'ol Pinion head 'ol Pinion head is offline
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Have used up extra 455 cranks in '69 YH 428 block several times.
Easy way to use up 2 bolt '69 428 blocks & extra 455 cranks, esp when customer has earlier A-body or 1st Gen Bird, doesn't need the motor mount bosses of the '70-75 blocks.

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Old 04-25-2022, 07:19 PM
blackadder blackadder is offline
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If I use 455 pistons will that not just be a 455? I have all the parts to build a 70 455 HO that are new less the block. A fella south of me has a 70 455 .030 over coded Hi HP block which will match the pistons I have but its rather $$. Cannot a 455 crank run 421/428 rods and pistons ? Or is the stroke an issue although I believe the deck height of all Pontiac engines is the same, but could be wrong. I am under the impression that adding just the crank from a 455 to either engine will yeild a 440 CID motor approx. Thanks for all the replies so far.

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Old 04-25-2022, 07:41 PM
'ol Pinion head 'ol Pinion head is offline
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.030 428 block will take std bore 455 pistons w 455 crank.
.060 428 is a 440.

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Old 04-25-2022, 08:47 PM
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The bore and stroke determine an engine’s displacement.

You’re swapping a 455 crank into a 421 or 428 block, creating a 455ish engine. The crank stroke increased from 4.00” to 4.21” when the 428 was upsized to become the 455, the slightly bigger 4.151” bore of the new 455 wasn’t the biggest factor in the displacement difference between the two engines it was the added stroke.

The 428 has a 4.120” bore and the 421 is 4.094”, both use the 4.00” stroke crank.

The piston pin location in relation to the piston top must be changed by half the additional stroke you’re adding. In the case of swapping in the 4.210” 455 crank you need pistons with the pin .105” closer to the piston top, because the rod length (6.625”} isn’t changing.

If you tried using a 428 piston with the 455 crank it would end up .105” further above the block deck than a 455 piston with the correct wrist pin location that matches the 455 crank.

The parts don’t care which block they’re residing in, a 428 block bored +.031” to standard 4.151” 455 bore size housing a 4.21” stroke 455 crank now becomes a 455 cubic inch engine.

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Old 04-25-2022, 11:18 PM
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Thanks B Man. I understand the Bore and Stroke displacement rule. Was looking for any points that generally do not get mentioned when doing the engine. I have a brand new set of 30 over 455 pistons but it seems I need to do a bit more investigation or perhaps just let sleeping dogs lie and stay with the stock cranck and rods/pistons. Just exploring some options. Thanks for all the help everyone.

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Old 04-26-2022, 12:29 AM
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Using the +.030 455 pistons will require boring the 428 block by about +.060 and the 421 block by about +.090, neither of which would be ideal.

Plan on buying some custom pistons should you decide to use the 421 or 428 block with the 455 crank. Bore the block you’re using to the minimum oversize that allows you to utilize piston rings that are commonly available.

Sell your 455 pistons or keep them for a future build on a 455 block.

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Old 04-26-2022, 07:39 AM
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The engine in my signature is a 1969 390HP 428 with a later thin counterweight 455 crank in it. I also used light weight forged "I" beam rods and ultra light JE pistons (.030" over) typically used for Super Stock type engines so they have thin ring packs.

It spins up like a 283 Chevy on steroids! The best engine combo by far I've had in the car, well had in the car because I sold it recently........

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Old 04-26-2022, 09:32 AM
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As stated already, a .030 428 with a 455 crank will be a stock dimension 455. Both blocks are getting hard to come by but the 421 is definitely more rare. I would put the 421 aside and use it for a true 4” stroke build up with tri-power. Would be great in a 2+2 clone or the real deal missing the original engine. Would be equally cool in a 65 GTO.

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Old 04-26-2022, 07:39 PM
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Quote:
Originally Posted by blackadder View Post
If I use 455 pistons will that not just be a 455? I have all the parts to build a 70 455 HO that are new less the block. A fella south of me has a 70 455 .030 over coded Hi HP block which will match the pistons I have but its rather $$. Cannot a 455 crank run 421/428 rods and pistons ? Or is the stroke an issue although I believe the deck height of all Pontiac engines is the same, but could be wrong. I am under the impression that adding just the crank from a 455 to either engine will yeild a 440 CID motor approx. Thanks for all the replies so far.
ALL 3.25” main Pontiacs run the same rods. The difference is in the compression heights of the pistons. To use a 455 crank you must use the appropriate 455 pistons or a custom piston with the correct compression height useless you are using standard bore 455 pistons (4.1510).

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Old 04-27-2022, 12:29 AM
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Andya I actually bought the 421 a year ago. Its a 1965 YJ code and I also bought a Tri Power from the same guy as he ws cleaning up lose ends so ho could move to British Columbia. I am just tossing around ways to use up some of the parts I have laying around. I was planning on usins the 421 in my 65 post body GTO as I restore it. I think that will be the path going forward. Probably pull a lot of the interal extrs I put into the 428 and put them in the 421. Things like the hi volume oil pump, crank scraper and windage tray I bought from SD Performance in the past.

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Old 04-28-2022, 12:27 PM
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I've got that same combination in my 69 GTO. 428 block, 455 crank from my race motor, ported 62 heads, COMP hydraulic roller, RPM manifold, try-y headers. Basically a 455 in a 428 case.

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