[dlloyd]

A Pontiac 421 has a standard bore of 4.09". How much can I bore this and still have a robust engine? If my block has already been bored too much, is there anything that can be done?

[GRX]

What do you mean by too much? Seems to me your 421 should be able to handle .090 over. Same as .060 over 428 bore which makes pistons easier to find. Sonic testing the block is always a good idea of course.

[dlloyd]

Thanks for responding GRX. with .09 over would put it at 4.180. So if the block was all ready at 4.180 what if anything can I do if it needs bored?

[Tom Vaught]

What do you plan on doing with the block, say if it was in good shape right now?

Tom V.

[mgarblik]

I ran a 66 421 block at 4.180" for years, street and strip. No issues. Any larger, sonic check for sure.

[dlloyd]

I would like to rebuild it and put a roller cam in it. I have a 1965 tri-power I would like to install on it as well.

[mgarblik]

If the block is 4.180" now, you need to have torque plates installed and measure the block to see what you really have to work with. If it is basically OK and .005" or so will make it perfect, the most cost effective thing to do would be have a set of custom pistons made and you are done. If it truly needs to go larger, then you need a sonic test for sure and bore and hone. If you had to keep it numbers matching, yes you could sleeve the entire block. Expensive, labor intensive, and never quite as good as original. Not a good choice. Recently worked on a 64 Plymouth factory lightweight car. Original numbers matching Hemi. 2 holes in the block where it tossed the rods, entire block way oversize. Welded up the block and put 8 sleeves in it. He had more in the repair of that stupid Hemi block than a new IA II block would cost us. It all depends on what you want to do. There are plenty of 421-428-455 blocks still around that don't need sleeves.

[dlloyd]

Thanks for the response Mgarblik. Can you tell me where I can get a 1965 or 66 421 block?

[mgarblik]

If you want to PM me, I can provide you with a few leads. I do not have a specific source. 65-66 421's were standard in 2+2's. Approx 25% of Bonnevilles had them as well. Optional in three different trims on all full size Pontiacs those years. Fair to say 100,000+ were made those 2 years. A 428 is just as good in my opinion. They were standard in all Bonnevilles in 1969, popular option in all full size 67-69 models. Great engines, lots of torque.

[61-63]

I have a .060 over '65 421 that I am taking to 4.180 after having it sonic checked, and the machinist is moving the #7 hole .015 toward the rear to give me a little thicker front wall on that hole. All of the side walls were fine but a couple of the front/rear walls less so. After dealing with this block I would not take one out to 4.180 without sonic checking it first.

I have a '65 or '66, I would have to check the year, .030 over 421 block and crank that I will sell for $850 if mgarblik can't help you.

[hurryinhoosier62]

Ultrasonic testing of the cylinder walls is the ONLY accurate way to determine their thickness. There are many factors that effect cylinder wall thickness (core shift, corrosion, errosion, coolant cavitation), so you just can't say "IF I bore it to _____, I'll be OK." IMO,EVERY block being considered for high performance use should be ultrasonic checked. I know of a case where a running 389 in a '66 GTO was rejected for machining because the cylinder walls were discovered to be .090 thick BEFORE boring.

[tempest1964]

Just to review, if a block is ultrasonic tested, what is the consensus from the more experienced engine builders for what is the minimum safe wall thickness after its machined. I am obviously a newbie, I have 1 block and want to do it right the first time.
Thanks,
Tony

[WDCreech]

From an article by David Vizard; "With the bottom inch or so filled, .125-.150" should be regarded as the lower limit." He had been refering to the thrust side of the cylinder in regard to the limits as you can go a little more on the non thrust areas. He didn't elude to a non filled block, so you will have to ask around if you aren't going to use a block fill at all.

[hurryinhoosier62]

Originally Posted by WDCreech

From an article by David Vizard; "With the bottom inch or so filled, .125-.150" should be regarded as the lower limit." He had been refering to the thrust side of the cylinder in regard to the limits as you can go a little more on the non thrust areas. He didn't elude to a non filled block, so you will have to ask around if you aren't going to use a block fill at all.

Paul Carter and I had a discussion off line on this subject. The old standard was .130 minimum on the side wall, .170 minimum on the thrust side. After doing a LOT of reading on the subject, I agree with Paul that a .125 minimum wall thickness on the side walls and .150 on the thrust wall is OK. The current thinking is the reciprocating parts are far lighter than they were 30 yrs ago, lessening the side loading effects on the cylinder walls. This allows for thinner wall thicknesses.

[hurryinhoosier62]

Originally Posted by tempest1964

Just to review, if a block is ultrasonic tested, what is the consensus from the more experienced engine builders for what is the minimum safe wall thickness after its machined. I am obviously a newbie, I have 1 block and want to do it right the first time.
Thanks,
Tony

Tony, a lot is going to depend on the service life of the block. If it was a well maintaned engine and tested OK on the ultrasonic test, I have no problems with a .125 minimum cylinder wall thickness on the side walls, .150 minimum on the thrust side as long as the reciprocating mass is kept as light as possible. BTW...what part of Indy?Former Speedway resident myself.

[tempest1964]

Hoosier, I actually Work in Speedway! A little place called Allison Transmission near 10th and Main St. I have a '66 421 short block that is currently .030 over, scheduled to go into my '64 Tempest Custom. I plan on doing a dual quad setup with the help of information from John Reeder (61-63 on this site). I will definitely get my block ultra-sound tested, just to be safe. But even at +.060, it sounds like I should be ok.
Thanks

[hurryinhoosier62]

Originally Posted by tempest1964

Hoosier, I actually Work in Speedway! A little place called Allison Transmission near 10th and Main St. I have a '66 421 short block that is currently .030 over, scheduled to go into my '64 Tempest Custom. I plan on doing a dual quad setup with the help of information from John Reeder (61-63 on this site). I will definitely get my block ultra-sound tested, just to be safe. But even at +.060, it sounds like I should be ok.
Thanks

Is the Dog and Suds still there on 10th? I used to love their onion rings and fried tenderloin sandwiches. The 421 blocks seem to have had better quality control than the 389 blocks of that era. You're probably OK with +.060, but an ultrasonic check will make it conclusive.

[tempest1964]

Yup, Mug-n-Bun is still there. Me and the Mrs' love to cruise down there. Great root beer too.