Big Block #3955272 Questions

[Rob V.]

Hi All--

I have two questions regarding a Chevrolet big block #3955272, ('69 396/402 ci), that I am going to build for my former/original (427 ci/390 HP) '67 Corvette.

1). The block appears to currently be standard bore. With that being said, can this block be safely bored to 4.25???

2). Can I use a 454 crankshaft in this block without relieving the bottom of the cylinders for clearance???

Thanks In Advance,
Rob

[Joe L.]

Re: Big Block #3955272 Questions

Hi All--

I have two questions regarding a Chevrolet big block #3955272, ('69 396/402 ci), that I am going to build for my former/original (427 ci/390 HP) '67 Corvette.

1). The block appears to currently be standard bore. With that being said, can this block be safely bored to 4.25???

2). Can I use a 454 crankshaft in this block without relieving the bottom of the cylinders for clearance???

Thanks In Advance,
Rob

Rob------


First of all, the 3955272 is a 396 block, not a 402. The 402 was first utilized for the 1970 model year and used a different block casting number (3969854). Therefore, the 3955272 is designed for a 4.094" bore. Using this block for a 427 application then requires a 0.156" overbore. You MIGHT get away with this if the block is a perfect casting. Sonic testing will reveal if it has sufficient cylinder wall thickness. Personally, I would not even attempt all of this; I'd find a block originally designed for a 4.25" bore.

If you were to use the 3955272 block, you would need to do some relieving at the lower end for clearance if you use a 4" stroke crank (i.e. 454).

[Rob V.]

Re: Big Block #3955272 Questions

Perfect Joe--I was hoping that you would respond. Thank you for answering both of my questions! I will forget about attempting to bore the block to 4.25; and will look for a 396/427 forged crankshaft rather than attempting to use the LS-7 454 crankshaft that I have. Hope you have a great weekend!

Rob

[Joe L.]

Re: Big Block #3955272 Questions

Perfect Joe--I was hoping that you would respond. Thank you for answering both of my questions! I will forget about attempting to bore the block to 4.25; and will look for a 396/427 forged crankshaft rather than attempting to use the LS-7 454 crankshaft that I have. Hope you have a great weekend!

Rob

Rob------


As I mentioned, you MIGHT get away with an overbore to 4.25" but, personally, even if I got away with it, I'd never be comfortable with the engine. I'd think that overheating or cracked cylinder walls would be in my near-term future.

As far as the relieving goes, if you obtain a 427 block, I would not worry too much about performing that to use the 4" stroke crank. It's pretty straight-forward and do-able.

[Rob V.]

Re: Big Block #3955272 Questions

Joe--

Thanks again! You make perfect sense about why not to overbore that 396 block. My understanding from all my years reading these posts, are the big blocks are more often than not, susceptible to overheating issues. I drive my 'Vette on the street; at cruises; and back and forth to work--always in traffic. That's also good to know that you don't think it is a big deal relieving the block for the LS-7 crankshaft. I would like to use the LS-7 crankshaft since it is new--never been installed in an engine. If I forge ahead just building the 396 for the car, could I still use the LS-7 crank with the block that I have (of course relieving the block as you advise)??? (I really can't afford or justify spending $4000 to $5000 for a bare 427 block). As always, I truly appreciate your knowledgeable and very excellent advice.

Rob

[Joe L.]

Re: Big Block #3955272 Questions

Joe--

Thanks again! You make perfect sense about why not to overbore that 396 block. My understanding from all my years reading these posts, are the big blocks are more often than not, susceptible to overheating issues. I drive my 'Vette on the street; at cruises; and back and forth to work--always in traffic. That's also good to know that you don't think it is a big deal relieving the block for the LS-7 crankshaft. I would like to use the LS-7 crankshaft since it is new--never been installed in an engine. If I forge ahead just building the 396 for the car, could I still use the LS-7 crank with the block that I have (of course relieving the block as you advise)??? (I really can't afford or justify spending $4000 to $5000 for a bare 427 block). As always, I truly appreciate your knowledgeable and very excellent advice.

Rob

Rob------


Yes, you could use the 454 crank in the 396 block. With a 0.030" overbore, this will yield about a 427 cubic inch engine. The big problem will be obtaining pistons for this combination of bore and stroke. I don't think that off-the-shelf pistons are available for this configuration (although it's possible they are). Otherwise, custom pistons will be needed and they can be expensive.

As far as a block goes, you don't need to spend 4K-5K for a block unless you want specific casting numbers and dates. There are 427/454 blocks out there with "undesirable" (from a restoration perspective) casting numbers and dates which are available for a LOT less than 4K-5K. The Mark IV big block was manufactured for PRODUCTION and SERVICE use until 1991. Even thereafter it was manufactured for SERVICE. For example, those blocks manufactured in the 1975-91 time period are not in much demand for restoration, the only use for which folks are willing to spend Big $$$, but they'll work PERFECTLY for your use. Plus, most of the 1970+ 4.25" bore blocks will be 454 blocks and will already be relieved for the 454 crank.

[Rob V.]

Re: Big Block #3955272 Questions

Thanks Joe--

I did not understand that I would need custom pistons for the 396 block / 454 crank application. I also was not aware that I could find a reasonably priced 454 Mark IV block for my use--every time I contacted an engine block supplier, and they found out that I wanted the block for my '67 Corvette, they immediately saw dollar signs and quoted ridiculous prices. I will look around some more. This is exactly the reason that I hoped you would respond to my posting--you saved me from making several foolish and costly mistakes. I truly appreciate all the help.

Rob

[Joe L.]

Re: Big Block #3955272 Questions

Thanks Joe--

I did not understand that I would need custom pistons for the 396 block / 454 crank application. I also was not aware that I could find a reasonably priced 454 Mark IV block for my use--every time I contacted an engine block supplier, and they found out that I wanted the block for my '67 Corvette, they immediately saw dollar signs and quoted ridiculous prices. I will look around some more. This is exactly the reason that I hoped you would respond to my posting--you saved me from making several foolish and costly mistakes. I truly appreciate all the help.

Rob

Rob------


Yes, when you told them that you wanted a block for a 1967 Corvette big block, they thought you meant a CORRECT block (i.e. correct casting number and date). That's BIG $$$$$$. In fact, probably about the most that anyone would have to pay for ANY correct Corvette block. But, if you don't care about "numbers" and "dates" (which you must not if you are considering a 3955272 block), that opens up a LOT more possibilities and WAY LESS expense to obtain one. Any 427/454 Mark IV block (except 10.2" deck height HD truck blocks) will give you about 99%+ correct configuration. Except for judging, no one's going to notice or care about casting numbers and dates or other possible nuances.

[Rob V.]

Re: Big Block #3955272 Questions

Hi Joe--

You are right on target as usual. Engine numbers and dates do not mean anything to me with respect to this car. While it would be great to find a correct block for the car, my intentions are to primarily drive it as I have the other 'Vettes I have owned over the years. Please don't misunderstand me, with these old cars, whenever possible I will attempt to find the correct parts for them, so that when they leave my possession, they are more correct than when I acquired them. However, with that being said, when I purchased this particular car, I knew that it did not have the original motor--and in fact, while it was originally a 427 CI / 390 HP car, when I got it, the car had a "scabbed" together '68 327 installed in it. Just so I do not confuse the issue, here is the background to this big block issue:

--About 2 1/2 years ago, my friend of 42 years wanted to trade me for some items that I had (we have been trading junk back and forth as long as we have known each other). Anyway, the stuff that I received consisted mainly of Chevy big block components; and the intention was to use these to build a big block for another 'Vette that I owned at the time. The engine was going to be built around an aluminum ZL-1 pre-production Can Am racing block and aluminum heads that he had, and which I got in the trade. The problem was that both the block and heads were damaged and needed considerable repair. While I am sure that they could have been repaired with copious amounts of money, I was not in a position to do this; and I was also not certain that they would withstand the rigors of my intended street driving. Anyway, I returned the block and heads two years ago, and have been pestering him to see what other suitable blocks he might have. Most of the big blocks he has left, are tall deck truck blocks (which I did not want to use for a multitude of reasons); a 454 block that had the deck "o" ringed for supercharger applications (and which he said would be a nightmare for me to use for my application. I guess there would have to be work to any head that I used on this block to fit the "o" ring?). Anyway, he does have the aforementioned 396 block that we have been discussing. So, I have been trying to figure out how I can use what I do have on hand to build a nice streetable big block for my '67 'Vette; and do it without breaking the bank. Obviously, it would be much easier if I just had a decent 454 block that would be compatible with my crank and balancer; or, a 396/427 crank to use with the 396 block. What I was attempting to do--either have the 396 bored to 427; or use the 454 crank in that 396 block were obviously very impractical ideas, and I am extremely thankful that you set me straight and put those ideas to rest.

Question: If I have asked you this previously, I apologize. Does Chevrolet still sell a finished Mark iV bare block that I can just install my components in and be good to go?

Thanks for all of the help, and great advice.
Rob

[Joe L.]

Re: Big Block #3955272 Questions

Hi Joe--

You are right on target as usual. Engine numbers and dates do not mean anything to me with respect to this car. While it would be great to find a correct block for the car, my intentions are to primarily drive it as I have the other 'Vettes I have owned over the years. Please don't misunderstand me, with these old cars, whenever possible I will attempt to find the correct parts for them, so that when they leave my possession, they are more correct than when I acquired them. However, with that being said, when I purchased this particular car, I knew that it did not have the original motor--and in fact, while it was originally a 427 CI / 390 HP car, when I got it, the car had a "scabbed" together '68 327 installed in it. Just so I do not confuse the issue, here is the background to this big block issue:

--About 2 1/2 years ago, my friend of 42 years wanted to trade me for some items that I had (we have been trading junk back and forth as long as we have known each other). Anyway, the stuff that I received consisted mainly of Chevy big block components; and the intention was to use these to build a big block for another 'Vette that I owned at the time. The engine was going to be built around an aluminum ZL-1 pre-production Can Am racing block and aluminum heads that he had, and which I got in the trade. The problem was that both the block and heads were damaged and needed considerable repair. While I am sure that they could have been repaired with copious amounts of money, I was not in a position to do this; and I was also not certain that they would withstand the rigors of my intended street driving. Anyway, I returned the block and heads two years ago, and have been pestering him to see what other suitable blocks he might have. Most of the big blocks he has left, are tall deck truck blocks (which I did not want to use for a multitude of reasons); a 454 block that had the deck "o" ringed for supercharger applications (and which he said would be a nightmare for me to use for my application. I guess there would have to be work to any head that I used on this block to fit the "o" ring?). Anyway, he does have the aforementioned 396 block that we have been discussing. So, I have been trying to figure out how I can use what I do have on hand to build a nice streetable big block for my '67 'Vette; and do it without breaking the bank. Obviously, it would be much easier if I just had a decent 454 block that would be compatible with my crank and balancer; or, a 396/427 crank to use with the 396 block. What I was attempting to do--either have the 396 bored to 427; or use the 454 crank in that 396 block were obviously very impractical ideas, and I am extremely thankful that you set me straight and put those ideas to rest.

Question: If I have asked you this previously, I apologize. Does Chevrolet still sell a finished Mark iV bare block that I can just install my components in and be good to go?

Thanks for all of the help, and great advice.
Rob

Rob------


No, they do not. The last cast iron Mark IV block was discontinued more than 20 years ago. The only cast iron big blocks currently available from GM are Gen VI type. Actually, some of these could be used for your application but you would sacrifice more in the area of original configuration. However, most Mark IV parts would work, including cylinder heads. While the blocks which are practical for your application are machined for one piece seal style crankshafts, you could use your crankshaft with an adapter. The most practical block for your application, GM part number 19170538, sells for about 1500 bucks. But, you should be able to buy a good, used Mark IV block for less than that.

[Rob V.]

Re: Big Block #3955272 Questions

Thanks for the info on the current GM blocks Joe--this is the kind of information that I absolutely need to know. I hope I haven't given you a headache; and I cannot express how much I truly appreciate your help with this matter!! I will eventually get a big block built and installed in that silly old 'Vette of mine.

Best Wishes,
Rob

[Rob V.]

Re: Big Block #3955272 Questions

Hi Joe--

I thought I would update you on the status of my engine block search efforts. My friend, Bob, found a "standard" bore two bolt main 454 block in Anaheim (CA). The block had been used previously in a jet boat; and the individual who was now selling the block had intended to use it in a '66/67 Nova "drag" car. Apparently, the seller decided he wanted a "bigger" engine so was getting rid of the 454. The seller couldn't tell Bob much about the block, but did mention that he had recently sold the forged pistons from the engine. Bob drove about 1 1/2 hours to the sellers home getting there as it was getting dark. The block appeared clean, so Bob pulled out his micrometers and measured the bores of the block, and then asked the seller what size the bores were, and the seller stated standard. Bob responded, right, standard plus .030. Anyway, Bob bought the block; took it home, and closer inspection revealed the thrust surface of the cylinders are scoured; and he is hoping that the block will clean up with torque plate honing at .040. So, I will go out to Southern California in January 2015, and pick the block up, along with some other parts awaiting for me. One question I have before I use many expletives to describe the seller, I thought all oversize pistons had the oversize number stamped in the top of the piston? If this is true, then how come the seller did not know what the bore size actually was??? Anyway, this saga will continue after January...

Best Wishes for a Happy Holiday,
Rob

[Joe L.]

Re: Big Block #3955272 Questions

Hi Joe--

I thought I would update you on the status of my engine block search efforts. My friend, Bob, found a "standard" bore two bolt main 454 block in Anaheim (CA). The block had been used previously in a jet boat; and the individual who was now selling the block had intended to use it in a '66/67 Nova "drag" car. Apparently, the seller decided he wanted a "bigger" engine so was getting rid of the 454. The seller couldn't tell Bob much about the block, but did mention that he had recently sold the forged pistons from the engine. Bob drove about 1 1/2 hours to the sellers home getting there as it was getting dark. The block appeared clean, so Bob pulled out his micrometers and measured the bores of the block, and then asked the seller what size the bores were, and the seller stated standard. Bob responded, right, standard plus .030. Anyway, Bob bought the block; took it home, and closer inspection revealed the thrust surface of the cylinders are scoured; and he is hoping that the block will clean up with torque plate honing at .040. So, I will go out to Southern California in January 2015, and pick the block up, along with some other parts awaiting for me. One question I have before I use many expletives to describe the seller, I thought all oversize pistons had the oversize number stamped in the top of the piston? If this is true, then how come the seller did not know what the bore size actually was??? Anyway, this saga will continue after January...

Best Wishes for a Happy Holiday,
Rob

Rob------


Most aftermarket replacement pistons do have the over-size stamped somewhere into the crown area of the piston. However, it's possible that some do not, especially those for "high dome" applications.

[Rob V.]

Re: Big Block #3955272 Questions

Thanks Joe; I will give the seller the benefit of doubt, and assume he really believed that the block was standard bore.


Rob