How to build a 427 sbc!

I started exploring building a low cost 427 sbc after I acquired the 400 short block. If the cylinders are bored out to the max 0.060" which is the absolute max bore. This ups the cid to 412. Naturally, this got me thinking, how hard would it be to offset grind the crank to pick up the extra inches to make it a 427 sbc.

The problem that I've run into is finding connecting rod specifications. After 3 weeks of searching, I found out that AERA Engine Rebuilders Association makes the Prosis database software ($500) of engine specifications and sells the Connecting Rod Specification 2001 Manual ($70). A search of their education links indicated that the Southern Alberta Institute of Technology, where I work, is a member institute. Another couple of days of searching at work (SAIT is a BIG institute!) located a copy of the manual. I'm now in the process of determining which combination of pistons and connecting rods will work. I consider a low buck 427 sbc to be the holy grail of small block chevys.

Here's what I've found out for an inexpensive 427 sbc (versus buying complete aftermarket parts).

  • 400 sbc block bored 0.060" over sized. This by itself will give 412 cid. The danger is in core shift in the cylinder walls. Core shift is the cast iron moving while pouring or cooling and giving unequal wall thickness. Not all blocks can be bored out this much, the cylinder walls may end up to be too thin for high performance applications but could be okay for the street

  • The crank has to be offset ground (grinded?) on the rod journals for 0.065" to give an overall stroke increase of 0.130" to 3.88". This will give you 427 cid. The rod journal final size will be 1.970" or 0.030" undersize for sbc small rod journals. You must start with a crank that has std rod journals.

  • Pistons and rods

    • 400 pistons

      You can go with 400 pistons that are 0.060" over and purchase custom aftermarket rods that will be basically sbc small journal 2.000", with a center to center of 5.500". The rods are 0.065" shorter than the stock rods to make up for the extra stroke. The small journal will allow for more clearance between the rod and the block/cam. Also, you can buy aftermarket rods that have additional extra clearance on the big end. Theoretically, there should be minimal clearance problems versus a stock 400 with this combo. This is choice is the best of the two but more expensive due to the cost of the aftermarket rods (US $800+).

    • 68-76 Olds 455 Pistons 0.060" over

      You can go with these OEM replacement pistons but they have a larger pin offset and piston pin diameter of 0.9805" which means a different rod must be used. They require a shorter rod 5.320" center to center distance. I found one OEM rod that fits the specs: 82-84 S10 2.0L (121 cid) 4 cylinder rod. It has a center to center distance of 5.320", journal size of 2.000", the same bearing width of 0.8420", the piston pin dia is 0.9055". So the S10 rod would work except for the smaller piston pin. You should be able to bore out the S10 rod end to 0.9805 (0.075 over) only if there is enough material at the little end.

  • There is the question of how strong a 121 cid rod is when used in a 427 cid engine. The S10 rod has been designed to be used in a engine where each cylinder sees about 30 cid, while the 427 sees just over 53 cid per cylinder - a 80% increase in cylinder size.

    Now for a street engine that is rarely raced or revved above 4000 rpm, this might not be an issue. The rod would have to be magnafluxed for cracks, all sharp edges ground down in the long direction and then shot peened.

    There has been much discussion about rod length and the rod to stroke relationship with longer being more ideal. This method uses a very short rod for a sbc (5.320" vs. a standard 5.7"). On the otherhand, small block Fords use rods that are even shorter at 5.100". Here's a website that discusses the mathematical advantage of stroke vs. rod length. Here's Dave William's site that lists a whole bunch of common engines versus their rod ratio and mucho interesting stuff...

  • There could be bdc clearance problems also. The 455 has a larger pin to top of piston distance so the bulk of the piston is actually farther away from the crank than a 400 piston. The stock 400 piston is 1.540" and the Olds is 1.720". I'm hoping that the 0.180 difference is going to help clearance problems at bdc instead of create them. Besides I always have a die grinder ;-)
Next step is to gather the parts and put this mess all together... This ain't gonna happen for awhile as I have so much else to work on.

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Copyright July 2011 Eugene Blanchard