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I have a 66 427/390hp driver. I am in the process of rebuilding a correct #, but not original engine. It is a 2 bolt main (3869942), and I plan to rebuild it close to the factory specs, but may up the cam and a few other items. I plan to use the dimple rods, roller cam and a steel crank. I have seen for sale several sets of 4 bolt main caps. How hard is it to convert the block to accept 4 bolt mains caps, What would this cost? Is this something a standard machine shop can handle? Is there that much benefit in strength? The original engine was rated at 390 hp with 2 bolt mains, I plan to add approx 500 rpm to the power curve and approx 50 more HP. Is this conversion something I should consider? Thanks - Joe
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Re: 427 2 bolt to 4 bolt conversion
Joe-----
The conversion is NOT necessary for your application. A 2 bolt big block will handle 500+ hp with no problem, at all, and you won't even be close to that by the sounds of it. However, the conversion is not all that difficult, either, and most automotive machine shops can handle it. It's simply a matter of drilling and tapping the block for the outer main cap bolts, machining the caps to register in the block, if necessary, and line boring the finished product. If a machine shop has the capabilities for line boring, then they'll be able to do all the rest with no problem. As far as cost goes for these operations only, I'd say you'd be talking in the $200, or so, range.
You don't need to purchase a set of used main caps, either. Very often, folks will want a king's ransom for these things. However, brand new ones are still available from GM. The rear cap is GM #14103156 and GM lists for $68.57/each. The other 4 caps are GM #14015334 and GM list for $22.31/each. From a competitive GM dealer you should be able to purchase the whole set for $130, or so. So, you sure as heck don't want to pay anywhere near that for a USED set. I've seen folks on eBay START THE BIDDING at $200 for a USED set. And, folks actually bid on them!
Also, keep in mind that the NEW caps are made with a little extra material so that they can be machined to register in the particular block they're going to be used with. Used caps have already been machined to register in the block THAT THEY WERE ORIGINALLY PART OF. There may or may not be enough material left to get them to properly register in your block. Usually, it's not a problem, but if it is then you've got yourself a set of interesting paperweights.In Appreciation of John Hinckley -
Re: 427 2 bolt to 4 bolt conversion
As mentioned, a 2-bolt block should contain 500 HP / 500 ft-lbs no problem. If you want even more confidence, machine your 2-bolt block for the new 30 degree angled 4-bolt caps. These caps will make a std 2-bolt block even stronger than GM's 4-bolt blocks, and everything hides within the factory oilpan for the purist.Comment
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Re: 427 2 bolt to 4 bolt conversion
Joe & Charles,
Thanks for the advice. I do want to maintain as stock appearance as possible, but I also like going with extra durability and performance when reasonable. The new caps sound like the way to go. That way if I decide to up the top end of the engine, the short block will accomodate. One follow up question. Since I have switched to a steel crank, do I need to a different harmonic balancer? From what I can tell the 390hp uses a 7" and the 425hp uses an 8"? Is that due to the rpm range or the steel vs. cast crank? Thanks - JoeComment
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Re: 427 2 bolt to 4 bolt conversion
Joe-----
Actually, all 1969 Corvette 427s originally used a forged crank; no cast cranks were originally used. There were some differences in the L-71 and L-36 cranks but it didn't relate to the material. The L-71 may have used Tuff-Trided journals and the L-36 not or it may have been a cross-drilling difference. I don't recall, at the moment.
The 7" versus 8" balancer use, in THIS case, primarily related to the peak RPM engine operation. You can install an 8" balancer with no problem.In Appreciation of John HinckleyComment
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Re: 427 2 bolt to 4 bolt conversion
Thanks Joe, One last question. Which would be correct for my 66 390hp, the 7" or the 8". JoeComment
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Re: 427 2 bolt to 4 bolt conversion
Joe-----
The 7" balancer is correct for the 66-69 L-36.In Appreciation of John HinckleyComment
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Re: 427 2 bolt to 4 bolt conversion
Joe,
Thanks for all the great info. I joined NCRS before buying my first Corvette this past summer. Knowing that NCRS, and this board, was out there gave me the confidence buy my corvette and to start this project. I greatly appreciate all support and info that this board provides. I could not do this with out NCRS. Thanks - JoeComment
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