427 blocks 4 or 2 bolt mains

[Joe L.]

Tom-----

A subtle point, but I believe that 400 hp L-68 engines were generally supplied with 4 bolt main blocks. 390 hp L-36s were all 2 bolt, though.

[Tom B.]

Re: 427 blocks 4 or 2 bolt mains

Joe, I'll most definitely take your word for it. I was only under the impression that the differences in the the 390hp L36 and the 400hp L68 were the intake configurations. TBarr #24014

[Tom B.]

Re: On second thought.......

Joe,

On second thought, are you sure about that? I don't want to have to drop the oil pan on my 67 L68 just to look. I've got someone interested in the car that's wanting me to sell it. :-) TBarr #24014

[Wayne M.]

Re: 427 blocks 4 or 2 bolt mains

Joe, Tom --- What Tri-Power model year are we talking about ? GM was so cost-conscious that I find it hard to believe they would supply 4-bolt mains for 400 hp L68 versions as a rule, rather than the exception (due to Tonawanda production errors, etc).

Assuming that the machining of the unique bosses and taps above the oil filter goes with the machining for the type of main cap, you only have to lift the hood and peek; no need to drop the pan.

[Tom B.]

Re: 427 blocks 4 or 2 bolt mains

Wayne, and Joe,

I hadn't previously known the way to identify the 2 bolt or 4 bolt block by the bosses on the left side, but I had some confusion when I looked at mine. Someone had previously plugged one of the bosses near the left motor mount with a brass hex head bolt leading me to believe something was installed there at one time. Since that's nowhere near the oil filter, I'm not finding a drilled and tapped boss there except for the oil pressure gauge line.

The other information I thought was clear to me is from Nolan's book about the 67 L68 that states other than the 3 x 2 Holley intake and manifold, the L36 and L68 are identical. It further mentions that the window stickers for the 67 L68 had an addition at the bottom reading "converted 390 to 400 hp". TBarr #24014

[Joe L.]

Re: 427 blocks 4 or 2 bolt mains

Tom and Wayne-----

I'm not certain, at all, that 1967 L-68s used the 4 bolt main configuration, but I am fairly sure that all or most 68-69 L-68s did. This fact "stuck in my mind" from some reading that I did some time ago. I recall thinking that it was contrary to what I had previously believed, too. But, my general recollection is that the source of the information was a credible one.

[Wayne M.]

Re: 427 L68 blocks; 4 or 2 bolt mains

Joe, Tom -- Tom, the best side-by-side pictures I've ever seen that compare the 2 and 4-bolt external drillings of the bosses above the oil filter are found in Colvin's '65-'69 book (page 56).

Now Colvin shows all 3 years of 400 hp L68's as having 4-bolt mains, but here's a bit I dug out of my literature file that may show otherwise.

GM 3-page fold-out comparing '67 427 Corvettes, Camaro SS 350's, GS-400's, 4-4-2's and GTO's: All 3 (427) engines have extra-thick bulkheads above each crankshaft bearing and wide-base caps. Four bolts secure each main bearing cap in 435 hp engine, two bolts do the job in 400 and 390 hp editions.

GM '68 Corvette showroom brochure: Rugged alloy steel crankshaft with five main bearings in all engines (specially hardened journals and special main bearing caps with four bolt attachment per cap in 435 hp version).

Now that's what GM "says" they did; Survey says...??? Ladies and gentlemen, pop your hoods.

[Joe L.]

Re: 427 L68 blocks; 4 or 2 bolt mains

Wayne----

A few comments: first, notwithstanding what Alan Colvin says, I believe that 67 L-68s probably didn't have the 4 bolt mains. I think that this change was made either for 68 and/or 69. Also, with respect to what the brochures say, this is primarily a product of MARKETING folks. For technical specs like this, they may not have had the correct information at the time of publication. Or, they may have chosen to "understate" the advertising claim if there was some uncertainty as to the actual PRODUCTION spec. That way, they couldn't be subject to claims of "false advertising"(i.e. someone who is suppossed to have a 2 bolt block is not going to complain if they find out that they have a 4 bolt block, but someone who has been told that their engine is suppossed to have a 4 bolt block might well complain, and demand replacement, if they find out that they actually have a 2 bolt block). Advertising folks tend to be very conservative about making technical and REFUTABLE product claims, but very liberal about making GENERALIZED(and fundamentally irrefutable) product superiority claims.

[Joe L.]

Re: 427 blocks 4 or 2 bolt mains

Roger and Jerry-----

Yes, and if you use 4, each of GM #14015334 4 bolt main bearing caps and 1, each of GM #14103156 rear main bearing cap when the conversion is made, the result will be even STRONGER than the PRODUCTION 65-71 4 bolt main bearing caps. And, to carry it one step further, if you use aftermarket BILLET STEEL 4 bolt main bearing caps, the bottom end will be stronger yet.

[Tom B.]

Re: 427 L68 blocks; 4 or 2 bolt mains

Joe,

I can vouch for at least one of the 67 L68's. Just to satisfy my own curiousity I went ahead and dropped the oil pan and found the engine to have 2 bolt mains. In case anyone is wondering, it is the non-rebuilt original engine. TBarr #24014

[Wayne M.]

Re: 427 L68 blocks; 4 or 2 bolt mains

Joe -- It's quite possible that marketing was covering their rear in '68 showroom brochure, (by saying only 435hp's had 4-bolt mains, when [maybe] the plant used 4-bolts also for 400 hp L68's). Since the '68 sales brochures are probably printed about July of '67, and as most '67 L68's had 2-bolts (Tom Barr's does), maybe GM's intention then was to continue the practice, but changed at the start of '68 model year production.

Potentially tied into this is the tach redline thread. Both '67 L36 and L68 motors redlined at 6000 rpm, but the limit was reduced for the 427 hydraulic motors in '68 and beyond. As 1967 was the introduction of the 5-year or 50,000 mile engine warranty, maybe there were too many failures at 6000 rpm, so it was lowered for future years.

Still, it would be interesting if others with L68's of all 3 tripower years would check and report on the configuration of the bosses above the filter.

[Jerry Clark]

Re: 427 L68 blocks; 4 or 2 bolt mains

In the for what it's worth department. When I rebuilt my 69, 400 hp, O31 built car it had two bolt main block, original engine, I was surprised by this, as all my previous 427s were of the 435 variety.

jerry

[Joe Peplinski]

Re: L68 Blocks; 4 vs. 2 Bolt External Differences

I don't have the Colvin book, but I do have a '69 L68. Can someone tell me what the drilled/tapped hole differences should be above the oil filter for a 2-bolt vs. 4-bolt main block? I don't know what to look for, but my original block has two drilled and tapped holes near the oil pressure line fitting. The one forward and above the oil line fitting appears to be about 7/16" in diameter, while the one to the rear and below the oil line fitting appears to be about 1/4' in diameter. Both tapped holes are open - No plugs are installed. What does this mean? Thanks.

Joe

[Wayne M.]

[PAT68]

Re: L68 Blocks; 4 vs. 2 Bolt External Differences

My 1968 L68 Corvette has a 2-bolt configuration.

PAT68