1969 Engine Block? Need Help!

[Joe L.]

Another Photo

Here's another of the casting date "I 27 8" (September 27, 1968):

[Joe L.]

Another Photo

Here's a photo of the casting number---3959512

[Joe L.]

Comments

Jonathan----

Well, this is a VERY strange situation; I've never seen anything like it. This engine was cast on September 27, 1968. However, the machining and assembly didn't occur until July, 1969. That's very unusual in itself.

The even more unusual thing is the block is the GM #3959512 casting. I've NEVER seen a verified case of this block being used in PRODUCTION. However, the case of this engine very much looks like that's what happened here.

Even more incredible is the fact that the 3959512 block was a small journal block. However, it may very well be that through machining operations it was converted to a large journal block; I cannot imagine that the factory would have installed a small journal block and 327 cid engine in a 1969 model year Corvette. Plus, the engine suffix code (assuming the last character is a "Z" which I cannot see in any of the photos) would indicate a 350 cid engine.

So, I would say that this is virtually a verified case of an anomaly that I've never seen before and, as far as I know, no one else has, either.

[Joe R.]

Re: Comments

Hi Joe:

I agree that this is all very interesting, and I hope that eventually we can figure out what happened here. For completeness, one thing I would like to see is a photo of the rest of the engine assembly code on the pad. Maybe Jonathon can send you a photo to post.

I have heard that sometimes replacement blocks were produced with either no stamping on the pad or with only the assembly code. Perhaps there are some scenarious that would involve that.

On the topic of the large vs. small journal situation, I have an observation and a question. I beleive that for 1967, the 3892657 block casting was machined for a small journal crank for the 327, but a large journal crank for the Camaro 350. In other words, the same raw casting could be machined for either set of crank journals. Perhaps the 3959512 block had the same characteristic, so that it could be machined for either the small journal or large journal crank. That might explain how this block could have been configured as a 350.

My question is this. Are the bolt spacings on the main caps of a large-journal block wider than the spacings on the mains caps of a small journal block? I suspect that the bolt spacings may have to be wider, since the additional .150" journal diameter would push the bearing opening too close to the bolts if they remained spaced for the original small journal design.

The reason I'm interested in this is a long story and I don't want to hijack this thread, so I won't try to explain here. The bottom line is that I'm trying to figure out how extensive the changes were when the 3892657 block was configured for large journals vs. small journals.

[John H.]

Re: Comments

Joe -

The bolt spacing on all five main caps is the same between the SJ and LJ blocks, but the rest of the cap machining and bearings are different.

[Joe R.]

Re: Comments

Thanks John. Do you think that this means that a small journal 3892657 block could be line bored for a large journal crank?

[Joe L.]

Re: Comments

Joe-----

It had to have been possible since the 3892657 casting was finished-machined for both size cranks.

[Joe R.]

Re: Comments

Hi Joe:

The thing that was bothering me was that the bolt spacing for the mains caps *could* have been wider when the block was machined for a large journal crank. This would make it impossible (or at least unwise) to line bore a finished small journal block to accept the large journal crank.

[Joe L.]

Re: Stamp pad pic

Jonathan-----

Your car's build date of early August, 1969 fits very nicely with the engine ASSEMBLY code of late July, 1969. Why a block CASTING that was manufactured nearly a year earlier was used is a mystery, for sure. However, I'll bet that there's a very interesting story behind this happenstance. I seriously doubt that it's totally unique to your car, either. I really don't think that the use of this engine and block had anything to do with the apparent "twist-and-turns" the order might have taken. I think it's just a case of this was the "HZ"-coded engine pulled out of the engine storage bay at St. Louis on the day your car was manufactured. How many others like it that might have been in there (or, in the engine storage bays at other GM plants) at the time, I suspect we'll never know, for sure.

There are a few pieces of information about this engine that I'm very interested in.

1) What is the configuration of the rear of the block in the area immediately adjacent to the distributor and slightly to the rear on the driver's side? There should be one of the following:

a) round cast boss facing upward with no machining;

b) round boss with a hole machined in the center and filled by a soft plug (i.e. like a "freeze" plug);

c) no boss, at all (VERY DOUBTFUL)

2) What is the configuration of the oil filter cavity machining? Most important, what is the installed oil filter adapter? It may have a casting number embossed on it near the base. Otherwise, could you photograph it? Obviously, you will only be able to do any of this when you remove the oil filter for replacement. However, when you do, you could easily remove the oil filter adapter for close inspection and photographing it. There are just 2 bolts that retain it and there should be no sealer or gaskets. Just remove the 2 bolts and out it comes. If you can see the casting number on the base of it, there would be no need to remove it, though.

[Dave P.]

Re: 3959512 Block

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[Joe L.]

Re: 3959512 Block

Dave-----

That's very good information. The description of the GM #3959512 block that you described is exactly how every other such block that I have knowledge of was configured. Also, the coding on the engine you described would indicate a SERVICE engine apparently cast in August, 1971. It was almost certainly a SERVICE engine manufactured at that time for some 1967 or earlier application. In 1971 GM was definitely still producing SERVICE engines of the 1967 and earlier configuration for SERVICE only. This 3959512 block is the one that was used for most of these engines manufactured after 1968. I do not know why GM went to this block casting and did not just use the earlier castings. However, and based upon the "revelations" provided by the engine that's the subject of this thread, it may be that the 3959512 was a "dual-use" casting that was designed to be machined into either pre-67 configuration or 1968+ configuration.

1969's did not use a road draft tube or crankcase ventilation system that otherwise required the orifice on the rear of the block. In fact, 1967 was the last year for that. 1968 and later blocks usually don't have provisions for that style crankcase ventilation system, although some may have "vestiges" of the earlier configuration.

In the case of the engine which is the subject of this thread, it certainly appears that this is a 3959512 that was built to 1969 350 cid specs.

[John H.]

Re: Comments

Joe -

I have annotated photos of LJ and SJ main bearing caps and bearings held side-by-side with the bolt hole centers aligned for comparison that show the machining differences; I can e-mail them to you if you like.

[Joe R.]

Re: Comments

Hi John:

I would be very interested in seeing the photos.