68 BB 390hp - Timing Belt Cover Queston
Does anyone if what size timing belt cover is required for a 390hp BB. My understanding is 7" is correct for a 390hp while the 8" cover was used on the higher house power. Does anyone know if this is correct? Also if anyone has a picture of a correct cover for a 390 hp that could be posted would be appreciated. I would also like to understand what the timing tab looks like as well. Thanks - Jeff
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Re: 68 BB 390hp - Timing Belt Cover Queston
Are you talking about the harmonic damper or the timing chain cover? I'm not aware that there were different sizes of the timing chain cover.
However the damper consisted of different sizes depending upon the horsepower of the engine. The 390 HP engine had a 7 inch harmonic damper.
Mark -
Re: 68 BB 390hp - Timing Belt Cover Queston
Jeff-----
All 1966-69 L-36 (390 hp) and L-68 (400 hp) engines used a 7" harmonic balancer. They also used a timing CHAIN cover with a spot welded-on timing tab specifically designed and placed for use with a 7" balancer. I believe this tab was a rectangular-shaped tab which was unique to 7" balancer applications and which also included many 396, 402, and 427 engines used in other Chevrolet applications. All L-78 (425 hp and 375 hp), L-72 (425 hp and 450 hp), L-71 (435 hp) and L-88 (430 hp) engines used an 8" balancer and a timing cover with a different tab and located at a 1/2" greater radius from the crankshaft centerline. All 454 applications also used an 8" balancer.
Beginning sometime in the early 70's, GM ceased the exclusive use of the welded-on timing tabs for big blocks and phased-in a bolt-on piece. Not too long thereafter, the timing covers with the welded-on timing tabs were discontinued from SERVICE and replaced by a cover with NO welded-on tab and used in conjunction with a bolt-on timing tab.
Curiously, there was only ONE bolt-on timing tab ever cataloged by GM for any Corvette big block application when used with the SERVICE no-tab timing cover. That tab is GM #3992073. However, this tab is designed for use with an 8" balancer. While it will "kind of" work with a 7" balancer, it's just not right for it as it's located too far from the balancer ring.
So, why didn't GM ever have available a bolt-on pointer for a 7" balancer application? Well, actually, they DID. But, they never cataloged it for a Corvette application and its availability is not widely known. In fact, I'll bet that if a person went into a GM dealership parts department and asked for a bolt-on timing pointer for a 1966-69 Corvette with 427 and 7" balancer, they would be sold a GM #3992073. That's assuming, of course, that the parts counterman didn't just tell them the part was not available. Period. This response would likely occur in more than 50% of the cases and, for the rest, the person would be told the 3992073 was the only pointer available. But, they'd be DEAD WRONG. There was and is available a bolt-on pointer for 7" balancer big block engines. It's GM #354407 and it's pictured below. Like I say, I'll bet there's not more than a few GM dealerships in the country that would be able to come up with that part UNLESS you ask for that specific part number. But, now you know it!
I also want to make clear that the 354407 is, with respect to "originality", NOT a configurationally correct pointer for any Corvette big block application with 7" balancer; those applications all originally used a welded-on pointer. It's a SERVICE pointer designed for use with a timing cover with no welded-on pointer (like a GM# 3975941 or GM #330850). However, it's a pointer that's FUNCTIONALLY CORRECT for a Corvette with a 7" balancer and FAR BETTER than the 3992073 which is designed for use with an 8" balancer.In Appreciation of John HinckleyComment
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Re: 68 BB 390hp - Timing Belt Cover Queston
Joe,
What is the finish on that 354407? It kind of looks like it might be chrome plated -- Flash chrome or course
TerryComment
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Re: 68 BB 390hp - Timing Belt Cover Queston
Terry-----
The picture doesn't show the finish very well, at all. The finish is zinc plated. It's identical in finish to the 3992073 used for the 8" balancers.In Appreciation of John HinckleyComment
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Re: 68 BB 390hp - Timing Belt Cover Queston
Joe -- your pic MAY have answered a question I've had since I bought this low-horse '961' May 1965 cast block about 10 years ago. It came with the 7" balancer and this chrome timing cover and bolt-on timing tab. The tab sure shows like in your pic, unless my view is off. Sure looks like chrome to me. Would GM performance parts catalog be offering this sort of stuff in the '80s for that aftermarket ?
There is 5/8" clearance between the tab and the surface of the balancer, which, IMO, seems to indicate application for 8" balancers
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Re: 68 BB 390hp - Timing Belt Cover Queston
Wayne-----Joe -- your pic MAY have answered a question I've had since I bought this low-horse '961' May 1965 cast block about 10 years ago. It came with the 7" balancer and this chrome timing cover and bolt-on timing tab. The tab sure shows like in your pic, unless my view is off. Sure looks like chrome to me. Would GM performance parts catalog be offering this sort of stuff in the '80s for that aftermarket ?
There is 5/8" clearance between the tab and the surface of the balancer, which, IMO, seems to indicate application for 8" balancers.
What you have here is the chrome-plated version of the GM #3992073. It's currently available under GM #12342001, aka Delco #141-201. These part numbers go back about 15 years and there may have been other GM chromed versions of this part before then. Or, it could be an aftermarket-equivalent of the 12342001 which go back way before it was GM-available. In the "old days", I think some of the aftermarket companies like Mr. Gasket used to obtain the exact same stamping that GM used for the 3992073 and chrome plate them.
The GM #3992073 (for 8" balancers) and the GM #354407 (for 7" balancers) have always been zinc plated.
By the way, the first picture in your series perfectly illustrates why the 3992073 is not the right bolt-on tab for applications with 7" balancers.In Appreciation of John HinckleyComment
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