I am completing the rebuilt of a 396. The cast Balancer pulley I assume is correct (#3863108) as all the books indicate. The water pump pulley calls for a 7 and 3/16 diameter pulley. When installing the water pump pulley it hits the Balancer pulley overlapping approx. 3/16 of an inch. I have the correct water pump #3856284. Any suggestions or insights would be appreciated.
-
-
Re: balancer pulley tolerance
David----
You have the correct pullies. The GM #3863108 is the correct balancer pulley. The original waterpump pulley was GM #3864480 which was later replaced by GM #3906656. Both pullies are 7-3/16" OD, although the "hat" sections are differently configured. Regardless of which pulley that you are using, there should be no problems created.
On a 1965-67 Corvette SHP big block, the waterpump and crank pullies will, normally, be quite close together. Quantifying this a bit, for a big block the distance between centerlines of the waterpump shaft and crank is 7.04". The RADIUS of the above-referenced waterpump pulley is 3.59" and the RADIUS of the crank pulley is 3.38". This totals 6.97". So, the expected clearance between the crank and waterpump pullies would be only 0.07" or about 1/16". The fact that you have a 3/16" overlap would seem to indicate that things are about 1/4" "off" somewhere. That's a lot.
I'm sort of baffled by this whole thing. I can't imagine that the balancer tappings for the pulley bolts could be that far off, but I suppose it's possible. If so, that would create other problems besides the pulley fitment problem. And, if the tappings are that far off, the balancer is defective. If the balancer is new, then that's a plausible explanation. If it's used (and, presumably, worked ok before), then that's not a plausible explanation.
The only other thing that I can think of here as a problem source that MIGHT create this sort of problem is a heavily line-bored engine. Line-boring changes the crank centerline and usually slightly reduces it. The amount you're off seems like more than I would expect a line-boring could cause. However, if the actual overlap is less than your estimate and considering the normal clearance between the pullies of about 1/16", I'd say it's possible, albeit still unlikely.
If there is a crank centerline problem created by line-boring, then you would have had to use a special reduced-length timing chain (or, you would have ended up with a "loose" timing chain). Did either of these things occur? If so, that's an important "clue" here.
In any event, if the problem is caused by a centerline problem caused by excessive line boring, I don't know what you could do to solve it. The only thing that I might suggest would be having the crank pulley perimeter slightly machined down. This shouldn't cause any problem other than, possibly, disturbing the pulley balance. I wouldn't do it, though, until I'd fully checked out all of the other possibilities (such as a defective balancer, etc.). Also, if there is, indeed, a full 3/16" of "overlap" between the pullies, then you'd have to have a 1/4" machined off the radius of the pulley and that's WAY more than I'd feel comfortable with.In Appreciation of John Hinckley -
Re: balancer pulley tolerance
If an engine was line bored that much, there might be some problems with piston to head or valve to piston clearance.
jonComment
-
Re: balancer pulley tolerance
Thanks for the information first off.
The only conclusion I can really come up with from the past (when the engine was running) to now is that I changed from what was on the engine a 338100 water pump to the correct 3856284 water pump. I have messured evry thing and the center line of the pulley to the center line of the bolts seams the same.
Thanks agai i will check back later. I have to figure this one out *groan*Comment
-
Re: balancer pulley tolerance
David-----
The waterpump casting that you're referring to must be 386100; there was no big block pump of 338100. The 386100 was used from 1973-1974 on Corvettes. The centerline on this pump is EXACTLY the same as the 3856284 and they can be used interchangeably. Although a 338100 pump casting can be used by rebuilders to build up a perfectly functional waterpump for any 65-70 Corvette, the original bearing and shaft used for this waterpump used a 3/4" pilot. That used for all 1965-70 Corvettes had a 5/8" pilot. I'm assuming that the pump that you took off was a 386100 casting that had been rebuilt to 65-70 specs with a 5/8" pilot shaft. If this is a case of someone using a pump with a 3/4" pilot and then enlarging the pilot hole in the pulley to accomodate that, then that could be your problem. When you go back to a 5/8" pilot waterpump, you would then have too large a pilot hole to properly center the pulley. You absolutely CANNOT rely on the 4 mounting bolts to center the pulley; it just doesn't work that way.
If the pilot hole in your pulley has been enlarged, then the only way that you can use it is to install a PRECISION 5/8"-to-3/4" bushing on the 5/8" pilot shaft of the '284' pump. Even then, it's risky because you don't know how "precision" of a "ream job" was done on the pulley.
If your pulley has a 3/4" pilot hole, then I recommend getting another pulley with the correct 5/8" pilot hole. These are hard to come by, but reproductions are available, although costly.In Appreciation of John HinckleyComment
Comment