Guys,

I found ONE ear broken on the male end of my oil pump drive shaft (the end that goes into the distributor) and I need to figure out what caused it. The drive shaft was still functioning on the other ear so no damage.

Anyone seen an issue with the SBC 400 oil pump drive? With the larger 400 main journals the hole that goes through the block and rear main cap is smaller then the 350 blocks. The directions that came with the new Moroso HD drive shaft says to enlarge the hole to 9/16" (currently its a hair over 1/2") My machinist says he has never had a problem and just checks the clearance. He must be good at checking clearance because I can't seem to get anything down there to get a warm fuzzy about the clearance. When I spin the motor over with the distributor and oil pump installed the new drive shaft does not rub.

Also, I run an oil baffle between the rear main cap and the oil pump to keep oil from sloshing up the back of the oil pan and block on launch. The baffle makes the drive shaft act a hair shorter . . . could this contribute to or cause the shaft to break?

The shaft was the one that came with my Melling pump. Perhaps Melling doesn't supply a very good drive shaft?

Any ideas?

Thanks

I like the ARP version that has a necked down shaft for clearance on the 400's. Used them in three different 400's ands never had an issue.

Jody

Eric,
I have always used the Melling steel shaft that is necked down to clear. Have never had a problem with the shaft breaking in any way. I think the part number is IS55E, but I would have to check in the AM to be sure.

Thanks guys. I think I will get a different shaft besides the Moroso then. ARP sounds good -- not sure I like the idea of using another Melling.

Do you guys think that maybe the baffle between the cap and pump could contribute? Maybe not enough engagement between the pump shaft and dizzy?

Like Jody, I use the ARP shaft. No problems so far, but it's kinda hard to inspect.

I too use the baffle with no problems. You may want to check with your distributor manufacturer for their recommendation for minimum distributor engagement. The longer the the lever arm (less engagement), the greater the bending moment applied. Can you tell by inspecting the distributor how much of the shaft was in contact?

I use the ARP shaft and the baffle, no issues whatsoever. The amount the baffle is thick (about 1/16) couldn't possibly have caused it, unless for some other reason you were right on the razors edge with another problem, but otherwise I'm just not seeing it. I have never heard of anything like that, and between my friends and I, there have been quite a few 406's put to the test.
How did the distributer gear side look? did it appear to have unusual wear? I think I'd write it off to a failed part, and put it back together with a close eye on detail. Not to mention being thankful you found it before it showed itself :eek: , that had to be a serious "pucker" moment, when you found it.

I had one shear off in my 350 about 10 years ago, started on cold morning and when cold she has about 100 psi oil preasure. Switched to ARP never had an issue again.

you dont happen to have a distributor with an adjustable slip collar do you? if the collar happened to slip or be in the wrong position and force the distributor to ride too high in the block??? this "could" cause the bottom of the distributor to not be "engaged "enough into the oil pump drive shaft, which could have caused the breakage. probly a long shot, but it was thought that popped into my head. good luck finding the problem.

OK, thanks guys. I took the Moroso shaft back and got an ARP shaft instead. The ARP has plenty of clearance -- a lot more then the moroso.

At this point I am chalking it up to a bad part.

Update,

If any of you guys run a Melling pump shaft you might want to check yours and ditch that shaft next time you have your engine apart.

I had a friend of mine (knowledgeable about metal fatigue, etc) look at the failed drive shaft and he said there was a stress riser that most likely caused the break.

The way the slot was cut in the male end (distributor end) of the shaft left the bottom square instead of rounded and there was a spot where there was clearly a stress riser on both sides. Too small to take a picture of or I would post one.