Oil pan bolt mystery

[Joe R.]

I got the my 427 / 425 block back from being rebuilt recently. The service manual says there should be three 1/4-20 bolts holding the oil pan to the timing cover. There are only two bolts installed there now. The one in the middle is missing. The oil pan has a hole for it. I can't tell if the timing cover has a tapped hole there because the gasket is covering it up. I got a bolt set from Corvette Central. It only has two 1/4-20 bolts in the set. In the book "How to Rebuild Big Block Chevy Engines" it says two conflicting things. It says to torque the 23 oil pan bolts (10 on each side, 3 for the timing cover). Later it says there should be two or three 1/4-20 pan to timing cover bolts.

Do I have the right oil pan? Should I pull the oil pan to see if the timing cover is tapped in the bottom center? Or is it OK the way it is?

Thanks,
Joe

[Cecil L.]

Re: Oil pan bolt mystery

Joe,
There was a service bulletin I ran across not too long ago that called out a change to the front pan to timing cover gasket on 396-427 engines. I believe it was in late 65 or early 66. As I recall it eliminated the center bolt and a rubber tit filled the center hole in the timing cover.

[Michael H.]

Re: Oil pan bolt mystery

Joe,
There was a service bulletin I ran across not too long ago that called out a change to the front pan to timing cover gasket on 396-427 engines. I believe it was in late 65 or early 66. As I recall it eliminated the center bolt and a rubber tit filled the center hole in the timing cover.

I may be wrong on this but I thought the center bolt continued through at least part/most of the 66 model year??

I remember the TSB that described the elimination of the bolt but I thought the date was later.

By the way, that center bolt was quite unique. It had a rounded attached/loose washer that conformed to the curved surface of the timing chain cover.

[Joel F.]

Re: Oil pan bolt mystery

Joe,

To be absolutely correct, I think your timing cover should have a weld-nut there to accept the bolt, regardless of whether a bolt was present or not. My 67 timing cover has the weld-nut, but by this time the bolt was no longer used. If it turns out no bolt should be present for 66, I do not think you'd be able to see if the weld-nut were present once the balancer is installed.

I do recall seeing this topic covered on the board at some point so I'd check the archives as well. Good luck and let us know what you find.

Joel

[Joe R.]

Re: Oil pan bolt mystery

After doing quite a bit of poking around, here's what seems to be the best info from Wayne Midkiff in a thread titled "C2 Big Block Oil Pan" posted August 23, 2008:

"here's a Chev News (May'67) item that indicates the thru-hole bolt was eliminated in PRODUCTION in Dec of 1966."


My car is from May of 66. It seems reasonable that the center hole in the pan should be there, but not the bolt. So, I don't need to do anything.

Thanks,
Joe

[Michael H.]

Re: Oil pan bolt mystery

After doing quite a bit of poking around, here's what seems to be the best info from Wayne Midkiff in a thread titled "C2 Big Block Oil Pan" posted August 23, 2008:

"here's a Chev News (May'67) item that indicates the thru-hole bolt was eliminated in PRODUCTION in Dec of 1966."


My car is from May of 66. It seems reasonable that the center hole in the pan should be there, but not the bolt. So, I don't need to do anything.

Thanks,
Joe

That doesn't necessarily mean that the bolt wasn't used through the end of 66 production in fall of 1966. From the TSB, all we know is that the hole was eliminated early in the 67 model year.

[Joe R.]

Re: Oil pan bolt mystery

Michael,

You're right, I was not thinking about it correctly. For '66 there should be a bolt there. I did a little looking, I was not able to find the special center bolt you mentioned in a previous post. Do you know of any sources? Paragon suggests ordering another bolt like the other two. That would get me close but not quite there.

Thanks,
Joe

[Michael H.]

Re: Oil pan bolt mystery

Michael,

You're right, I was not thinking about it correctly. For '66 there should be a bolt there. I did a little looking, I was not able to find the special center bolt you mentioned in a previous post. Do you know of any sources? Paragon suggests ordering another bolt like the other two. That would get me close but not quite there.

Thanks,
Joe

This is what they look like. I'm not sure on the head mark but I assume there were probably a few different vendor sources.
The washer is conical.
I suppose a similar bolt with c/sunk head could be found and, if necessary, a flat washer could be slightly formed into a conical shape. (even though that shape isn't physically correct for the application)

[Wayne M.]

Re: Oil pan bolt mystery

My car is from May of 66. It seems reasonable that the center hole in the pan should be there, but not the bolt. So, I don't need to do anything.
Joe

Joe -- I've re-read that Chev Service News announcement and I agree with Michael H. and your re-assesment, that your '66 should have the hole in the pan front seal flange AND the bolt (# 3856691, Screw, w/lock washer 1/4-20 x 5/8, front center hole). Change of front seal design didn't occur in production until 21 Dec 1966 (approx. '67 VIN 06500).

Now what I would like to know, is when (if) the front captured nut in the timing cover was changed to a hole only. From the Service News, it sounds like four changes were made simultaneously on 21 Dec. (new seal design; center bolt hole deleted (pan side), center captured nut in timing cover deleted, and screw eliminated). Chances of that happening in actual fact are slim.

I have a low-horse BB timing cover (no dowel holes) with NO front seal nut, and I also have a proper big block oil pan, except it has no hole in front seal area. I'm trying to figure out the posssibe dates around these items.

I had asked (back in my August '08 post) what the judging guides had to say -- 1967 and 1968, but got no response.

[Terry M.]

Re: Oil pan bolt mystery

Wayne the 1968-69 TIM&JG says nothing about the timing cover to oil pan fasteners for 427 engines.

I likely missed your earlier post because I tend not to look at most posts where the title indicates mid-year subject matter. I looked at this one because I recall attending an Illinois tech session many years ago with three 425hp cars all of which we put on a lift, and examined this center bolt. All I remember for sure is that they were all different. Sorry that doesn't help much. I was hoping to find the answer here -- it has bugged me for years.

[Joe R.]

Re: Oil pan bolt mystery

Wayne,

Thanks for the info and the part number. I was able to track the bolt down using the GM part number. Did you get the part number from the AIM?

Joe

[Wayne M.]

Re: Oil pan bolt mystery

Did you get the part number from the AIM? Joe

Joe -- got the # from my Chevrolet chassis parts books (P&A30 series). First issue of these for a model year is usually October of the previous year, to coincide with new cars requiring service. The 1967 issue (Oct '66) shows it for '65 thru '67 (as they didn't know that it would be eliminated on Dec 21st). The 1968 (Oct '67 book) shows it for 1966 Corvettes only [no '65 396, probably an omission], as for service the new gasket 3909800 was in effect for all previous BB's. You could argue that they should have eliminated the screw entirely by then, unless you were replacing screw only, without dropping the pan.

This info wouldn't appear in the AIM, as the complete BB engine assembly arrived at St.Louis from Tonawanda. Don't know whether the engine plant would have an equivalent AIM, since the directions were quite basic. Maybe Terry McM. knows.

[Joe L.]

Re: Oil pan bolt mystery

Wayne,

Thanks for the info and the part number. I was able to track the bolt down using the GM part number. Did you get the part number from the AIM?

Joe

Joe-----


The GM #3856691 screw is a 1/4-20 X 5/8" hex head screw with conical washer. It was available in SERVICE until March, 1988 when it was replaced by GM #9439771. The latter is still available from GM. Is it exactly the same as the 3856691? I doubt it. However, it is a 1/4-20 X 5/8 hex head screw with a conical washer. I expect the hex head configuration may be slightly different and, almost certainly, the headmarking will not be typical of those used during the 65-66 period. Later SERVICE examples of the 3856991 probably weren't, either.

[Terry M.]

Re: Oil pan bolt mystery

The only Tonawanda engine assembly manual I have seen is for 1971 -- so it won't help with this discussion.

[Michael H.]

Re: Oil pan bolt mystery

Joe-----


The GM #3856691 screw is a 1/4-20 X 5/8" hex head screw with conical washer. It was available in SERVICE until March, 1988 when it was replaced by GM #9439771. The latter is still available from GM. Is it exactly the same as the 3856691? I doubt it. However, it is a 1/4-20 X 5/8 hex head screw with a conical washer. I expect the hex head configuration may be slightly different and, almost certainly, the headmarking will not be typical of those used during the 65-66 period. Later SERVICE examples of the 3856991 probably weren't, either.

The pic that I posted earlier is of NOS 3856691 bolts that came from GM in the mid 70's. I don't know if they're exactly the same as originals from 65-66 though.