Transmission to bell housing bolts -C2 with M20

[Grant M.]

Hello,

Looking for information on the bolts that secure the front of the M20 transmission to the bell housing (1965 L75 car). The AIM lists two distinct part numbers for the upper two bolts depicted in the drawing but does not show numbers for the lower two bolts. I didn't dis-assemble the car, but I have the four bolts: three of the same length and one about 1/4 inch shorter, all with lock washers. What I need to know is which hole the shorter bolt goes into and, if known, why is there a difference? I tried looking at what the suppliers (Ecklers/Corvette Central) offer as replacements, but theirs are all the same length (similar to the alternator bracket to exhaust manifold bolts they offer--two of the same length, whereas the originals were different length).

Can anyone help, please?

Thanks,

Grant

[Keith B.]

Re: Transmission to bell housing bolts -C2 with M20

try AMK for bolts

[Grant M.]

Re: Transmission to bell housing bolts -C2 with M20

Thanks Keith, but I'm not looking for the bolts, (i have them), just information on which size (short/long) goes in which hole on the transmission.

Grant

[Mike B.]

Re: Transmission to bell housing bolts -C2 with M20

Hi Grant,

For my '65 and '66 the bolts that attach the transmission to the bellhousing are all the same. They may show different head marks but the dimensions are the same. As per the measurements found in GM Engineering Standards all four of these hexagon bolts #100161 should be:
1/2" - 13 x 1 3/4" plain steel, grade 2, no lines on head, just the head mark. Typical head marks for midyears are "WB", "TR", "A" etc.

The 1/2" inch heavy duty helical spring lock washers #103329 are also listed as plain steel. These bolts and washers are often found plated. Usually cad or zinc. But on occasion I've seen them black phosphate/oxide. Here's a '66 AIM page showing these part numbers. I'm also attaching a nice shot of my buddy's '67 "TR" bolt example. The repros of the "TR" headmark are inferior.

'66 AIM Transmission to Bellhousing Bolts.jpgDavid's TR Transmission Mount Bolt.JPG

There was a recent thread that dealt with 1/2"-13 thread bolts that was quite good also. I tried to create a link to it but was unsuccessful. Here's the thread title. You'll find it on about page 8 of this Technical Discussion Forum. Enjoy, Mike

David L. Bolt Thread.jpg

[Joe L.]

Re: Transmission to bell housing bolts -C2 with M20

Hi Grant,

For my '65 and '66 the bolts that attach the transmission to the bellhousing are all the same. They may show different head marks but the dimensions are the same. As per the measurements found in GM Engineering Standards all four of these hexagon bolts #100161 should be:
1/2" - 13 x 1 3/4" plain steel, grade 2, no lines on head, just the head mark. Typical head marks for midyears are "WB", "TR", "A" etc.

The 1/2" inch heavy duty helical spring lock washers #103329 are also listed as plain steel. These bolts and washers are often found plated. Usually cad or zinc. But on occasion I've seen them black phosphate/oxide. Here's a '66 AIM page showing these part numbers. I'm also attaching a nice shot of my buddy's '67 "TR" bolt example. The repros of the "TR" headmark are inferior.

[ATTACH=CONFIG]101584[/ATTACH][ATTACH=CONFIG]101585[/ATTACH]

There was a recent thread that dealt with 1/2"-13 thread bolts that was quite good also. I tried to create a link to it but was unsuccessful. Here's the thread title. You'll find it on about page 8 of this Technical Discussion Forum. Enjoy, Mike

[ATTACH=CONFIG]101583[/ATTACH]

Mike-----


1966+ Corvettes with manual transmission did use the 1-3/4" GM #100161 for all 4 bolts securing the transmission to the bellhousing. However, 1964-65 Corvettes used a different bolt, GM #106342, at the lower left position with the 100161 for all other positions. I have no idea of the reason for a different bolt at the lower left position, especially since the 100161 was used for all 4 bolt positions after 1965..

[Mike B.]

Re: Transmission to bell housing bolts -C2 with M20

Hi Joe,

Thanks for this information. Unfortunately I'm on business and don't have a copy of my '65 AIM with me. I do have a complete '66 AIM and my GM Engineering Standards with me now.

When I was researching last month's thread between David Liukkonen and you about 1/2"-13 bolts I'm sure I compared my '65 AIM UPC page 7 sheet B3 shown in my previous post here. I'm almost positive I spotted GM part #100161 only in my '65 AIM. I must've overlooked the reference you and Grant are referring to here?

And I'm also basing my references here due to my experience with my own '65 Corvette. I'm the 2nd owner of the car and no prior work was ever performed on the M21. When I removed the unit it had a mix of "TR" and "WB" #100161 bolts. I'm quite sure all four bolts were 1 3/4" inches long.

This is very interesting new information to me and I would really like references showing this. So this is what Grant was talking about in his opening post to this thread? Grant, would you be so kind as to show a pic of what you based your statements on? Thanks, Mike

[Don H.]

Re: Transmission to bell housing bolts -C2 with M20

Both the 64 and 65 AIM show the bolt in lower left as Joe L describes. GM #106342


64 AIM pg 157.jpg. 65AIM pg 161.jpg

[Mike B.]

Re: Transmission to bell housing bolts -C2 with M20

Thanks Don!

I think I figured out my problem. I've got a VERY poor copy of a '65 Mid America AIM. You posted your pages from Graphic Co. AIMs. Don't think my AIM shows this. Now I need to find a copy of a '65 Graphic Co. AIM! Mike

[Mike B.]

Re: Transmission to bell housing bolts -C2 with M20

Grant,

I just looked up the GM dimensions of this shorter #106342 bolt. My reference shows it as grade 2, plain steel, 1/2" - 13 x 1 3/8" inches long as measured from under the head to the tip. Curious to find out what your shorter bolt measures? Would you also let us know what your head marks read?

Thanks,
Mike

[Grant M.]

Re: Transmission to bell housing bolts -C2 with M20

Don, Mike, Joe and Keith,

Sorry to be long in getting back; I've been off-line for a bit. My shorter bolt measures 1 3/8 inches and has a headmark of "P". My other three bolts are the longer, 1 3/4 inch bolts and they have an "A" headmark.

Joe, you've answered my main question and I guess we'll have to wait and see if anyone has more insight on the 'why' for the shorter bolt (and its specific position.

Thanks all for your input and responses. I have been off the Board for a few years and some of the names are new, but the terrific help and support is as it's always been! From Joe's "in Appreciation" line in his posts, I sadly conclude that John Hinckley is no longer with us. I had numerous exchanges with him and remember him as one of the most giving and helpful people on this Board. He even drew on his career experience as plant manager at Lordstown in the late 60s to help me with a VIN issue on a Firebird we were importing. RIP John...

Grant

[Joe L.]

Re: Transmission to bell housing bolts -C2 with M20

Don, Mike, Joe and Keith,

Sorry to be long in getting back; I've been off-line for a bit. My shorter bolt measures 1 3/8 inches and has a headmark of "P". My other three bolts are the longer, 1 3/4 inch bolts and they have an "A" headmark.

Joe, you've answered my main question and I guess we'll have to wait and see if anyone has more insight on the 'why' for the shorter bolt (and its specific position.

Thanks all for your input and responses. I have been off the Board for a few years and some of the names are new, but the terrific help and support is as it's always been! From Joe's "in Appreciation" line in his posts, I sadly conclude that John Hinckley is no longer with us. I had numerous exchanges with him and remember him as one of the most giving and helpful people on this Board. He even drew on his career experience as plant manager at Lordstown in the late 60s to help me with a VIN issue on a Firebird we were importing. RIP John...

Grant

Grant------


No, thankfully and happily, John is still around. He had some health challenges but he's overcome them. My "in appreciation" footnote is just what it says----in appreciation for his contributions to this DB and the Corvette hobby.

By the way, I have little doubt that your 1-3/8" bolt is for the lower left position.