1966 Big Block Valve Cover Bolts for a December '65 built L-36. Need your help.

[Mike B.]

Good Evening Guys,

Sure hope everyone had a great Thanksgiving! I need the help of '66 experts out there. I'm selecting which valve cover bolts to use on my 1966 L-36 December 1st, '65 built car. VIN #72XX. I scoured the archives for the proper type to install, but I couldn't pin an answer down. Here's my question:

When did the factory begin using the flat hex head bolts vs. the recessed hex head type? I confess that I'm still using my '65 and '66 Judging Manuals from the mid-nineties. Those books only reference the recessed head type with "RBW" or "Clover" head marks. The aftermarket vendors sell "65-'66 Big Block "RBW" 1/4-20 x 1/2" recessed head bolt sets with the 3 radial lines. I bought a set and they're nice. But recently I've decided to go the extra mile and use real bolts from my collection.

Should I be using the recessed hex head or the flat hex head type valve cover bolts for where my car lays within the production year?

And then there's the question of which head mark to use? I've heard from various sources that for the recessed head type to use "RBW", "E", or "Clover". And for the flat hex head to use "TR". I've also heard to use the small "TR" variety?

I sure could use some help here! Especially from anyone with a '66 L-72 or L-36. Could you please tell me what bolt types came with your car and what month the car was built? I'm including some shots of some of the bolts I've collected through the years. Thanks a lot for any help you all can provide. Mike

[Edward B.]

Re: 1966 Big Block Valve Cover Bolts for a December '65 built L-36. Need your help.

Use one of each to engender conversation among the "experts."

[Michael M.]

Re: 1966 Big Block Valve Cover Bolts for a December '65 built L-36. Need your help.

Mike, 7/16" recessed hex-head bolts that are painted engine color with triangular reinforcement plates. Known head marks are " RBW " or a three-leaf clover which you both show in your post.

[Mike B.]

Re: 1966 Big Block Valve Cover Bolts for a December '65 built L-36. Need your help.

Hi Michael,

Thanks for your response. That's what my old '66 Judging Manual says. Both "RBW" and "Three Leaf Clover". I've never seen a "Three Leaf Clover" bolt before. Anybody care to post a shot of one? Or was this a mistake that's been corrected in the newer revisions? As you can see, I show the bold "Four Leaf Clover" and the small "Four Leaf Clover with raised dot" varieties in my photos. Also, I show an "RBW" without the 3 radial lines and one with the lines. The repros have the 3 lines. I wonder if the factory mixed the 2 types of "RBW" bolts?

I'm very interested to know when the factory switched from recessed to flat hex head bolts. I've got two NCRS buddies that have '66 big block cars and they both have the small "TR" flat hex head bolts on their cars. Does the new(er) '66 Judging Manual state this head mark now?

In case anyone is curious, the bolt measurements that I've posted here are:

Recessed hex: 1/4 - 20 x 1/2" inch length. All "RBW, "E", and "Clover" bolts shown have a 3/8" inch head.
Flat head hex: 1/4 - 20 x 9/16" inch length. All "TR" bolts shown have a 7/16" inch head.

Mike

[Mark L.]

Re: 1966 Big Block Valve Cover Bolts for a December '65 built L-36. Need your help.

Mike, do yourself a big favor and purchase the new 66 Judging Manual. It is much more comprehensive than earlier versions.

[Mike B.]

Re: 1966 Big Block Valve Cover Bolts for a December '65 built L-36. Need your help.

Here's a shot of some other possible applications for the later 7/16" flat hex head 1/4-20 x 9/16 length bolts. The earlier recessed hex heads are 3/8" with 1/4-20 x 1/2" length bolts. I spoke to a buddy that has a stunning '66 L-72 Nassau Blue coupe which has the 7/16" flat hex head bolts. His car is an April '66 car. My car is 4 months older (December build) and I'm wondering which bolts to install.

My question is:

When did the factory begin transitioning from the recessed type to the flat head type big block valve cover bolt? So, would anyone with an L-72 or a L-36 please be so kind to post what type bolt and which head mark your car has. No need for photos, but they would be excellent! And it's very important to state the month of production of your car. Thanks so much for any participation. Mike

[Donald A.]

Re: 1966 Big Block Valve Cover Bolts for a December '65 built L-36. Need your help.

I have a Jan 66 L72, it has the flat head type RBW bolts, they don't look repro but that's just my best guess

[Ronald L.]

Re: 1966 Big Block Valve Cover Bolts for a December '65 built L-36. Need your help.

Dec 13 car, Oct Engine small recessed ones
April 28, engine April, TRW.

and the TRW have been in repro for so long bad bolts are out there for 25 years. It likely to never want a grade 3 bolt going into cast iron, they will snap off. Look at all the other fasteners going into the cast iron block, they are grade 5 with 3 bars, so those you see with no bars are likely those that have been changed at one time.

[Mike B.]

Re: 1966 Big Block Valve Cover Bolts for a December '65 built L-36. Need your help.

Good Afternoon Guys,

Thanks for the last 2 responses. Don, your RBW flat hex heads sure look good to me. I don't think anyone is reproducing these type bolts. But I've heard that one of the big bolt guys out there found a large cache of this head mark. I'm gonna look him up. Thanks Don.

Ronald, I've never heard of that head mark before. I'd love to see a photo of those you mentioned in your post. Good to hear from you. Mike