Anybody know the size and thread type of the bolt that attaches starter brace next to the block on a 71 small block? My assembly manual only referes to it as a UPC6. BTW, does anyone know of a bolt/nut translation table that would give you the bolt and nut sizes to match the part or UPC numbers in the assembly manual? I find this part of the manual to be useless. Thanks. John

 

Actually, John, UPC 6 is the entire engine section in the AIM; in the 70 AIM, the specific id for that bolt is Item 11 (Part No 3973388) on Sheet B2 of UPC6.

I really didn't expect my parts book to show the part number, but I checked it and confirmed that to be true. There are so many fasteners, that to list them all would probably have the effect of doubling the size of the parts book.

The best way to identify an unknown thread size is to simply have on hand a range of cap screws, bolts, or nuts that can be tried as a known sample. Most of the chassis fastener sizes on old Corvettes will be 5/16 NC (some), 3/8 NC (most), 7/16 NC (a lot), 1/2 NC (a lot), 9/16 NC (a few), larger sizes (they do exist). NC stands for "national coarse" and, for each fastener diameter, implies a standard number of "coarse" threads per inch.

The main concern that you will have in using this trial and error method is that THERE ARE SOME FINE THREAD FASTENERS (NF) USED ON OLD CORVETTES. If you attempt to put a NC thread in an NF hole or vice versa, you will RUIN THE THREADS. So there is a little observation that has to be done before you automatically stick a coarse threaded bolt into any hole. With experience, you will be able to quickly determine if the required fastener should be coarse or fine thread; the smaller the fastener, the more difficult it will be to make this identification. We can just be thankful that the early cars don't also mix in the metric thread pitches.

When you try a fastener, make sure the threads in the hole and on the bolt are clean and lightly lubricated. If the fastener starts easily and quickly, turns with little resistance and with consistent effort, and the fastener is definitely threaded with very little side-to-side slop, then you have a match. Use that size to buy the class and length bolt you require. If you cannot get the fastener started or you have a lot of difficulty getting it started, the turning effort varies during turning, or the fastener seems to have a lot of slop within the threaded hole, you probably have the wrong thread size...try again. NEVER FORCE A FASTENER.

The only exception to the "never force a fastener" rule if for locking nuts (what do they call them? prevailing torque or something.) This type of nut will have either a "triangular shaped" threaded hole in the center or some type of nylon insert. These nuts are intended to be installed using substantial torque (they don't turn easily), and once installed, THEY STAY THERE. If removed, they are generally discarded and replaced. There are a few of these locking nuts on old Corvettes, but they are easily recognized since they are generally zinc plated.

Now that we have covered more than you wanted to know, the probable fastener size on that bolt is 3/8NC X 1" (estimated). If you have the engine block handy, it should be fairly easy to confirm my guess. Another simple way of making this determination may be to go to your local Chevy dealer and ask for the bolt by part number (3973388) and see if they can cross reference the part number. The probability that they still sell the bolt is higher than most Corvette parts, because the fasteners were used on many more applications and vehicles.

I know of no data table that lists fastener size, length, head stamp etc. by part number or application.

 

Chuck,

As usual, you have generously provided a wealth of informtion. I agree with what you said. What I should have said was that I took the darn bolt out a couple of weeks ago and can't find it! So much for organization. I usually try the hit and miss approach very carefully so as to not ruin the threads. I am at work now and was hoping to take the easy way out to finding the correct bolt so I could pick one up on my way home. I am almost positive that it would be NC threads since it has been my experience that most of the bolts that go into the block are NC. I pick up a 3/8' NC on the way home and see if it fits. Thanks. John

 

Your bolt is probably on the floor of your garage...use a strong flashlight, get down on your hands and knees with your eye at floor level.

Give your Chevy dealer a call first...it may save you a lot a trouble.

Bolts are cheap. Buy a 5/16 NC, 3/8 NC, and 7/16NC (Grade 5) to cover the most likely range, and save yourself a trip. Good Advice: shop an auto parts store for your bolts...don't mess with that imported junk at Home Depot. You have no idea what the strength is for those fasteners...suitable only for porch swings and sandboxes. Black oxide will probably be a more original looking finish than zinc, but you may not have a choice.

I agree with your assessment of the thread...on the engine block, it almost a virtual certainty to be coarse threads (I can't remember any fine threads on the engine; suspension, chassis is a different matter). Required length is probably 1" or 3/4", but you really need to measure the depth of the hole.

 

It's a 5/16"-18 x 1/2" bolt - the "correct" one has an indented hex head and a captured external-tooth star washer.

 

Thanks for the info, John.

An interesting thing about my car was that it did not have the starter brace installed, and I am convinced that it came from the factory that way.

The reason it didn't have the brace is because it came with another starter number that was used on another GM car (Camaro, Chevelle). This starter has a cadmium plated solenoid. It also didn't not have the long through bolt with the little stud on the end that retains the brace, so the assembly guys just left the brace off. I am convinced the starter is original because the date fits perfectly in the context of the car's build date.

It was the end of the model year, they ran out of the specified starter, so they just slapped another "performance car" starter on it and kept on trucking. I should check with guys on Juliet's registry with late 350/300 starter numbers, and see what they have on their cars. I can't wait to see how many point deducts it nets me in judging. :nonod: :nonod:

 

Thanks for all the advice guys! I have had it shut down for a couple of months to fix timing chain seal, rear main seal, oil pan gasket, new plug wires while I am at it. Trying to get it running in time for the cooler weather here in south LA! Thanks.

 

John Z was correct. I picked up a 5/16 - 18 X1/2" bolt last night and it fit perfectly. Thanks again.