lars

Repairing the Corvette Holley 9/16-24 Float Bowl Inlet Threads

by Lars Grimsrud
©2017 Lars Grimsrud.
This article may not be published or distributed without the written permission of the author


Mid-60's Holleys on Corvettes, and many of the aftermarket Holley-based carbs, use an odd 9/16-24 thread at the float bowl fuel fitting inlet. This tech paper will discuss a procedure I have developed for the repair of stripped inlet threads on those carb float bowls with the odd 9/16-24 inlet threads. These float bowls are not available new from Holley, and there are no “HeliCoil” thread repair inserts or “Keensert” inserts available in this thread size from any source. Once the threads strip out, the float bowl is junk. This is a problem for C2 Corvette owners who wish to preserve the original carb, or for C3 owners who wish to run one of these carbs on their later model Vette.


Overview
Holley carbs on the mid-60’s Corvettes, such as the List 3124, used a 9/16-24 inlet fitting thread in the float bowls. This is an odd thread size, and no thread repair kits offer this thread size “Helicoil” insert. As these carbs keep getting older, and as the castings deteriorate, many of the float bowls are stripping out at the inlet threads. This renders the float bowl unusable.

Seeing more of these problems, I have designed and developed a repair for the problem.


Process
Here is a typical early Holley 4150 as used on mid-60’s Corvettes. Note the small inlet fittings: These have a 9/16-24 thread into the float bowl. This is not a common thread size, and was soon eliminated by Holley in favor of the more robust (and also non-standard) 7/8-20 thread size:




Due to the very short thread engagement and the small thread size, these threads will strip out easily with age. This, effectively, destroys the float bowl with no repair possible:






Seeing more of these issues with the carbs I receive for rebuild, I designed a special aluminum insert with some special thread sizes to allow the repair to be performed:


The custom machined inserts have the stock 9/16-24 threads on the inside diameter, and a special 5/8-24 “extra-fine” thread on the outside diameter. This allows maximum wall thickness of the insert, while removing the minimum amount of material from the float bowl thread bore in order to retain the stock inlet fitting gasket integrity. The inserts are made from 6061-T6 aluminum, which is significantly stronger than the bowl zinc die cast material:


To do the repair, the bowl is removed and disassembled. A 9/16-24 tap is threaded into the remaining original threads in order to establish the original thread position/alignment:


With the 9/16-24 tap in the inlet threads, the tap is inserted into a drill press chuck, and the bowl is then clamped into a drill vice in the non-restrained condition to assure exact thread alignment:


Once aligned, the tap is removed, and a tap drill for the special extra-fine 5/8-24 thread tap is installed in its place:


The float bowl is drilled out to the new tap size:


Without shifting the alignment, the tap drill is removed and the special extra fine 5/8-24 tap is installed in the chuck:


The tap is turned by hand by rotating the drill press chuck as the drill head is lowered to follow the tap progress. This assures perfect alignment of the threads to the original thread bore:


New threads in the float bowl:




The insert is test fitted into the new threads: A perfect fit:


The insert threads are coated with JBWeld to assure that the insert will not move or rotate after installation. The epoxy is not the inlet seal: The seal will still be provided by the inlet fitting gasket:


The 9/16-14 tap is used as the insert installation tool, threading the insert into the bowl until it is perfectly flush with the original inlet fitting gasket sealing surface:


Insert installed:


Fitting re-installed and carb ready for service:


I'm not selling these inserts due to the machining requirements to do the repair, but if you're interested in having this done, you can e-mail me for info:

Lars
V8FastCars@msn.com

71VetteLover

I have always used ordinary JB Weld to permanently attach the inlet fitting into stripped threads. I just coat the threads with the JB Weld, screw the inlet fitting in snug, let it cure overnight, and the next morning it can be put back into service. The little sintered bronze fuel filter can't be used but it's not a problem because an inline filter can be used. This last summer I used JB Weld to permanently attach my inlet fittings into the un-stripped threads of my #3310 just because I was tired of the constant seepage around the gasket. And as I use a one-quart spin on filter back at my tank I have plenty of filtration.

lars

The carb shown had the fittings JBWelded in place, which is why they failed and I had to do the repair the right way. You can see the failed remnant JBWeld in the 3rd photo.

Lars

mrvette

EH, put DPFI on it.......

but gotta admit Lars, NICE work.....pass any resto exam.....

69L46vert

Very nice repair. Thanks for the detailed post. Finished product looks great.

gkull

Lars we always used these at the shop


http://www.timesert.com/

lars

George -
Those are nice - I've used them, too, but they're not available in the 9/16-24 thread size. There is no source for 9/16-24 inserts.

Lars

Greg

Permanent and reusable, way cool.

'75

that's a nice looking repair. Here's an adapter that might work as well if a gasket was used between the adapter and fuel bowl. Available in stainless or black steel, could borrow it from your AR15 accessories.
https://www.brownells.com/rifle-part...nt=100-015-977

7t2vette

Great job Lars!!!

lars

Dean -
I found that one, too, but it would give the carb a non-factory appearance with the lip/ridge feature. The larger major diameter of the adapter's 5/8 thread/pitch combo would also remove more of the carb's stock gasket sealing surface, making it risky to use. The "extra fine" outside thread on my adapter removes the minimal amount of material from the stock float bowl, leaving enough of the original sealing surface that the stock inlet fitting gasket still seals up.

Thanks, Bruce!

gkull

I need to drive back over and see you again

lars

Darned right! I still have your sunglasses on the bar...

Lars