Washers for carb-to-manifold bolts

[markedmondson]

Did the factory originally use washers (flat, lock,...) on the four bolts (sometimes two front bolts, two rear nuts/studs) that hold the Q-Jet to the manifold? 

If washers were used, what type and finish?

The 1970 Corvette assembly manual does NOT show any washers, but that seems odd.

[carmantx]

I don't recall washers being used on the original bolts.  The original bolts had shoulders on them.

[markedmondson]

What about the rear studs/nuts, which don't have a shoulder?

I don't believe washers were used originally.  But it seems odd for two reasons.  First, washers would protect the soft potmetal of the air horn from being damaged from the bolt head.  Second, it seems a lock washer would help maintain tension given the low torque used.

[Ethan1]

I don't recall washers being used on the original bolts.  The original bolts had shoulders on them.

 Agreed. I do not remember seeing any washers either.

[markedmondson]

I found a post on the NCRS Forum dated 2010 by Joe Lucia that states:

"Here's the deal: 1970 Corvettes with Q-Jet carbs used unique-to-that-year front retaining bolts for the carb. These were special bolts of GM #3967476. These bolts were 5/16-18 X 3-5/8". They had a flanged (washer) head. I do not know the material grade, finish, whether or not the head was trimmed or untrimmed, and what the across-the-flats (wrench) size was for these bolts.

So, why did they use these bolts for 1970? Well, it had to do with the NA-9 (California emissions) option. For cars equipped with this option, the front carb bolt holes were equipped with insulators. I expect that to improve retention of the insulators, the flanged head bolts were used. However. perhaps to simplify things, the same bolts were specified for all Q-Jet applications and not just those equipped with NA-9.

For early 1971, these bolts were replaced by GM #9432450. These are, from what I can tell, very similar bolts except that they are 3-3/4" long. They have a flanged head, phosphate finish, GM 280M material grade (SAE grade 5--3 lines), and untrimmed ("indented") head. I believe these are also the bolts used on the Oldsmobile applications that clem referred to.

In later 1971, these bolts were replaced in PRODUCTION by GM #9419047. These bolts are the same thread size and length as the 9432450. However, they are a NON-flanged head bolt. They were phosphate-finished and of GM 280-M material grade. I do not know if they had trimmed or untrimmed heads.

Why GM went back to the non-flanged head bolts during 1971 or why they ever used them for 1968-69 Q-Jets, I will never know. I TOTALLY agree with clem; the flanged head bolts should ALWAYS have been used with Q-Jets to avoid damage to the carb air horn. It was a DUMB thing for GM to use non-flanged head bolts for this application----just as dumb as using non flanged head bolts for small block intake manifolds and non-flanged head bolts for head bolts on both small and big blocks. DUMB!

By the way, all of the above-referenced bolts are discontinued and, as far as I know, not reproduced."


Other posts go on to agree that no washers were used by the factory with any of the four hold-down bolts or nuts.  Of course, this information is specific to Corvettes of this era and may or may not apply to other models/years.

[bry593]

I use AN washers from Fastenal.  They are thick, but have a small OD.  Also useful for exhaust manifold screws.