4323 Carb to Manifold Bolt

Classic short-frame models

Re: 4323 Carb to Manifold Bolt

Postby dr dick » Mon Sep 07, 2020 9:12 am

being a hardware junky the xlch manifold bolt has been on my radar many decades because it has never been documented correctly in the parts books.
that said i strongly believe this bolt is 3/8-16 x 1-3/16.

hd lists part number for a bolt of that size: 4348.
(4354 is found in big twin parts books at 1-3/8 long. thats to long it will hit the lower carb screw head.)

as for the 4348 its the bolt that also fastens the rear motor mount to the crankcase.
there are two versions of this bolt early ones marked 1038cp and the later which are unmarked.

every non molested bike ive seen use the same bolt on motor mount and carb, with the 1038 used till at least e1960.
ive had my hand on a few good 64s and they always had the unmarked versions.

when the change from 1038 to unmarked needs more research.
dr dick
Posts: 105
Joined: Mon May 26, 2014 9:01 pm

Re: 4323 Carb to Manifold Bolt

Postby bill pedalino » Sun Sep 13, 2020 10:03 am

I received the 4354B bolts and the length is perfect. After repeated tightening over a two-day period (to compress the white insulators) I measured about 0.050" between he end of the lower bolt and the body screw. The top bolt extends into the cast iron body until its just about flush at its end. I would feel uncomfortable with a shorter bolt, as you have to repeatedly tighten the bolts to keep compressing the insulators which 'creep' in compression over time, thus allowing further bolt tightening.

I assembled the insulators off of the bile as a 'block using Permatex HighTac between them. I placed the block in my vise and squeezed the assembly moderately with two end plates and allowed them to set up for a full day.

I then installed the block onto the manifold (which was left on the bike) and bubble tested the intake assembly a 2nd time, this time including the insulators and paper gaskets. The manifold O-Rings were leak-free at a positive 15 psi although the insulators showed very slow bubble formation. I cranked down on the bolts a bit more and the leaks slowed to almost non-existent.

I believe that the new insulators (OEM) are a little pourus by nature and at a positive 15 psi they do weep a little. However, the system will never see a negative 14 psi ('perfect' vacuum is -14.7 psi) in operation, so this shouldn't cause a lean condition after the carburetor is adjusted.
bill pedalino
Posts: 22
Joined: Sat Mar 21, 2020 9:52 am


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