Carb Allen Screws

[5twins]

One of the things I do when rebuilding a set of carbs is replace all the original Phillips screws with stainless Allens. I used to use regular Allens everywhere but have changed now to buttonhead Allens on the tops and sides, and regular Allens on just the bowls and that angle bracket across the bottom. I think the smaller, more rounded off buttonheads look better and blend in better with the carb body .....

So, with all the carb work I've been doing over the last few months, I thought it was time I made up a list so I don't have to re-measure stuff all the time, lol ..... and here it is .....

..... and some closer shots of the BS38 and BS34 sections .....

(sneaking this in here, ggg)
BS38 SCREW SIZES
DIAPHRAGM COVERS
with bracket 5) M5 X 16
W/O 3) M5 X 14
CHOKE HOUSINGS 6) M4 X 12
CHOKE LEVER 1) M4 X 12 UP TO 16, VARIES
CHOKE LOCK TAB 1) M4 X 8
BOWLS 6) M4 X 14
BOWL UNDER CHOKE 2) M4 X 13.5 CAREFUL!
ANGLE BRACKET 4) M6 X 16


BS34 SCREW SIZES
DIAPHRAGM COVERS -
NO BRACKET 2) M5 X 10
ONE BRACKET 4) M5 X 12
TWO BRACKETS 2) M5 X 14
BOWLS 8) M5 X 16
ANGLE BRACKET 4) M6 X 16
Thanks to 5twins!

same info as a JPG

Some notes and explanations will follow in the next post.

 

[5twins]

OK, now I got some 'splainin' to do, lol. As the note at the bottom of the BS34 list incurs, there are special lock washers made just for Allens .....

They have a slightly smaller O.D. so they fit better under the smaller diameter Allen head, and then are made slightly thicker to compensate for that. The top two below are the high collar washers .....

Next, let's talk about the BS38 float bowl screws. The originals were 12mm long and frankly, that's too short. They don't reach all the way through the hole and use all the threads available .....

So, for 3 of the 4 bowl screws, I switch to longer 14's. These fit perfectly in my mind, making it all the way through the hole by a thread or two .....

But, you can't use a 14 under the choke housing, it's too long and will bottom out before the bowl is tight. Keep tightening it and you strip the threads out. So for this screw, I custom cut 16's down to around 13 or 13.5mm .....

Then I test fit them using a slip of paper as a "feeler gauge" to insure there is clearance .....

Last BS38 note concerns the choke lever screw. The depth of the mounting hole can vary from carb set to carb set so you will need to measure it. You may need a screw as short as 12mm but it may take one as long as 16mm.

The only note pertaining to the BS34s concerns the carb top screws. There are two sets of lengths listed, one for the original thin "tin" tops and one for if you swap on thicker alloy tops (all screws 4mm longer) .....

 

[arcticXS]

Just a minor comment. I see that Torx screws are becoming increasingly common. Which may be a good alternative to Allen screws in some higher torque applications. Countersunk Allen screws are especially prone to stripping out the hex, due to small hex size and the added friction from being countersunk. Also, the "low cylindrical head" type Allen screws can have the same issue, again due to a smaller hex than the regular type.
Lately, I have been buying some SS button head screws in M4, M5 and M6 for my sailboat. All these were Torx. So now I need to keep a 1/4" socket set with both Allen, Torx and regular sockets on the boat.......

 

[jpdevol]

Countersunk Allen screws are especially prone to stripping out the hex, due to small hex size and the added friction from being countersunk.

Countersunk any head screws are prone to "freezing" from the friction area you cite. The worst offenders on bikes I've found are speedometer drives on old Hondas - compounded by the fragile drive that won't stand impact drivers. Anti-seize on the "cone" is best practice.

 

[Grewth]

I've found quite a few stripped threads on mikuni carb bodies caused by people overtightening allen screws.
Yes they're much smarter than chewed up crossheads, but harmful in the wrong hands

 

[Kevin Werner]

The concern of rounding out the allen hex recess is valid except I do not crank them tight. These fasteners are mostly snugged down with a thumb driver (?), a small knurled wheel that fits a hex/JIS/phillips bit. I did my second set of carb float bowls a few weeks ago and took one screw to the hardware store to get 8 SS allen head cap screws. @5twins is very kind to catalog this info for all of us.

 

[jetmechmarty]

Just a minor comment. I see that Torx screws are becoming increasingly common. Which may be a good alternative to Allen screws in some higher torque applications. Countersunk Allen screws are especially prone to stripping out the hex, due to small hex size and the added friction from being countersunk. Also, the "low cylindrical head" type Allen screws can have the same issue, again due to a smaller hex than the regular type.
Lately, I have been buying some SS button head screws in M4, M5 and M6 for my sailboat. All these were Torx. So now I need to keep a 1/4" socket set with both Allen, Torx and regular sockets on the boat.......

I don't believe the bolt head is a worry. The socket head bolt can transmit a lot more torque than a crosspoint, making it easy to strip the threads in the carburetor bodies. Torx or allen, care is required

 

[arcticXS]

I don't believe the bolt head is a worry. The socket head bolt can transmit a lot more torque than a crosspoint, making it easy to strip the threads in the carburetor bodies. Torx or allen, care is required

Completely agree! My comment was not specifically directed towards carb screws, but just a general comment about torx drive replacing Allen drive in especially countersunk and button head screws. I guess mainly because those have a smaller hex than cylindrical head Allen screws.
I also have a thing for reduced hex flanges bolts and reduced hex flanged nuts. I guess I could consider myself "a nutcase"