[Healeys] Tapping SUs for air filters.

Peter & Veronica greylinn at ozemail.com.au
Thu Nov 22 15:14:56 MST 2018

...you’re really not going to do any harm tapping the mounting holes. Just use the smallest diameter that you can to thread the existing holes. The alloy is no softer than eg an aluminium sump as per MG T type, which have tapped holes where the bolts do have to be done up tightly. And you’re not (I assume) going to be taking the filters on and off every 5 minutes! I think the reduction in aggravation when you do have to remove them is well worth it!


Peter Linn
Brisbane Oz

Austin Healey 100 V6
MG TC/Q type replica

Sent from Mail for Windows 10

From: Michael Salter
Sent: Friday, 23 November 2018 6:47 AM
Cc: healeys at autox.team.net
Subject: Re: [Healeys] Tapping SUs for air filters.

Better still get some of these:


On Thu, Nov 22, 2018, 12:48 PM Bob Spidell <bspidell at comcast.net wrote:
I've not tried this, but my first thought is that the aluminium bodies of the SUs would be too soft; eventually, you might strip the threads (esp. if trying to get a good seal with the filter).  Why not do what you're likely to end up doing anyway: installing thread inserts (Helicoil or similar).  I think 5/16" bolts would be sufficient.
On 11/22/2018 6:15 AM, simon.lachlan at alexarevel.plus.com wrote:
I’ll be removing my carbs soon in order to achieve clearance for raising (the front of) the engine. (Harmonic balancer installation).
So, it struck me that I may give my 3 HD6s another go. Swap them with the current setup.
Thinking it through and looking at the big box of bits, I recalled what a pain it is to get the nuts and two washers onto the bolts that hold on the air filters. I have French manufactured K&N lookalikes. The bolt starts inside the filter, goes through the carb and gets two washers and a nut. Very awkward to deal with those nuts and washers. PITA.
I thought that I might put in helicoils then I thought “Why not tap the holes?”
The holes on my HD6s are 11/32” or 8.8mm.
I am no engineer but it seems to me, from looking at tables on Google, that I could use a 25/64” drill bit and tap them out to 7/16”. I have both the drill and the tap but would like some expertise applied to the issue before I start drilling.
Any input would be much appreciated.

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