Forum

[John R]

I'm about to attempt my first Helicoil on a 5/16 BSW hole. I have the helicoil kit but it specifies drilling out with an 8.3 mm drill bit. The nearest I have is an 8. Wiggle it around a bit or get the proper bit, do you think? 

[JOHNNY SINGLES]

As it is BSW, I assume it is in aluminium, if so an 8mm drill will be ok, do not wiggle it about, you would brobably make the hole too big. Just make shaw you tap the new thread slowly, two turns forward and then one back to clear the cutting edge.

[Dennis C]

As Johnny says,


also if it alloy use WD40 or parafin  as a cutting agent or it may just pick up and remove all the alloy to the size of the tap.


Don't ask how I know.

[Alan R]

Hi there John R-------If I may put some additional to the other lads }---Check your drill bit for the correct angle for Aluminium and make sure the cutting faces are sharp.  Are you drilling free-hand ? Get someone to give you a line -of-sight to keep the drill at 90 degrees to the surface. If it's a blind hole remember to clear out the swarf from the bottom of the hole at regular intervals---don't let it get compacted down there.  Have a couple of practice sessions on a scrap casing or similar, just to get the "learning-curve" sorted out first. Once you've done a few you'll wonder what all the fuss was about. Have a nice day !! Alan R (no relation )

[Midge]

Look up a decent tool supplier such as tooled-up.com or google 8.3mm drill bit, they are easily available.


Correct size drill is critical for success, its not the sort of thing to make a mess of for the sake of a couple of quid.

[MadMike]

John, like Midge says the correct size drill is critical with a helicoil insert. You are not simply cutting a 5/16BSW standard thread form. You are going to tap an oversize non-standard thread form. this is designed to allow the insertion of the insert, which at the start is like a spring with all coils touching. The insert will follow the tapped hole you create and in so doing will open up the coils, The outer part of the insert will lock into your tapped hole, and the inner shaped part of the insert will in fact re-create the 5/16BSW thread form that you require.


An oversize hole may allow the insert to come loose, and an undersize hole may create a thread form which will not allow the bolt or whatever to wind into the finished insert.


 


All the advice above about cleaning the hole etc is spot on incidentally. If you have some spare inserts then you would be well advised to do a couple of trial insertions before comitting to the final piece.


HTH.


 

[John R]

Not allowed to be simple.


The bolt is one of the handlebar clamp bolts on my Bullet. The original is threaded only on its lower portion, so the helicoil would have to sit low in the hole. But the tool has a flange that stops it getting the insert in any deeper than the top of the hole.


 So I could use a different bolt or countersink it with a larger bit, say 9mm. "Different bolt" seems the simplest idea - but it's a shame since the original was a polished  stainless job.

[MadMike]

John what you have is a bolt as oppsed to a set screw. Now what sized is the clearance hole into which the bolt fits before it engages with the thread? If it is close to the insert tapping size which is, as we have discussed before, greater than the bolt diameter then you may get away with drilling out the remains of the thread, and then helicoiling into the top portion of the hole and simply replacing your bolt with a slightly shorter stainless steel set, bolt and polishing the head.


Have you managed to completely strip the threads in the original hole? Or could you retap them to repair them?


Where are you located? I am in Leicestershire if I can be of help.


Are you sure about the 5/16 BSW thread size incidentally? An original Redditch Bullet would probably have had BSCY (Cycle) threads with a 26 TPI thread form. I am guessing that an Indian Bullet is more likely to have a metric thread form. 


   

[PeteF]

Mike, Even late Indian models have all sorts of threads on them. Metric, BSCY, BSF - you name it.


John, Are you sure you can't get a longer bolt and get a nut on the back?

[Dennis C]

Hi John


I did one of these last year for a friend, the alloy is like chocolate take great care and use plenty of cutting fluid when tapping the hole, if your insetion tool has a fixed collar just use a length of round bar old bolt etc with a slot cut in the end to insert it with.


It sounds like it is one of the top bolts that is stripped this is quite common as they are easy to over tighten, the idea of these is to just nip them up then use the studs and nuts on the bottom of the clamp to take the main part of the strain as these have a better thread, and are designed to do the main clamping, when these are tight they exert a sideways pull on the top bolts locking them in place. so ensure when drilling for the coil that the top part of the hole is not enlarged, if you have already enlarged this hole you will need an insert making to take it back to the correct size.