Thread size

[iansoady]

I'll probably be drummed out of the Velo club for saying this (BSA owners are much more liberal) but with the normal tolerances we would be working with the 5o difference is negligible.

In the same way 1/4 BSF and Cycle are essentially interchangeable.

[Brian]

Might be time to bring up the subject of thread pitch and throw a bit of reality in.

Yes BSW and BSF have a 55 degree thread angle and UNC/UNF are 60 degree as is BSC (cycle thread).

Now the reality of it all, unless you are buying proper machined threads which these days are nearly impossible to find then you are buying bolts/screws with thread angles anywhere between 50 and 65 degrees. Most modern bolts are mass maufactured and the threads are rolled and basically are of very poor quality from an engineering point of view. If you get a quality set of thread gauges and look at some bolts under a magnifying glass you will see what I mean.

A lot of people use cap head (allen head) screws to hold on timing and primary covers, no matter what the seller told you then they will be UNC, anyone had a cover fall off yet because there is a 5 degree difference in the thread angle ? Basically if you use UNC in the place of BSW in places like side covers then it does not matter. For anything inside the engine and for things like head bolts most definitely use the correct threads with correctly machined bolts but for sidecovers and other general use bolts it is quite acceptable to use UNC in place of BSW. The same as using 1/4" BSF in place of 1/4" cycle is quite ok.

No doubt some will disagree with me and thats ok as we all have our opinions but I have been substituting these threads for as long as I can remember and have yet to have a problem.

[Hubie]

I didn't use loctite in the end, there's no way those inserts are coming out anyway!  I think I'll have to run a 5/16 tap down the insert to open it up a bit or trim down the thread on the bolts I have.

Cheers,

Dave.

[Brian]

I would trim the bolts down Hubie. Those inserts are hardened and if you screw a tap in you may find it will jam in and when you try to screw it out the thread insert will come out with it.

I have had this problem with inserts myself, it doesnt usually happen in alloy but is quite common in steel. I make all my own bolts so I can make them to suit.

[BSA_54A10]

Well there is hard and there is hardened.
Heli coils are generally substantially harder then the material they are inserted into.
As such they put a greater load on the tap which will generally break. This is particularly so if you are using carbon steel taps and not the stronger HSS taps or even stronger taps for tapping harder materials.
Then you have the contact area.
If you look closely at a Helicoil you will see that it is not the correct thread angle, it is square ie 90 deg.
That means that tap is required to cut a lot of material off both sides, not something that a tap was designed to do.
Now you can do it, with a lot of lubricant and a very light cut more like an agitation than a tap , but it is a lot easier to take a bit off the bolt.

[Hubie]

Well I learned the hard way that a standard 3/8 tap is not the same as a recoil/helicoil tap.  I purchased one today, problem all sorted.  Need two fresh 5/16 whit allen bolts now and I'm ready and waiting for my crank.

Cheers,

Hubie.

[iansoady]

If you look closely at a Helicoil you will see that it is not the correct thread angle, it is square ie 90 deg.

Not sure about that - Helicoil website says:

"HeliCoil Inserts or Wire Thread Inserts are made of cold rolled 18-8 grade Stainless Steel to AMS 7245 , MIL-I-8846 & DTD 734. This wire is cold rolled to a diamond cross section..."

Now it doesn't say what the angles are but I would guess 60o as that would suit most applications.

[chaterlea25]

Hi Hubie,
 ???? ???? ???? ???? ???? ???? ???? ???? ????
Did you tap the crankcase with a 3/8 BSW tap and then put in the inserts ????
3/8 BSW is 16 threads per inch
5/16 BSW is 18 threads per inch
If you then jacked in a helicoil tap (18tpi) there wont be much metal in the case left to hold the thread insert

Regards
John O R

[Hubie]

Bit of loctite permanent strength will see to that I'd reckon.

Cheers,

Hubie.