Threads, pitches and spanners.

Biscuit · Post by **Biscuit** » Sat Aug 12, 2006 10:21 am

Don't much about helicoils, but if the 'tapping' drill will fit down the rocker box cover hole, do it with the cover in situ, it will keep the drill square (essential) and contain the swarf.

TommoT · Post by **TommoT** » Sat Aug 12, 2006 11:57 am

Hi Mick,
I did this very job on my G80 and I used a 5/16"BSF kit from A. R. Sheldon tools. Not a complicated job, but acuracy in getting the tap lined up properly is necessary. Actually I used my drill press to ensure square insertion. I also took the opportunity to repair a few suspect threads at the same time, and the helicoiled threads take the bolts with much more precision than before.

good-hifi · Post by **good-hifi** » Sun Aug 13, 2006 5:01 pm

I'm probably going to use my bench drill too. The only problem is that I am scared to remove the head in case I unearth even more horrors!.

Maybe I might be best to use the rocker box cover as a guide as Biscuit said.

To cap it all I found a previous owner changed the main jet on my little Yamaha (to make it faster!) and two strokes don't like a weak mixture.

Now I have nothing to ride again.

Must pass my test and then one day I might buy a bike that will actually work for at least a month....

Cheers

paul knapp · Post by **paul knapp** » Mon Aug 14, 2006 2:12 am

Mick,... if you are going to use a helicoil to restore the thread, avoid using any power assisted drills as a "dig in" may result with more damage. There will be very little metal to be removed for the helicoil tapping drill which is easily done by turning the drill by hand. I did a friends G80 recently using a hand drill and a square made from a block of hardwood (as a guide to keep the drill square in the hole) without removing the head.
Paul

Biscuit · Post by **Biscuit** » Mon Aug 14, 2006 10:29 am

Difficult to describe without pictures, but if you take off what is effectively the top rake from both flutes of the drill, it will not dig in. With the drill held in line with the grindstone take a bit off of the cutting edge until you have a land or flat approximately 40-50 thou wide (depending on the diameter of the drill)

jim501 · Post by **jim501** » Mon Aug 14, 2006 11:24 am

Nothing to add to the excellent advice.

Just a few cautionary general thoughts. Aplogies to those who know all this.

BSW and BSF are standard British thread forms. BSW doesn't often appear on bikes because it's usually found on bolt sizes above 1/2". The Flying Scotsman and London Bridge are held together with BSW bolts. Probably the most common thread form on bikes is BSC (British Standard Cycle) which is not interchangeable with BSF. Also don't forget the Americans gave us UNC and UNF threads and some of these nuts and bolts could have found there way on to your bike over the years. Just to make it more interesting you'll also find BSG (British Standard Gas) on fuel and oil pipe connections. Don't forget 12mm is also uncomfortably close to 1/2"

What's all this mean. A replacement nut or bolt pulled out of the junk box may look right and could be the right length and diameter but could have one of six different thread forms. If it doesn't easily screw in for three or four threads either the thread is damaged or it's the wrong thread form. Force it and you'll strip it. Strip the thread in a blind hole in an alloy casting and you end up with the problem which started this thread.

cheers
jim

Biscuit · Post by **Biscuit** » Mon Aug 14, 2006 12:38 pm

British Standard Gas - properly known as BSPT - British Standard Pipe Thread. Whitworth bolts and screws can be found on Burman gearboxes (cover screws) and Lucas magnetos (mag. to platform bolts), at least they knew the correct threads for aluminium. Although theoretically incorrect, for all practical purposes 1/4" BSF is interchangeable with 1/4" cycle.

deshollier · Post by **deshollier** » Mon Aug 14, 2006 8:59 pm

Also, for all practical purposes UNC 3/16, 1/4, 5/15 & 3/8, are interchangeable with BSW

Biscuit · Post by **Biscuit** » Mon Aug 14, 2006 9:11 pm

Didn't know that Des, I'm thoroughly Imperial. I'll file that away, and some time in the future I'll pretend I knew it all the time!!

Eric · Post by **Eric** » Tue Aug 15, 2006 12:46 am

I have had a quick check the pitches are the same for Whitworth and UNC but as with 1/4" BSC and 1/4" BSF the flank angles are different e.g. UNC 60 whilst Whit is 55 I you would probably get away with it but things would get a bit tight one way or the other.