Stripped thread repair advice please

One of the 1/4" BSF tapped holes retaining the changewheel quadrant on my old ML7 has been stripped by a previous owner, and the other is not great.

Can't comment on Myford lathes as I do not have one, but whenever I have one of these problems I drill a larger hole, tap with a suitable thread and insert a piece of screwed steel using Loctite Stud locker or similar. For additional security I sometimes centre pop at 4 locations around the joint thus providing some distortion at those points. Finally, I then drill and retap the correct size into the inserted screwed steel.

Regards,

Peter G. Shaw.

A helicoil repair will be the easiest and best solution as it will retain the original spec. thread that will be much stronger than the original tapped hole in casting.

Hi there, Jon,

I've found that those Woodruf keys can be easily removed using a pair of side-cutters. The problem can be that once they start to come out, they can fly so I'd suggest that you sweep the workshop floor first!

As to repairing the stripped thread, using a Helicoil is going to be a bit expensive for just one hole unless you happen to already have a Helicoil kit for that size (or access to borrow one).

It's possibly worth mentioning here that the illustrated parts lists (aka 'exploded diagrams' ) for the ML7 are all available on the new Myford's web-site. (I wrote 'all' but I couldn't find one for the ML7 counter-shaft clutch.)

Best regards,

Swarf, Mostly!

Woodruffe keys should be a tight press in fit to the shaft but fairly easily removable for all that. I normally use a small soft drift on one end of the key and either try tapping down so that the key pivots on the curved back section and comes round and out, or try tapping the key along the shaft so that the curve lifts the key out as if sliding up a ramp.

Once the key is out and the quadrant off, then repairs are easier to carry out and the method will depend on ow much metal there is surrounding the thread, and what equipment/skills you personally have. Using a thread insert (helicoil et al) would be my preferred way as the least "invasive". Making my own insert would depend on what taps/dies I had available and how much metal there was to play with in the original. If you have an engine re-conditioner close at hand it might be worth having a word with them, as they will most likely have the correct taps for the thread inserts to hand as it is a regular part of their experience. Buying the helicoil kit for one or two damaged threads is expensive.

A thread insert will have the same tpi/pitch as the thread that is damaged so one possibility might be to make a tap to suit. You can find the details of the recommended drill sizes for tapping for the insert and also the sizes for the outside of the tap on the internet. Actually at 1/4 BSF (26tpi) you might find that a 5/16 Model Engineer 26tpi tap could do the job, to take the insert with some loctite.

Keith

Edited By Keith Long on 19/02/2014 16:27:57

Jon,

if you go the helicoil route you must use the correct special tap to suit the helicoil and be sure to drill the correct size hole for this tap.

A helicoil kit may be a bit expensive so you might be able to borrow one from your local club.

Eric

Thanks very much for the quick replies and suggestions.

I'll try removing the woodruf key and take it from there.

A 1/4 BSF helicoil kit is about £25 delivered and a set of carbon steel taps for 1/4 BSF is about £10 delivered. So either approach would be in-budget. I've a pretty good selection of metric coarse taps and so making my own insert ought to be quite possible and especially attractive since it'll be the cheaper option.

Many thanks again

Jon

Jon,

The helicoil kit will contain all that your require - you will not need the 1/$ BSF tap set.

Eric

the problem with helicoil sets is they are expensive and you only get a set for one thread, I bought an M6 kit and used it once, Plugging and re drilling is the best method.

Martin P

Thanks again.

The quadrant is off - the woodruf key just popped out easily

Having cleaned everything up I think the plug/redrill and tap will be pretty straight forward. I just need the 1/4 26 BSF tap(s).

I'm quite tempted to create the thread on the outside of the plug and drill the BSF tapping hole concentric before installation. Does this make sense or am I making the plugging more difficult?

Jon

Hi Jon, you can always use a large enough bolt and drill and tap it in the lathe with your 1/4 BSF tap and then screw it into your new hole with a little thread loc using the hex head and then cutting it off flush once the thread loc has set.

Regards Nick.

I am surprised that M7 x 1mm is mentioned as a replacement thread. M7mm is a non preferred thread in the metric system. It does exist but not in general use I am afraid.

The answer here is a Helicoil insert. If you cannot afford the kit then cast about some of the local Engineering firms and get a quote. i charge £5.00 for 1 M5 Helicoil replacement. The helicoil kit will come with a special tap and an insertion tool with a recommendation for the tapping drill size. The Helicoil kits I buy come with 10 spare coils.

One other answer is make a plug and thread that into the arm and drill that to suit the ripped out thread.

Clive

It's not common in general use as said but it does crop up regularly in automotive applications. You'll find it on BMW and VW cars and Suzuki motorcycles that I know of and probably loads more I don't have personal knowledge of.

hi john,

i would go up to 5/16" BSW and tap the lathe accordingly and turn up a stepped stud 5/16" BSW and with the plain shank 1/4" dia and 1/4" BSW on the end. you need quite a coarse thread for cast iron.

am sure you can borrow some taps and dies if you havent got them.

cheers,

julian

Thank you again for the helpful advice and replies.

I've made my decision and have ordered some 1/4 BSF taps and will drill and tap a couple of lengths of M10 stud on the lathe and install these into new tapped holes in the casting with Loctite. M10 should allow for enough meat between the thread crests of the 1/4 thread and the core of the new stud - I was a bit worried that M8 would be cutting things a bit fine.

I've made myself a hardwood drilling/tapping guide to ensure that my tapped hole can be made square and concentric with the existing holes and equidistant from the leadscrew as these'll have to done freehand.

Many thanks again

Jon

Virtually all of the methods above will work. It's dependant on what equipment and tooling is at the OP use.

One thing isn't mentioned and that is where the OP is located as someone local may have the taps or helicoils saving unneeded expense.

BTW M7 x 1 is virtually the de-facto standard now for brake pipe fittings so if that size tap is available tap the housing, screw a brake pipe nipple in from your local friendly garage / car parts shed, then tap the pre drilled hole in the nipple M5.

Twenty way to skin this cat, just depends which way the cat likes best.

M7 x 1 as JS says is used a lot, Citreon 2CV 's are mostly made from them ( and the odd M9) Best is only need two spanners 11 a/f and 14a/f. The brake pipe nipple seems the way to go for me.

Hi John,
I fully agree with you over lack of approximate location of the OP. I find it frustrating when helping people with electrical problems (Like the recent Warco motor problem.) having posts going backwards and forwards over several days when the problem could be sorted in a few minutes with a visit from someone with some electrical knowledge. The OP could be living in the next street to me or someone else with that knowledge.

Les.