Damaged thread - advice needed!

[Dave Herrod]

I have done a very foolish thing....

 

Nearing the end of building up an engine for my TR3A rebuild, I temporarily installed the thin belt crankshaft pulley to see how everything lined up. I then had to use a puller to remove the pulley to fit the timing cover. In doing that, I am embarrased to admit that I have damaged the first couple of threads in the end of the crankshaft. ( I'm not usually that ham fisted)

 

I could run a 5/8" tap into the hole, but I am worried that it may start cutting a completely new thread and make matters even worse. Alternatively, I could attempt to grind out the damaged threads, creating a taper into the undamaged ones. Before I cause more damage, I would welcome any advice.

 

Dave.

[Ian Vincent]

I would try to re-cut the thread with a tapered tap but only cut a tiny amount each time before checking that it wasn't cutting a new thread.

 

I guess the worse that could happen is that you would have to helicoil the thread if it became completely bu**ered.

 

Rgds Ian

[AlanT]

As long as you have a proper taper-tap it should find it's way.

 

If there is a big burr on the first thread you could remove it with a counter-sinking tool.

[oldtuckunder]

Get a new or old bolt with correct thread, ideally the crapiest softest bolt you can find and grind a chamfer on the leading threads and then use it to gently work into the damaged thread using some lubricant, it will be way softer than a tap and have little chance of damaging the existing threads but will normally gently remove any burs in the thread. Work it in and out until everything feels smooth, and the try the real bolt again, if it feels like its binding revert to the chamfered bolt and rework again.

 

Unless you have completely buggered the thread this will usually clean it up.

 

Alan

[Guest]

Will 'Uniburr' help? Have a look here: http://www.tr-register.co.uk/forums/index.php?/topic/54752-niburr-might-come-in-handy/

 

And then there's the much cheaper NG1700 tool. Just google it.

 

Menno

[AlanT]

I'd got the idea this was a female thread.

 

The right solution on a damaged male thread is a "restoring file".

[RogerH]

Hi Dave,

just to clarify are you talking about the hole that bolt #69 fits into http://www.moss-europe.co.uk/shop-by-model/triumph/tr2-4a/engine/engines-components/internal-engine-components.html

 

isn't this bolt bigger than 5/8".

 

If a taper tap will not fit then have you got a Dremmel type device to delicately clean up the threads so the tap will fit.

Or - You could grind a taper tap so that the taper is longer - the taper will eventually bite before the meaty bit takes hold.

 

Or - if we are only talking of a couple of threads in the hole why not simply counterbore the hole and remove the damaged item.

 

 

Roger

[D Murton]

Get a new or old bolt with correct thread, ideally the crapiest softest bolt you can find and grind a chamfer on the leading threads and then use it to gently work into the damaged thread using some lubricant, it will be way softer than a tap and have little chance of damaging the existing threads but will normally gently remove any burs in the thread. Work it in and out until everything feels smooth, and the try the real bolt again, if it feels like its binding revert to the chamfered bolt and rework again.

 

Unless you have completely buggered the thread this will usually clean it up.

 

Alan

I have used this trick quite successfully on occasion (although not on a crankshaft). If you also file a groove down the length of the thread of the bolt it gives any burrs that break off somewhere to go rather than picking up on the thread.

 

Dave

[Dave Herrod]

Many thanks for the replies - I still feel stupid, but less lonely!

 

We are indeed talking about the female thread that that bolt 69 goes into. I am pretty certain it is 5/8" UNF (18TPI)but I am currently about 100 miles away from the engine so can't double check. At least being this far away does mean I can't do any more damage.

 

Dave

[RobH]

Yes indeed it is 5/8 UNF Dave.

[BlueTR3A-5EKT]

A thread that big you may be able to chase the damaged threads with a bit of tool steel ground to cut LH internal threads - Work from inside the damaged area out towards the damage and start of the thread by simply holding the tool and rotatting it while the cutting tip is in the good bit of thread. Easing the tool round as if scraping. I am finding this process difficult to explain.

 

image of RH internal thread cutting tool http://littlemachineshop.com/products/Images/480/480.1895.jpg

 

Sale of LH thread cutting tool http://www.ebay.com/itm/Internal-Tool-31-1255-Left-Hand-Threading-Tool-Min-Bore-290-Max-Depth-1-000-/371489573677

 

Perhaps easier is as already suggested remove the damaged threads with a small grinder ie dremmel.

 

Peter W

[Dave Herrod]

Interestingly, someone suggested a similar approach, but using a smaller diameter tap having the same TPI - in this case probably 5/16 UNC (18TPI) - to work out from the good thread.

 

I am pretty certain that I have only butchered the first thread so, thanks to all the responses, I'm pretty sure it can be rescued. Thanks for the help, chaps.

 

Dave.

[MikeF]

This may be a red herring, but I was in B&Q yesterday looking for plumbing stuff and saw a device called a tap re-seater. It has a variety of selectable tapered cutting threads arranged small to big on a single shaft with a turn handle at the top. IT quoted the sizes but I couldn't remember the exact details on this thread (Boom boom), and didn't have my mobile to go online. Now I have it but the signal here is variable G2 to no G so can't really check. I can't recall precise re-seater details but they seemed to be the range discussed. Might be worth a Google.

 

Mike

Edited December 10, 2015 by MikeF

[AlanT]

I have one of these. The threaded part is soft.