Distorted head stud thead in block

[Fingolfin]

Hi all,
Coming to the end of my 1275 rebuild! While threading the new head studs into the block, I discovered that one of the threaded bores in the block deck was distorted, such that a head stud wouldn't thread in very far - no more than one turn or so. The others screwed all the way down by fingers. I tried switching studs around, in case I had a bad stud, but no joy.

How should I go about fixing this? Is it as simple as getting a correct-size tap and chasing the threads back into shape?

(Apologies for the mess in the background...)

1105171747.jpg (1.02 MiB) Viewed 1830 times

[firedrake1942]

Not a scoobie, but if that is a concertina bypass hose, get rid of it and get a solid one before you put the head on!

[philthehill]

Get the correct tap (3/8" UNC Plug) coat the tap in grease and put the tap down the thread.
Small arcs of movement for the tap is the way to go backing off frequently.
The grease will collect any metal.
Did you countersink the top of the thread hole?
It is important to do so as when the head is tightened down the top of the block is pulled up around the stud- see schematic below:-

[oliver90owner]

Taps are for cutting threads, but will clean up a damaged thread. A thread chaser may be a cheaper option as they are only needing to clean up an already installed thread.

However, do you know that all the studs were OK before the rebuild? There is always the possibility of previous non-standard mods...

I once drilled and tapped a Landrover cylinder block with an over- sized thread. Required a stud, instead of a bolt, but was a 'perfectly' satisfactory repair for me at the time. The 'proper' alternative would have been drilling oversize and fitting a helicoil insert (but they were expensive at the time - and unobtainable for a farmyard repair at a weekend).

[philthehill]

Instead of purchasing a thread chaser - if you have a spare stud you can file a flat on the UNC thread and screw it into the stud hole in the same way as you would use a tap. Lock two nuts together on the 3/8" UNF thread to provide drive. The flat on the stud will do the same job as a thread chaser in that it will not cut a new form of thread but clean up the thread already there. If you can do the job without having to resort to using a 3/8" UNC tap so well and good.
Personally I would have run a used 3/8" UNC tap down all of the female threads and countersunk the stud holes in the top of the block before starting to assemble the engine. If there was a problem that required a rethink over the quality and type of thread it could have been resolved early in the rebuild.
Use a powerful vacuum cleaner to catch any crud generated.
As regards the stud threads - always use a 3/8" UNC die nut to clean up the threads as die nuts do not cut or damage the existing threads.
You can never have too many taps and dies and die nuts. Thread files (various) are a good item for the tool box

[GavinL]

For taps at a reasonable price try Tracy tools

http://www.tracytools.com/taps-and-dies ... 16-tpi-unc

Carbon steel tap should be ok for cleaning threads, and are cheaper than HSS, but carbon taps can snap if not used with care . For the extra few pounds I would go for HSS.

[Fingolfin]

Good suggestions, everyone!

Firedrake - already on the list. I knew someone would comment on it as soon as I took the picture!
Phil - I was unaware countersinking these holes was suggested! But, thanks to your diagram, I can certainly see your point about why it's necessary. Please keep in mind, this is the first time I'm rebuilding an engine, so it's all a brand-new experience - and sometimes no amount of workshop manual and forum reading catches these little tips. I'd like to have a nice tap and die set - but it will have to wait for when I have funds.
Oliver - I think I still have the old studs somewhere, so I'll look for old mods. I may also be able to use the best of the old studs for Phil's suggestion about a homemade thread chaser.
Gavin - thank you for the lead, but I'm in the States, so I imagine I can source them locally for less.

[philthehill]

The majority of taps and dies I own have been sourced off 'e' bay S/H or NOS at greatly reduced prices.
You do not have to have a full set of taps and dies. Purchase only those that you require.
Having had a Carbon Steel (CS) tap break in two when refurbishing the damper mount threads in the swinging arm of my Honda motorcycle I can with experience say only use High Speed Steel (HSS) taps for peace of mind.