Morris Minor Owners Club

I have a cylinder head with damaged* screwthreads on the two holes for the heater tap. the previous owner has drilled them out so they now have no thread at all. I have purchased a tap set, but for a 6mm tap I dont drill a 6mm hole do I? It has to be slightly smaller doesnt it? any ideas please!

Picky

*Mod by dull old square Andrew again - and sorry, again

i used a 5mm drill then taped it out very cearfully to say the lest

I will try 5mm then and see how it goes.

Apologies Andrew for my language!

Picky

just make sure you do it slowly and use oil to lub it all while you do it and make sure it is straight as it very easy to go wrong
good luck have fun
(i ant started the charger thing yet ) will do once the convertible finished

picky wrote:I will try 5mm then and see how it goes.

Apologies Andrew for my language!

Picky

Not a problem for me, but it is for the powers that be Fair enough, really - if we're gonna play in their garden...

Hello Picky,

"6mm tap"

don't you dare, it should be 1\4" UNF. However if the previous owner has drilled them out the hole will be too large. The best bet for such cases is to buy a 'Recoil kit' which good engineering tool shops can supply. This gives you an oversize tap and an insertion tool to screw in a coiled steel 'spring' which will then accept the original sized bolt.

If you want to know more about tapping drill sizes get hold of a little booklet called 'Zeus Tables' This gives all sorts of information and data about threads, tapping drill sizes, conversion factors and more. Even Smiths used to sell them but you may now need to go to a decent bookshop.

Alec

Ive done it now, I had to drill the hole out to 7mm, and tapped it to 8mm. worked ok, except that the heater valve will need bigger holes in it now. never mind.

Im afraid I only had metric taps available! (younger generation, tut tut, etc etc)

Picky

Hello Picky,

taps are not just for new threads but are very useful to clean existing threads so a set of UNF and UNC can be invaluable. I know they can be expensive so look out for second hand but try and avoid high carbon taps, look for high speed steel ones (HSS)

Alec

Thanks for advice alec, if I see a cheap imperial set then I will grab it.

Picky

Get them very cheaply in Canadian Tire - if you have/know anyone living in/going to Canada !! Your 8mm will be fine - but you may need to use Allen Cap head screws - may not be enough room for the bigger hex head screws

I don't think 8mm or 5/16" nuts will fit with the heater tap. I would suggest drilling out to either 8.50mm for a 10mm or 3/8" UNF tapped thread (both the same tapping size drill from Zeus). Screw in a suitable sized bolt or screw tight with some Thread Lock Loctite, cut off flush with the head and file carefully flat for the gasket face.

You can now drill 2 holes, 5.50mm or 7/32" diameter in the original positions and tap the original size of 1/4" UNF.

If I don't know what size to drill before tapping , I use a nut of the same thread size and pick a drill that only just goes through.

its a good idea to get a set of BSF taps and dies as well- these can be bought very cheaply from boot fairs or autojumbles. Also worth getting 2BA sized tap for the screws on the minor that are 2BA.

Zeus tables are available on-line.

just make sure you do it slowly and use oil to lub it all while you do it and make sure it is straight as it very easy to go wrong
good luck have fun

VERY important to get the tap started straight. If you feel like the tap is even the slightest degree off from perpendicular to the surface then STOP and try again. If you break the tap you might as well throw the head away. What i like to do is get the tap started by using a drill press and that way its pretty much guaranteed to tap straight. You probably have the head bolted on the engine so you just have to be extremely careful.