tap new thread in cylinder head?

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Ardenfour

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The third hole which takes a thermostat housing bolt on my MD1 has corroded and said bolt no longer grips. I plan to drill the hole and tap an M8 thread for a new bolt (up from the original M6) The remaining metal thickness is about 5mm and opens into a water channel. I'm confused about what kind of tap I need - a quick look on ebay shows second tap, spiral, tapered etc - can someone advise on the type required please? Thanks
 
Ardenfour said:
The third hole which takes a thermostat housing bolt on my MD1 has corroded and said bolt no longer grips. I plan to drill the hole and tap an M8 thread for a new bolt (up from the original M6) The remaining metal thickness is about 5mm and opens into a water channel. I'm confused about what kind of tap I need - a quick look on ebay shows second tap, spiral, tapered etc - can someone advise on the type required please? Thanks
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Tapered gives a very gentle start to the cutting. Second cut is tapered, but not as much as tapered. Plug gives a thread almost to the bottom of a blind hole.

It's best to go taper -> second cut -> plug but if you can't and are careful second cut -> plug will probably work. Plug on its own almost certainly won't.

Have you thought of using a helicoil? The recommended drill size for M6 is 6.3mm, just a wee bit less than 6.8mm, which is the tapping drill size for M8.
 
Thanks for the info, I'll try the tapered. The last 4 yrs had the housing and hole secured with epoxy putty, last season sealed with blu-tac, which worked when cold but went soft when hot...so any kind of bolt will be a big improvement
 
Just to clarify the tapered tap is good when you can pass the tap right through the materia and clear out the other side so that the proper thread cut at the base of the tap cuts where the bolt will sit. My guess is that your hole is a blind hole. So unless the hole is very deep compared to the depth of the bolt you will need a plug tap to cut right to the bottom of the hole as far as the bolt will reach. So if I am right you will need both a taper and a plug tap. good luck olewill
 
I'm with Jumbleduck. A thread insert would require far less drilling (basically, you just take out the 'peaks' of the old threads, which by the sound of it have pretty much gone, anyway). I'd suggest Recoil rather than Helicoil: simpler and cheaper. Kits are freely available on-line.
 
Use a thread replacement system, Helicoil or whatever I always think it better to stick with original sizes if possible.
 
William_H said:
Just to clarify the tapered tap is good when you can pass the tap right through the materia and clear out the other side so that the proper thread cut at the base of the tap cuts where the bolt will sit. My guess is that your hole is a blind hole. So unless the hole is very deep compared to the depth of the bolt you will need a plug tap to cut right to the bottom of the hole as far as the bolt will reach. So if I am right you will need both a taper and a plug tap. good luck olewill
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Agree with above, don't use an insert (bodge) unless you have no other option.
 
Also bear in mind that if you DO decide to go with the 8mm bolt option, make sure the mating part that the bolt passes through and holds to the cylinder head, has enough "meat" round it to allow it's hole to be opened out to (say) 8.5mm AND that there's enough space round it to get the larger-headed fastener (an maybe a spanner or socket on to it).
 
macnorton said:
don't use an insert (bodge) unless you have no other option.
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A thread insert is far from a bodge. In fact it's the thread of choice for high loads in softer metals such as aluminium. In the marine environment it's also the only truly reliable means of preventing corrosion between steel fasteners and aluminium. (I appreciate, of course, that the OP is probably tapping into steel, where either a larger thread or an insert should be structurally sound.)
 
macd said:
A thread insert is far from a bodge. In fact it's the thread of choice for high loads in softer metals such as aluminium. In the marine environment it's also the only truly reliable means of preventing corrosion between steel fasteners and aluminium. (I appreciate, of course, that the OP is probably tapping into steel, where either a larger thread or an insert should be structurally sound.)
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+1
Used extensively on Porsche and many other high performance engines, if it's good enough for Mr Porsche it's good enough for our engines
 
I too was surprised too hear an insert is a bodge but in engineering there are many chains of thought, which is why I often refrain from contributing to these threads as it can often lead to information overload for the OP. This thread being an example.
 
If anything, they're a more expensive manufacturing option. Putting a steel fastener straight into a tapped hole in aluminium is the "bodge" really! It's widely done because it's cheaper / faster and for most applications "does the job".
 
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