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Daimler 15: stripped thread

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captain bobo
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Re: Daimler 15: stripped thread

Post by captain bobo »

The photo shows what greeted me after I removed the cylinder head today. It looks like there was a helicoil already in use which failed. It doesn't look long enough to my inexperienced eye.

The challenge now is to find out what size and length thread insert I should use given that the stud hole in the block will have been enlarged to accept the (now failed) thread insert coil.

The thread on the stud is 3/8 inch BSF. Can anyone tell me what most likely size the hole will have been enlarged to?

I've never had to install a thread insert coil before, so all comments and recommendations are very welcome!
Attachments
IMG_20210815_180028_3.jpg
Best wishes, Dave B

Marcel Renshaw
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Re: Daimler 15: stripped thread

Post by Marcel Renshaw »

The thread inserts have their own tap. You might be able to just screw in a new insert. Preferably you need to aim for a length of 1.5 times the diameter of the thread you are repairing.

NickDeAth
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Re: Daimler 15: stripped thread

Post by NickDeAth »

That picture looks more like the stud has pulled the first 2 or 3 threads out of the block. You can't easily break a Helicoil and if they come out they are usually like a stretched spring. Helicoils are made out of what I would imagine is something like spring steel.You need to buy a 3/8 BSF Helicoil kit which usually have a variety of lengths of insert in the kit.

Nick
"Nick - do you think you will ever put that old car back together again?"

Rolf B.
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Re: Daimler 15: stripped thread

Post by Rolf B. »

I only can agree Nick, this is not a Helicoil. It looks like the original threads of the block. The material of Helicoils is a lot more thick than than this string is.

Rolf

Chris_R
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Re: Daimler 15: stripped thread

Post by Chris_R »

Nick is correct. You need a 3/8" helicoil kit. The supplied drill and tap will be some size larger than 3/8" and drill out the hole large enough to be tapped for the helicoil to be screwed in but the result after you finish will be a 3/8" screwed hole into which your bolt will screw. Make sure you buy the correct 3/8" TPI (threads per inch) kit. UNC is 16, UNF is 24, Whitworth is also 16 but a different thread angle, BSF is 20, also a 55 degree angle.

Stan Thomas
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Re: Daimler 15: stripped thread

Post by Stan Thomas »

I doubt the material clinging to the failed stud in your picture is the original thread from the block, as the block is cast iron - which does not form into a "wire-like" spiral if stripped.

You might be able to remove the old helicoil insert with a helicoil tap if that be the case, but the resolution might be more difficult if not.

What you will need to do is turn a "stepped" stud of the correct diameter for the cylinder head nut etc, but with an enlarged diameter to accept a similarly enlarge threaded hole in the block, or even the "next size up" helicoil for which you could dimensionally consider a metric size.

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captain bobo
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Re: Daimler 15: stripped thread

Post by captain bobo »

I removed the "spring" from the stud and took this photo. I guess that it's a threaded insert judging by the consistent thickness and the cleanliness of the edges.

I've examined the hole in the block as closely as I can (strong torch beam and magnifying glass); the thread looks to be in good order.

When I screw in the head stud it turns with almost no resistance and there is very slight movement when fully screwed in which suggests to me that it has already been resized to accept a threaded insert such as a helicoil.

Can anyone suggest anything else I might consider and check before I buy and try to fit a 3/8 inch helicoil?
Attachments
IMG_20210816_182133_2.jpg
Best wishes, Dave B

Stan Thomas
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Re: Daimler 15: stripped thread

Post by Stan Thomas »

Your almost home and dry!!

Given you have identified that the block had been helicoiled before and you have removed the old insert, you merely have to put in a new insert - and given the stud was passing through the cylinder head means the position of the old insert was dimensionally correct.

So, first measure the depth of the hole in the block, as that will be the length of insert to use (and you can cut them down). Now all you need to do is verify the thread size - which I believe has been verified by others to be 3/8 B.S.F. - which is 20 tpi. (threads per inch).

Has no-one on the forum got a helicoil kit that size, as all you need is one insert which you can buy seperately, rather than the costly kit which includes drill and tap etc. In fact, if you bought just the insert you could make the tool which allows you to screw it into te block.

Stan,

Rolf B.
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Re: Daimler 15: stripped thread

Post by Rolf B. »

I don't believe the removed "spring" is part of a Helicoil because Helicoils are made of a round wire and of thicker wire material than this one. And this removed "spring" wire is flat shaped at the outside.

Rolf
Attachments
tempart2-b7211-25.png
tempart2-b7211-25.png (106.39 KiB) Viewed 464 times

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captain bobo
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Re: Daimler 15: stripped thread

Post by captain bobo »

Rolf, not all thread inserts are round in cross section: this extract from the web page of a UK based thread insert supplier explains that theirs are diamond shaped cross sections (the article goes on to explain why this cross section gives better performance for many engineering applications). I may be wrong, but I'm pretty sure what I removed from the stud was a thread insert with a non-round cross section.
Attachments
STI.jpg
Best wishes, Dave B

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