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Stripped Centre Main Thread!


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#16 mike.

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Posted 14 June 2009 - 08:43 PM

Just been watching some helicoil how to videos on youtube. it doesn't look too bad to do yourself. Although i'm not sure.





Do helicoils come in longer lengths? Because the coils you get in the kits look about 5-7mm long, whereas the main cap thread is quite a deep thread...


Just looking incase the helicoil repair from a machinist is rediculously pricey...

Edited by minimadmike, 14 June 2009 - 08:43 PM.


#17 MRA

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Posted 14 June 2009 - 09:33 PM

No you use the same size of bolt..... but how will a helicoil fit in your block ? you will need to drill oversize and tap to a helicoil thread size which will be bigger :(

#18 Shifty

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Posted 14 June 2009 - 09:59 PM

Just to be clear......


Its the longer bolt that was supplied with the centre strap that has stripped?

Chances are that its the bolt thats knacked not the actual thread in the block, stick up a pic of the bolt and the threaded hole in the block that you think is suspect.

#19 mike.

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Posted 14 June 2009 - 10:37 PM

Right well i'll take some picks earlyish tommorow.

Yep, its the longer centre main bolt that has stripped. But just by looking into the block, its clear that theres a large area of thread stripped out of that too.

As you look down the caps dowl, the first couple of threads are there but look damaged, then theres an area where 4-5 threads are missing, Then deeper into the hole, you can see the rest of the threads look ok. So i'm basically missing the first load of thread.

I haven't got a tap the right size, but by looking down the hole with the torch, it looks like most fragments of thread came out with the bolt anyway. Like i said, when i pulled the bolt out it had bits of thread from the block stuck to the oil I put on prior to inserting them.

#20 Cooperman

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Posted 15 June 2009 - 08:36 AM

You need a pillar drill to bore out for the Helicoil, as it is vital that you go in square with the block face.
It might be an idea to speak to the manufacturers of the Helicoil for their advice. I find that the technical support departments of companies extremely useful when you want to use their products for a high-spec application. The Helicoil suppliers could give you the contact details.
It really is your only straightforward route. The alternative could be to drill and tap the block to a much larger size, then fit a machined solid insert threaded on the outside, weld that in place around the top, grind back locally, drill & tap in exactly the right place with a jig-borer for the bolt then counterbore for the locating dowel. Now, that would work, but be very expensive as you would need a precision engineering shop.
The only question I have is whether the tapping drill for the helicoil is a larger diameter than the diameter of the locating dowel. If it is, you would have to have the locating dowel counterbores in both block & cap machined out and fit a larger diameter location dowel which has to be very accurately done.

#21 mike.

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Posted 15 June 2009 - 08:48 AM

Yeah, i thought i'd need a pillar drill. I don't really fancy doing it myself anyway.

I don't think the drill is bigger than the dowel, the drills just there to clear out the remains of the old stripped thread.

I'm going to ring the headshop in a minute to get a price...

#22 mike.

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Posted 15 June 2009 - 08:56 AM

Right, just rang the headshop, he sounded abit unsure and said he's never helicoiled anything like that before. But said that doesn't mean he can't be done.

So i'm going to take the block down to him today and see what he says.

He said if its all ok to do, it'll be £30 + VAT so the sort of price i expected/wanted to hear :thumbsup:

#23 Cooperman

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Posted 15 June 2009 - 10:14 AM

I can't check that the drill will be OK as it's not there just to clear the old thread, it's the correct size for the tap which puts in the new thread for the insert, whatever size that is. As I don't have a block here in bits at the moment, I cant measure the dowel hole diameter, which is what is important. For what it's worth, I think it'll be fine.
It takes about 1/2 an hour to set up and drill, tap and fit the insert.
I also think that longer inserts are available than the ones you get in the off-the-shelf kits. If I were you I would make a few calls to Helicoil suppliers to check this out in view of the application. It's 7/16" UNF by the way.

#24 Ethel

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Posted 15 June 2009 - 10:30 AM

A quick nose on the web suggests UNF Helicoils are "next size up" so you'll be looking at 1/2 inch clearance & a 29/64 tap drill.

Sounds like a goer as the dowel must be more than 1/32" thick. :thumbsup:

#25 Cooperman

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Posted 15 June 2009 - 03:06 PM

Actually it doesn't matter if the drill is slightly larger than the intyernal diameter of the locating dowel, so long as there is still a slight step to keep the dowel in its correct location after fitting the Helicoil and as you say, Ethel, there almost certainly will be. If I had a 1275 block in pieces this week I would go and measure, but I haven't. The only one I have is a Mk. 1 Cooper 'S' which has studs, not dowels and bolts - a much better arrangement IMHO.

#26 mike.

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Posted 15 June 2009 - 07:00 PM

Right well, the latest news.

I've been down to headshop UK, great place, the guys there are very clued up. Also saw pictures of sprokets mini up on the notice board...

He took a look at the block, and said it could be helicoiled. He asked if I was fitting a strap and if I had the bolts i was trying to use. I brought out the centre main strap and the stripped bolt. He took one look at the bolt and said 'your not using these are you' Then said 'they're not long enough, no way'

He said that the bolts i've been given with the centre main strap are way to short. He said once they've gone through the cap, the strap and the dowel on the block they only give about 6mm thread engagment. As seen in this picture below:

Posted Image

So he said, since these are poor quality bolts, he said theres no way so little threads could take that much torque and thats why its stripped both the bolt and the block. He measured the depth of the thread in the block at 42mm! So he said the bolts i've got should have another 30mm of thread at least!

So basically, i've tryed to put 63ft/lbs of torque on about 4 threads. He was shocked that such small and poor quality bolts had been supplied for such an important part. He said i should have some bolts that have about 40mm of thread after the dowel in the block and should have washers, with a recess machined into the strap for the washers to sit in.


Anybody else have bolts like the ones i've got with there centre main straps?


Anybody know how long those bolts i've ordered from america are?

Edited by minimadmike, 15 June 2009 - 07:04 PM.


#27 mike.

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Posted 15 June 2009 - 07:24 PM

Hmm. Just looking at the item picture for the high grade bolts i've ordered off ebay.

From the picture they look nothing better than the ones i've got here. Like ethel has said, If these bolts have no deeper thread engagement, whats to say they're not just going to rip out of the block like these last ones have done? If they're no longer than the ones i've got, they're going to be no better.

Seriously regretting even bothering with this centre main strap >_<

#28 MINI 1

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Posted 15 June 2009 - 07:35 PM

dont worry , it will be fixed in the end >_< at least you can fix it !

Edited by MINI 1, 15 June 2009 - 07:36 PM.


#29 mike.

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Posted 15 June 2009 - 07:38 PM

Well yeah i suppose thats one way of looking at it, it could be alot worse.

But still, i could of have most of the engine built by now, but instead, i'm going to be waiting weeks for some bolts from the US... I hope they're worth the wait!

#30 stardude

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Posted 15 June 2009 - 07:49 PM

ive ordered little things from america before and most of them take about a week or less, as its airmail




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