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P.I. manifild: Stripped thread!


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Good evening, fellow TR6ers... as some of you might recall, this time last week I was asking for your excellent advice regarding 'weeping banjos' on my TR6 metering unit. Today, I have tackled that problem (seemingly successfully, so far...), & fitted new 'o' rings to the aforementioned banjo bolts. However in the process of re-bleeding the injectors, I've found that the thread (for the bolt that holds the injector retaining plate) in the injector 1&2 manifold has stripped. I had recut this thread (5/16'' unf) last year, as a p.o. must have had similar problems, & he (or she) had wound in a fairly nasty metric bolt. I have made a temporary repair by winding some tape made by 'Scotch' as a kind of thread lock 'round the offending bolt, but I am looking for a more permanent soloution (without having to have my manifolds re-conditioned, or even removed from the car, if possible..). Perhaps using chemical metal, or something similar,& re-cutting the thread into that, Or just opening the hole out & cutting a bigger thread (for a bigger bolt!)? Any ideas? Your advice, as always, is much appreciated,

Cheers, Paul.

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Bolts don't need to be tight.

Even without the keep plates, injectors won't fall out.

 

Ivor

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Like John says:

UNC.

This is also a much more suitable thread for the soft alloy.

Waldi

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Paul,

 

There seem to be two types of casting on the injector retaining plate bolt holes, an open hole where the bolt can protrude out of the bottom and a closed off hole. If a slightly longer bolt is used on a closed hole the bolt bottoms out and either pops the casting or strips the thread.

 

If the "bling" stainless retaining plates are used, these are thinner and the original bolt penetrates lower into the threaded hole and can bottom out before the plate is "tight" and leads to over-torquing.. If you need to do work on the castings it may be better to ensure the fixing hole is drilled through to allow for the accidental fitting of a longer bolt.

The injectors are fitted in the pi inlet manifold not the combustion chamber as are so many modern systems and the retaining plates only need to stop the injector either vibrating out or being pushed out with a backfire and need only to be finger tight'ish.

 

Alan

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Thanks chaps,sounds like good advice. My manifolds are the ones with the open hole ( and I was using a longer bolt; D'oh!!!). I cut fine threads, as the parts lists from Rimmers & Moss say 5/16'' unf.....

Cheers, Paul.

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Quote, "the parts lists from Rimmers & Moss say 5/16'' unf....."

Can't trust anyone these days.

 

New thread will do the trick, but it must be a bigger bolt, else the 'new' thread will only be where it doesn't cross the old, and will soon fail again..

8mm is a little bit bigger than 5/16, 3/8 a lot bigger.

 

Peter, you crossed out your glued-in stud idea. What's wrong, apart from the cries of consternation and dropped knitting from the originality brigade? You might have to slide the retaining plate onto the stud with the injectors as they go in, fiddly but not done often.

John

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Quote, "the parts lists from Rimmers & Moss say 5/16'' unf....."

Can't trust anyone these days.

 

New thread will do the trick, but it must be a bigger bolt, else the 'new' thread will only be where it doesn't cross the old, and will soon fail again..

8mm is a little bit bigger than 5/16, 3/8 a lot bigger.

 

Peter, you crossed out your glued-in stud idea. What's wrong, apart from the cries of consternation and dropped knitting from the originality brigade? You might have to slide the retaining plate onto the stud with the injectors as they go in, fiddly but not done often.

John

The plate has to slide sideway to free the injectors. As you say,with a stud all three would have to be lifted, or refitted, together.

Peter

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Hi Peter/John, I must say I liked the stud idea, 'til you pointed out the shortcomings; helicoil sounds like the best answer to the problem.

Cheers, Paul.

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You can use studs but the retainers would need modification to allow them to pivot.

The standard bolt is UNC. Removing the washer sometimes allows the bolt to grip the last of the good thread at the bottom, not ideal but will last.

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Hi Peter/John, I must say I liked the stud idea, 'til you pointed out the shortcomings; helicoil sounds like the best answer to the problem.

Cheers, Paul.

Paul, Try gluing in a rivnut ? Peter

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Normal engineering practice is course threads for ali and cast iron as I was taught when I was an engineering apprentice in the 1960's and have always followed this practice. As others have said use UNC only.

 

Bruce.

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Hi Paul,

To add to the discussion I had the same issue with one of my CR manifolds. A local machine shop put a heli-coil in and it has been perfect ever since and visually you would never know.

 

Best Regards, Peter

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