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METEOR/MERLIN, BROKEN CYLINDER STUDS

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Dear All,

Meteor tank engines (and later Merlins) are prone to snapping their 'corner' cylinder studs that hold the cylinder head /cylinder block onto the crankcase.  The studs appear to snap in service and not just when being tightened up.  It is thought to be a stress corrosion problem.  The studs are an interference fit in the crank case and are very difficult to unscrew even when complete and with the possibility of have two nuts secured on the top of the stud.  About 16" of leverage is required to get them to unscrew.  Unfortunately, the snapped studs can leave only a 7/16" diameter stud only 3/4" high or less! 

The engineering solution that I have devised is to drill and tap the remains of the stud 1/2" UNF Left Hand. (Thanks to TSB for that idea).  Then a left hand threaded bolt can be screwed in and the remains of the original stud will unscrew.  I found that the remains of the stud unscrewed quite easily.  This probably because the threaded remains of the stud 'relax' slightly.  If the remains of the stud does not unscrew, the replacement stud can be turned down and threaded 1/2" UNF, LH , and screwed in place with Locktite to stop it from unscrewing when the cylinder head nuts are torqued up. I made up a range of special jigs to facilitate drilling the remains of the stud absolutely square and concentric.  This is essential in case the stud will not unscrew and a turned down stud has to be resorted to. The manual makes it clear that the studs cannot be bent even slightly.

Photos of the tooling can be seen on other posts of mine on the HMVF forum.

John

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Very interesting, John. Sounds exactly like stress corrosion cracking, which is surprisingly common for high tensile bolting under a lot of stress and seeing moisture and heat. 

I guess studs were larger diameter below broken 7/16" section since you drilled and tapped for 1/2"?

What is the torque on these studs? 

Malcolm

 

Malcolm

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Malcolm,

I believe that you are right in suggesting that it is a combination of stress, moisture and heat.  All three.  The studs other than the corner studs are running covered in oil from the rockers and do not suffer from this problem.

The studs are much larger diameter where they are screwed into the crankcase 11/16 BSF or maybe bigger still.

The cylinder stud nuts are torqued to 115 ft lbs. The studs are necked down so that they are quite small dia for most of their length.  This means that the stretch and stress will remain constant even though the aluminium cylinder block has expanded with the heat.

John

 

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John,

i worked at a nuclear plant, so steam, heat, and highly stressed bolting. We had a very good metallurgical department who identified failure causes and we saw our fair share of bolting SSC.

115 ft-lb on a 7/16" stud is 140,000 psi stress in the stud. High quality grade 8 bolting has an ultimate tensile stress of 150,000 psi. So those studs are extremely high tensile material. No wonder they were susceptible to SCC. And no wonder you had difficulty drilling and tapping it! 

 

 

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Malcolm,

Not only did I have to use HSS cobalt drills I also needed to use an HSSE tap which is HSS cobalt apparently.  Even so, the tap could only manage about three tappings before it lost its edge.  Initially it was like going through EN1A with lots of lead!  Even so, if the tap can solve the problem at £35 a time it is worth it.

Most of the studs are covered in oil passing back from the valve gear.  The corner studs in question are "dry".  They are sealed at the bottom and there is a special washer at the top.  I doubt that the temp would exceed 100 degrees C.  Coatings on the stud do not appear to be successful.  I was wondering about filling the void with engine oil and grinding a groove in the top washer to relieve any pressure. That way the air would be excluded so the studs would not be in a corrosive environment..

What do you think?

John

 

  

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1 hour ago, attleej said:

 I was wondering about filling the void with engine oil and grinding a groove in the top washer to relieve any pressure. That way the air would be excluded so the studs would not be in a corrosive environment..

What do you think?

John

 

  

John,

I think that that is an excellent idea, it is certainly very hard to see that it could do any harm. The worst that could happen is that the oil oozes out and then you are no worse off than the standard arrangement.

David

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Yes indeed. I am in close touch with Peter and have  discussed this problem.  That is why I know that it is common to both Meteor and Merlin engines.  I am very keen to share the techniques that I have developed.  That is the beauty of a forum like this.

John

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6 hours ago, attleej said:

Malcolm,

Not only did I have to use HSS cobalt drills I also needed to use an HSSE tap which is HSS cobalt apparently.  Even so, the tap could only manage about three tappings before it lost its edge.  Initially it was like going through EN1A with lots of lead!  Even so, if the tap can solve the problem at £35 a time it is worth it.

Most of the studs are covered in oil passing back from the valve gear.  The corner studs in question are "dry".  They are sealed at the bottom and there is a special washer at the top.  I doubt that the temp would exceed 100 degrees C.  Coatings on the stud do not appear to be successful.  I was wondering about filling the void with engine oil and grinding a groove in the top washer to relieve any pressure. That way the air would be excluded so the studs would not be in a corrosive environment..

What do you think?

John

 

  

Cobalt drills or OK'ish but more easily broken than a standard HSS drill.

A few years ago I had to drill many holes , only abt. 3/6" dia. and then C'sk them.  I had no long lasting luck with Cobalt .  I had been off the tools twenty + years and had never used TiN.    I happened to be going to a specialist coater who did for cutting tool manufacturers (as well as gold coloured SS urinals for residents of Saudi Arabia - their motor yachts , apparently crew quarters) .   Explaining the problem - I was given a stash of misc. sizes .  No need for Rocol cutting paste (RTD)  -  a single stubby did hundreds of holes dry.  Then I used a 'gold' TIN drill to countersink.  The drills were at a production stage not marked with makers name of size.  The ones I have used since for difficult jobs are the ones grey or purple finished - I believe these must be  TiAlN Coated Drill Bits

Edited by ruxy
spelin

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18 hours ago, attleej said:

Malcolm,

Not only did I have to use HSS cobalt drills I also needed to use an HSSE tap which is HSS cobalt apparently.  Even so, the tap could only manage about three tappings before it lost its edge.  Initially it was like going through EN1A with lots of lead!  Even so, if the tap can solve the problem at £35 a time it is worth it.

Most of the studs are covered in oil passing back from the valve gear.  The corner studs in question are "dry".  They are sealed at the bottom and there is a special washer at the top.  I doubt that the temp would exceed 100 degrees C.  Coatings on the stud do not appear to be successful.  I was wondering about filling the void with engine oil and grinding a groove in the top washer to relieve any pressure. That way the air would be excluded so the studs would not be in a corrosive environment..

What do you think?

John

John, the metallurgists told me the factors in SCC were bolt material, bolt stress, temperature, and environment. You're stuck with bolt material, bolt stress, and temperature. So the only variable,you have any control over is bolt environment. Surrounding the studs with a protective layer of oil to keep moisture and halides out has to help. 

Malcolm

 

 

Edited by sexton

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The corner studs break due corrosion . Getting them in and out requires heat on the base with the banks off . There was a mod that had a seal between the bank and the block ... that made the situation worse as it trapped any moisture CB in . Best apply a thixotropic wax from time to time to prevent corrosion . 
other studs don’t have the problem as they are the return path for rocker gear oil .

I used specialized drills to make a hole and inserted a stud remover . With heat apply pressure to remover ... when temp right it will crack and come out easily . 
if you don’t use heat when replacing you will never get it down to the correct height ... bloc the to top of stud .

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There are two problems with heating up the crank case.  The first is that the EMER says to use an oven which I do not have access to.  To use localised heat is to risk distortion. The second is that even with heat the stud might not come out.  I had a scrap crank case which I could afford to heat up very hot and local to the stud.  Nevertheless, the stud still would not come out!  I also made a special tool for undoing the studs.  It consisted of a tube with a bi-hex socket at the top for engaging with the double nuts locked together at the top.  At the bottom it was hex so that I could use a large FO combination ring spanner.  This avoided the problem of the stud tending to bend all over the place under the load.

A really good point about the risk of the stud not screwing all the way down.  I will be careful.  If necessary, I have a suitable tap and something that was not available in the 1930s when the engine was designed- Locktite!

I think I will fill the corner stud voids with OC 600 worm gear oil.  If it leaks past the seals it will only do so slowly.  I will grind a pressure relief slot in the washer at the top which will also allow the void to stay 'topped up' with engine oil.

 

John

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John, the guys at the nuke plant used to bring in an electrical discharge machining contractor when they broke a stud on a critical piece of equipment. Don't know the price but it didn't seem to be too expensive. 

Malcolm

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Atleej 

I and my friend have done quite a few engines. We use Oxy on the casting bulge inside the crank case . While one is applying pressure to the stud extractor the other should heat the crank case. It does not require so much heat so that it would damage the crankcase or change the alloy properties. 

You will need heat to refit the stud or you will never get it down. If you use lock tile the next poor soul will have even more trouble than you getting it out. 
 

No need to seal it with anything as the corner studs are not a return path for oil . If they are left without anything but a little thixotropic wax of some sort ( Ardrox) they are easy to have a look at on a regular basis. 
I you grind a slot then the oil on the rocker gear will leak out through the corner studs...I don't think that is a good idea. Best just paint the studs and brush them down with  preservative. 

Seriously when you get the temp right they crack and come out easily . 

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