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Hi All

Planning ahead to some winter maintenance, I am going to machine a jig to drill and tap for helicoils in the trailing arm, to hub flange.

I have seen the figure of 4.25" quoted for the pitch circle diameter, PCD. of the 6 studs.

Can anyone confirm this figure?

 

Thanks.

Paul Bond

 

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Hi Paul,

I have already done this . You may borrow my jig if you wish.

I also have the Helicoil 5/16 UNF kit if you need it.

 

There is discussion as to the best way to go -

1 - stay with the original 5/16 UNF.Helicoil They lasted 40 odd years and with the helicoil should last a good 50 years more.

2 - Go to 5/16 UNC Helicoil - would normally make sense in Ali Alloy but the UNF lasted very well.

3 - Go To 3/8 UNC - also makes sense but the backing metal is now getting thin.

 

Where abouts are you.

 

Roger

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Roger.

 

 

Thanks for the kind offer.

 

It will probably be a protracted period before I get to do both sides, so making a jig will be more convenient.

I will probably go for UNF helicoils, although in theory UNC should be more appropriate for alloy.

The option of larger theads or Keenserts is probably not appropriate given the amount of material surrounding these studs.Having used Helicoils and Timeserts in the past, l have found Helicoils as strong if not stronger than Timeserts and much simpler to install.

 

I was not impressed with the quality of some replacement studs available. do you know of a source of decent quality items?

 

Paul

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Hi Paul,

I will measure my jig tomorrow and post the dim's.

 

There has been quite some discussion about the various threads to use.

 

I still believe that a 5/16UNF helicoil is far stronger than the original thread direct into the Ali.

 

You can still borrow mine if you wish. I have no further use for it - famous last words.

 

TR the TRShop for the studs.

 

Roger

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"The option of larger theads or Keenserts is probably not appropriate given the amount of material surrounding these studs"

 

Paul, a 5/16th UNF Helicoil requires to be drilled .328 thou before tapping to receive the Helicoil, I believe a Keensert or Timesert (other brands area available) also require a dia hole of about the same dimensions.

 

Counter intuitively a larger 3/8" UNC thread form tapped directly into the parent alloy trailing arm only requires a .312 dia drill or even no drill as the 5/16th (.312 ) stud when it strips removes all the material and you can just tap the 3/8th UNC thread straight into it.

You will require a dual dia stud with 3/8th UNC on one side and 5/16th UNF on the other to use the hubs with their standard diameter 5/16th holes. These are available through Classic Driving Developments http://www.classicdrivingdevelopment.co.uk/or you could have some made up yourself. The varying diameters and threads need blending carefully to avoid any stress raisers and attendant problems if not done correctly.

The 3/8 studs against the 5/16 studs can be made longer on the trailing arm side allowing them to be drilled deeper into the trailing arm parent metal, giving a useful .3 or .4 more thread than standard which bed into virgin non drilled or tapped material.

 

It is important the drilling and threading of the trailing arms are carried out correctly at 90 deg in two planes. The holes in the hub for the studs are a close fit upon their shafts and using a pistol grip to drill freehand ( I am a time saved engineer with no compunction about using pistol grip drills where necessary) and then tapping freehand it's very easy to lean some of the studs which are accentuated by any lean on the other studs (we are talking max 5 thou here) such as to prevent the hub from being fitted. Drilling out the hub holes larger to allow leaning studs to pass through introduces a bending moment into the stud shafts, which is to be avoided even though they are only tightened to a wussy 16 lb ft. So using a jig is an excellent idea and will allow a "proper job" to be done here.

If you use the search box (always a good idea to review the previous thoughts and methods) you'll find quite a few threads with posts on this.

 

PS: PCD is 4.250 Dia

 

Mick Richards

Edited by Motorsport Mickey
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Roger, Mick

Thanks for additional info.

I have a Bridgeport milling machine in the workshop, however the trailing arms are an awkward shape to set up to machine, so as you note Mick, I will need to ensure the jig I use, supports the drill correctly.

On your comments,Roger, re strength of Hellicoils.Whilst not in alloy, I was interested in the strength of helicoils for cylinder head studs. The reason was, I fitted ARP studs who gave a torque setting of 80 ft,lbs somewhat, greater than standard, plus this torque setting is with their moly lubricant, which will increase the tensile load on the stud significantly and hence thread in the block.

I tapped a block of mild steel stock with a standard 7/16 UNF thread and a 7/16 UNF Helicoil

Using some original studs and nuts with a hardened washers under the nuts, I replicated the thickness ofthe head.

With a liberal coating of ARP lubricant the studs were torqued up in stages to 80 ft,lbs on dismantling there was no sign of problems. The studs were re lubricated and taken to 110 ft, lbs again no problems on dismantling.

As the torque wrench I was using only went to 110. I used a socket extension and applied a further 180 degees to the nuts, at this stage there were signs that the studs were stretching.

 

A further 180 degrees was needed to cause failure. Both studs failed just under the nuts with a typical "cup and cone"tensile failure.

Both the threaded holes showed no damage with the broken studs being able to be removed by hand

 

Whilst the mild steel is not the same as the cast iron of the head, I think it demonstrates the strength of helicoils.

 

Paul

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Roger, Mick

Thanks for additional info.

I have a Bridgeport milling machine in the workshop, however the trailing arms are an awkward shape to set up to machine, so as you note Mick, I will need to ensure the jig I use, supports the drill correctly.

On your comments,Roger, re strength of Hellicoils.Whilst not in alloy, I was interested in the strength of helicoils for cylinder head studs. The reason was, I fitted ARP studs who gave a torque setting of 80 ft,lbs somewhat, greater than standard, plus this torque setting is with their moly lubricant, which will increase the tensile load on the stud significantly and hence thread in the block. this is actually lower than standard. The use of lubricant may well equalise this to the standard dry set up

I tapped a block of mild steel stock with a standard 7/16 UNF thread and a 7/16 UNF Helicoil

Using some original studs and nuts with a hardened washers under the nuts, I replicated the thickness ofthe head.

With a liberal coating of ARP lubricant the studs were torqued up in stages to 80 ft,lbs on dismantling there was no sign of problems. The studs were re lubricated and taken to 110 ft, lbs again no problems on dismantling.

As the torque wrench I was using only went to 110. I used a socket extension and applied a further 180 degees to the nuts, at this stage there were signs that the studs were stretching.

 

A further 180 degrees was needed to cause failure. Both studs failed just under the nuts with a typical "cup and cone"tensile failure.

Both the threaded holes showed no damage with the broken studs being able to be removed by hand

100Lbs/Ft + 180 degrees lubricated is a very high torque setting - no wonder the studs failed. Good result for the threads tho'

 

Whilst the mild steel is not the same as the cast iron of the head, I think it demonstrates the strength of helicoils.

 

Paul

 

That is why I always state that there is no need to go to UNC or 3/8" is using helicoils.

The original 5/16UNF lasted more than 40 years without problems. With helicoils they may last the next 100 years

 

The holes can be free hand drilled BUT you MUST use a rigid/fixed guide.

 

Roger

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If you buy the CDD Studs they will also hire you a Jig to do the job properly,tried and tested and cheaper than making your own for something you’ll only use once.

As Niall says if you buy the CCD studs they'll loan you the jig http://www.pattonmachine.com/TAK.htmto drill out the stud holes in the trailing arm (see the website, the jig attaches to the arm) unless of course you just fancy doing some machining for a one time use.

If fitting a Helicoil the outer thread is of course larger than the inside thread and also coarser which is why it's thread root thickness is more and it has a greater resilience to torque and stripping. They certainly work on Rolls Royce truck engines which had them fitted from new and put out 350 hp upwards with no problems of pulling or stripping.

 

Mick Richards

Edited by Motorsport Mickey
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Hi Mick,

because of the angle between the stud thread and the helicoil thread the possibility of stripping the thread is greatly reduces.

As the stud is pulled upwards under tightening it is forcing the helicoil outwards and making it grip better.

 

In soft material (Ali, Cast Iron etc) you can have far greater torquing figures than otherwise would be obtainable.

 

Roger

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Good read above, you fellow engineers????.

I would also use helicoils to maintain 5/16 UNF.

What a pity mine are not broke!

Waldi

Edited by Waldi
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Just (from memory) to add that that CDD loan two jigs. One for drilling and one for the tapping pocess. They are not like the Patton ones but they make the job very straightforward.

Edited by peejay4A
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The tapping process is even more difficult to keep at 90 deg in two planes, given that most people do this with it on the car, ssooooo easy to stray from square.

I've two trailing arms bought cheaply because of angled helicoiled studs fitted I'll reclaim by doing it properly.

 

Mick Richards

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  • 2 weeks later...

Hello Paul

Further to comments above re CDD studs.

I uprated the studs on the rear hubs of my TR4A a little while ago. I went to Classic Driving Developments - a small engineering firm based in Shipston on Stour Warwickshire. They not only have the uprated 3/8 UNC studs but will hire you the 2 jigs - one for drilling, one for tapping, and the drill and tap so you can do the job in situ on the car and get holes at right angles to the face. They also provide the lock tight for the studs. The link to the firm is below. What you should have is also a torque wrench that reads low enough for torquing up the nuts when refitting the bearing carrier. Do not exceed the recommended 12 to 14 ft lb and you won't strip out the aluminium. I know this doesn't sound enough but that's the torque setting in the workshop manual (not the Haynes). CDD were really helpful and the job went very well. Oh one final tip - make sure you don't drill initial hole too deep!

 

http://www.classicdrivingdevelopment.co.uk/index.asp

 

Keith

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I really believe the threat of a crooked hole is a little overblown, especially for one with some experience with power tools. If buying a jig were my only option, I'd do it by hand. OTOH, a jig is pretty easy to make. Here's mine.

 

Ed

post-12543-0-27282200-1508945989_thumb.jpg

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Interesting contradiction there but as has been said above, you dont need to buy a jig. Either borrow Rogers or use the CDD one loaned on purchase of their stud upgrade kit.

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I really believe the threat of a crooked hole is a little overblown, especially for one with some experience with power tools. If buying a jig were my only option, I'd do it by hand. OTOH, a jig is pretty easy to make. Here's mine.

Ed

Don’t think you’d make that to handy with basic Tools.
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I agree Ed, the drilling isn't so much the problem, I guess you missed the other post

 

"The tapping process is even more difficult to keep at 90 deg in two planes, given that most people do this with it on the car, ssooooo easy to stray from square"

 

On 6 holes which are a tight fit on studs on converging centres (ie a PCD) it's VERY easy for an owner who's tapping the holes with the trailing arms fitted on the car to "lean" the tap which causes an interference with the hub. ie 3 thou lean one way meets 3 thou lean diametrically opposite...you ain't getting the hub on, I'm afraid we are not all engineering gods !

 

Mick Richards

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Mick--

 

I agree it may be an acquired skill, but I believe even tapping can be done straight with a sharp taper or plug tap. The tap wants to follow the hole, and will unless it's pushed off course.

 

If there is any doubt, use the jig.

 

Ed

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Ed,

 

I'm a time served engineer, you can't shake a stick at the number of holes I've tapped, too numerous to count.

 

I had no hesitation at making a jig for myself when contemplating redrilling and retapping not just one but complete sets of holes in each trailing arm. Not worth messing an arm up by having a threaded hole go in off square, between the two arms there are 12 of them ! I fear encouraging a "gung ho" attitude in the less skilful will lead to hub flanges with studs leaning like drunks, only a few thou out in any direction and the hub won't go on.

That then leads to opening out the hub holes to allow fitting the hubs which then inputs a bending moment onto those leaning studs which leads to possible failure points, it's wise to err on the side of caution here.

 

Mick Richards

Edited by Motorsport Mickey
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