There is not enough alloy material in the trailing arm to install 3/8 UNC stepped studs with heli-coils. Just enough to use Heli-coils with 5/16 UNC studs. My stepped studs go straight into the alloy.
Cheers Ian A
Cheers Ian A
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I have not studied my trailing arm castings and the last time I had one apart was my TR5, but if the holes are badly corroded has anybody re drilled them mid way between the existing? or had the original holes ally welded then re drill?
just a passing thought.
PhilComment
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You do need to reread the previous posting made in post #11Originally posted by stag8manod View Post
"It is good engineering practice to repair the threaded holes in the trailing arms, either by helicoiling to accept 5//16th UNF standard studs ( they have 5/16 UNF threads on both ends ), or now favourite practice ( well it is with me) is to uprated the holes by tapping to accept a 3/8UNC parent thread. Happily there are 3/8UNC X 5/16UNF threaded studs which will allow the hubs to retain their clearance ( only about 5 thou per hole) on the 5/16th ends allowing standard fitting procedures"
The clue is in the wording "either" which points out that the two options are to Helicoil at 5/16 UNF retaining standard studs or to increase the stud size to 3/8 UNC but tap it as PARENT THREAD ie direct into the material of the trailing arm. As pointed out in the same post the 5/16 UNF threaded original holes when they fail leave a not surprisingly dimensioned hole of ...err 5/16 dia (or .312) which happily is the tapping size for 3/8 UNC.
If you review the 5/16th UNC Helicoil dimensions that's overall dimensions is BIGGER than the 3/8th UNC, even requiring a .328 tapping size drill (which removes MORE core material from the trailing arm as against the .312 tapping size drill needed by the 3/8 UNC tapped PARENT THREAD (ie tapped into the trailing arm direct).
Bottom line is the 3/8 retains more core strength AND more material in the holes than a Helicoil at 5/16 dimension, that's why I said it's counter intuitive, it don't seem possible ...but it is. As said previously the reason is the double thread required in the Helicoil an inner AND an outer thread which requires a greater dia than the single dia 3/8th UNC thread...I hope that clears up the understandable confusion.
Micky
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Hi Phil,Originally posted by Phil S View Post
If you reread my post #11 you'll see I mention
"Also the studs are threaded into small square blocks located in the casting unhappily not always accurately and it's not unusual to see the threads breaking through the sidewalls in some areas."
What I didn't mention was the guy in the foundry who put these blocks for the studs to the rear of the hub mounting face into position in the mould where these trailing arms were cast was a YTS new starter of about 3 months standing and he was DRUNK. Hence they are skewiff on the pi$$ and barely in position on some arms I've seen, because they vary greatly it suggests the arms were made in relatively small batches and the mould and it's blocks for the stud retention placed in individually.
Because these blocks are behind the hub mounting face badly positioned,they are barely big enough to take Helicoils or increased dia studs used for greater purchase. Also because these studs go through the hub mounting face into the positioned block behind in 6 positions there are no other blocks or material behind that can be utilised by rotating the hub and redrilling through, so the PCD of the studs have to remain where they are.
I've never investigated welding and redrilling the holes back on the same PCD but would be wary of introducing "hard" areas susceptible to vibration (plenty there) and the hammering effect (even if damped by being clamped by studs) of the road wheels which could cause localised fracturing.
MickyComment
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I replaced drive shafts this year and am also a non mechanic but willing to learn. My problem was iffy U.J.s with staked joints more than wheel bearings but Robsport did a good deal on fully re-conned units that I just switched over, My problem was getting the old shafts out after undoing the flange bolts. Had to thump hard on the inboard end of the shafts both sides to split the hubs from the castings but there was no corrosion. One point. You do not have to wire up both halves of the shafts as per the Haynes Manual as the spline is keyed to only fit one way. Much easier to separate the two halves in my humble opinion. Luckily I had help and advice from a more experienced Stag owner . JohnOriginally posted by ipc View PostComment
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Just to re-enforce Micky's point opening point, the rear hubs and front stub axles are a vital safety component and worth replacing with quality non-fatigued metal, IMO, such as CDD units. Plus decent tyres and brakes.
Mk 2 1975 TV8 Mimosa
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Thanks for all the input and advice. I’ve spoken to Alisdair at CDD and plan to buy their
uprated CV drive shafts and uprate the studs as per Motorsport Micky’s advice and convincing argument.
This’ll be a job for the New Year so hopefully it will go as smoothly as people suggest - otherwise I’ll be asking for more help!!
Merry Christmas to all!Comment
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If the tapped holes in the trailing arm are corroded/damaged I see no problem in dressing them back and have them tig welded by a competent welder. First make a drilling jig to retain PCD and square.
Cheera Ian AComment
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