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Stripped thread in alloy head

Submitted by chas_goupillot on Mon, 03/02/2014
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Heavy Twins

Hi Guys,

One of the 3/8" studs into the alloy head of my Mercury has stripped. What thread form is in the head (3/8" Whit?). Engine No. is 18SS/129215/P.

Regards,

Chas

Submitted by chas_goupillot on Mon, 03/02/2014

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Previously Gordon Johnston wrote:

Correct 3/8" Whitworth. Gordon.

Thanks Gordon, will get some Helicoils ordered Chas

Submitted by chas_goupillot on Mon, 03/02/2014

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Hi Again,

Checked thread repair kit suppliers all reference 16TPI for 3/8" Whitworth. Just checked the stud that was removed - appears to be approx 22TPI as for as I can ascertain with a ruler. The mystery deepens! Chas

Submitted by gordon_johnston on Mon, 03/02/2014

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16 TPI is UNC and WW, 20 is BSF, 24 is UNF. Could the fact that the stud has stripped be due to the fact that someone has forced the wrong threaded item in? Not altogether unknown. Buy a new stud and then get the helicoil to suit - can't go wrong then!

Submitted by david_cooper on Mon, 03/02/2014

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The stud should have a coarse Whitworth thread on one end (to enter the alloy head) and a fine BSF thread on the other (to take the steel nut). If yours has the same thread both ends then that's your problem.

Submitted by chas_goupillot on Mon, 03/02/2014

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Removed the other 2 studs to check.They are boththe same as the stripped stud (ie approx 22 TPI). Thecorrespondingthreads in the head are undamaged so studs and tapped holes match. The "nut " end of the stud has approx 28TPI. All very strange! I take Gordon's point that if I buy new studs, which I intended to do anyway, then I can buy Helicoils to suit the new studs.

Maybe strange head studs are Matchless/AJS items? Engine was built in 1968 at Plumstead.

Contacted Norvil :- for reference, Head studs are 3/8" BSF.

Regards Chas

 


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