Cautionary Note for New Head Studs, associated Dome Nuts, and head gaskets on 3 1/2 litre pushrod engine

The chromed “acorn” nuts for securing the cylinder head on the 3 1/2 litre engine are known to have some variation in their internal thread depth. Too short a thread depth can lead to the nut bottoming inside the head before torque specification is reached.

Today I put the head back onto my 3 1/2 litre Mark V engine using new head studs as supplied from Alan Gibbins and Worcester Classic Spares. I am grateful to both for the supply. Having received the head studs, I bought extra and checked them for Young’s Modulus and to what torque they stayed within elastic performance. Both the long and short studs supplied remained elastic to 90 ft.-lbs torque.

The short studs:
C.339 Stud, securing Cylinder Head Plate No. H.24 No. per Unit. 6
all had the correct BSF 7/16"x18 TPI threads on both ends. These studs, however, all had slightly too large threaded diameter which required me to run them through a die on the short-thread end four times each to achieve hand tightening capability in the block. These short bolts were all a blackened color, but as the long-thread end was fine and the short-thread end has too much material in the threads, I suspect a die-process difference between the two ends.

The long-thread end of the C.339 worked fine without changes for the nuts. The long studs:
C.340 Stud, securing Cylinder Head Plate No. H.25 No. per Unit. 8
all fit fine for thread. These were all shiny steel color.

Now, on fitting the studs to the block, the studs were set to fully in the block and then turned back one quarter turn. The head then was placed on using a new gasket. Of course, over the years people have used different thickness gaskets, as have I. The present gasket may be a bit thinner that what Doug Nantes had provided me for the last usage.

With the new gasket, three of the “acorn” nuts:
C.341 Nut, Dome, for Cylinder Head Studs Plate No. H.27 No. per Unit. 7
bottomed out internally when merely hand tightening. I did not measure the length of all the new short studs. However, the combination of new gasket and new studs led to three of the acorns bottoming out internally at merely hand-tightening. I don’t want to use nuts where the final torque figures are driven only by torsional stud action different from intended forces. I changed the three questionable acorn nuts to spare “non-acorn” nuts (the ones found under the rocker cover) and proceeded satisfactorily in my mind. The external appearance of these nuts will not meet concourse judge standards but my engine now is torqued up correctly on all head studs.


Its not just a problem with Acorn nuts for those motors either

I had this issue with a 4.2L engine, and the threaded internal depth of the new nuts varied by ~1mm = .040"

Enough that at least one didnt bottom out at all, and the torque on others would be suspect

Kudos to you for checking carefully, and no assembler should take it for granted !

I got lucky to notice that one on mine, take them all back off and measure

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Same problem on a 3.4 years ago. Undoing head nuts, a few came out with the studs. There was no corrosion and the seizure was due to the nuts bottoming on the stud. I then did a non-tightening dry run of all nuts to check they would all run down to the head without washers. About four or five failed so I cut the threads in the nuts deeper with a tap. Yes, it is a trap for the unwary, and I’ll be checking from now on, all similar applications where a capped nut is used.

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We should note to use what is called a bottoming tap, which has cutting threads almost all the way to the end. It is made for finishing the threads at the bottom of blind holes. Unlike a starter tap which has visibly tapered threads.
They are often sold in sets of three.
Tapered tap for starting, plug tap for thru holes, and bottoming tap for blind holes.

To use these properly, you have to remove the chips as you go, don’t let them build up in the bottom of the hole.


Is the turn-back something specified in the pushrod engine FSM? That’s not something I’ve ever seen done with studs. Finger tight, certainly, but never backed out from that point. That would add about 14 thou to the exposed length.

I’m not fond of acorn nuts in critical applications. What I’ve used in their place was high alloy stainless ARP 12 point jet nuts. Many racing organizations ban acorn style lug nuts.

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Thats a mod that works, and it looks cool to

probably more economical than crappy new chrome acorn nuts made in China

I later found out it was cheaper to get the old nuts rechromed

Would re-used acorn nuts have a compromised internal thread stretch situation,
or does that apply more to the stud ?

Good to discuss head stud reassembly process. I’d love to hear further from from Mike and others. The FSM says (e.g., at Page B.32) only the usual “Reassembly is the reverse of the above procedure.” There is no FSM instruction to rotate head stud one quarter turn back from thread bottoming on block surface.

The dismantle instructions include (e.g, Page B.27) “Slacken nuts only a quarter of a turn each initially, until they become free, to prevent strain on studs and distortion of cylinder head.” I interpret that advice was given realizing people might reuse the head studs, so keeping the studs within elastic range was encouraged by this sentence.

Why do I insert the head studs into the block and then back them out one quarter turn before nut application and torque? This is based a bit on conjecture so opinions are well worth further remarks by everyone.

When the final torque on the head stud/nut combination is done there are axial and torsional forces in place on the stud. As an experimental physicist, I’m making a conjecture that the stud/block interaction around the threads has fewer force hotspots approaching yield if the threads are not bottomed out at block surface. I conjecture this makes lifting at the head surface less likely. Anecdotally, the few times I have examined blocks where the quarter turn relief was done had no block distortions needing resurfacing. Also anecdotally, some machinists who work on pre-war engines also follow this quarter turn relief step. I’ve never made measurements of the force and torque distributions in the studs and block. Nor have I done finite element analysis on this situation (though I have done it for other mechanical circumstances with nuts and bolts involved).

You are correct that the quarter turn relief does raise stud exposed length at the top. Yes, that enhances the chance one of the “acorns” will bottom out and force additional torsional force on the stud in the final torque application. The C.2138 Nut, Cap, Assy., for Cylinder Head Studs avoids the “acorn” internal bottoming out problem and is readily at hand in my supplies. Your ARP solution looks superior when the ARP threads can be used. I use one of the C.2138 with the cap taken off to measure elongation when testing the Young’s modulus. That measured elongation agrees with your thread TPI calculation method for expected elongation.

In the end, the combination of any head resurfacing, head gasket thickness, and “acorn” domed nut internal clearances all add up to needing to be cautious in the assembly process. On these pushrod engines the studs are BSF threads so supply choices are limited both for the studs and the nuts.


Very true. You also need to accommodate the change in stud length due to stretch when torqued. That stretch is directly proportional to the working length of the stud, so the studs in a long-stud 4.2 stretch a lot more than those in a short stud 3.8. Was there a difference in part numbers for the dome nuts between those two engines? There are two easy ways around this, open end nuts and hardened washers. I did both to minimize the number of exposed threads. I used two hardened, ground, ARP washers per stud, the top one stainless for appearance.

That reminds me of something. Mine came to me with mismatched washers, various thicknesses, a couple were helical lock washers. Who can ever guess why. I put on all new stainless flat washers on the exhaust side, a bit thicker than the original plain steel washers which I used on the intake side.
Mark V exhaust

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I too found an assortment of washers on my 2.5L MK V engine. I replaced all with hardened washers and use two if it looked like the acorn nut was going to bottom out.

Much useful @Roger_McWilliams, thank you for this . I am having a similar problem with my XJ6 series 3.
I ordered chrome nuts from SNG and checked each stud protrusion, but unfortunately not acorn nuts thread deep, as I though nuts were the same as thr original ones But they are 3mm shorter. I only suspected that when I realised one nut was twisting back when final torque was being applied .

My question now is, are all studs the same thread ,BNF 7/16 18, skso for the 4.2 XK lster emgined used in rbe S3 XJ6 ? I made the mistake of tapping two nuts with a 7/16 20 TPI makimg them unusable now . I wanted to reuse the old nuts …

The pushrod engines in the pre-xk cars are all BSF threads.
With the XK engine they went to ANF threads, or later on it was UNF threads.