Classic British Bike Magazine

Triumph TR25W Trophy

Rockerbox threads stripped and repaired.


▲ This is what a helicoil looks like up close. It's just a metal coil formed to give the correct internal and external thread. The tool (shown) is used to wind the helicoil in. It's a secure system, and is easy to fit. Get a helicoil kit for your bike and keep it close.

The Problem


This is the story of how one problem led to another, which is often the case when working on classic bikes. My Triumph TR25W Trophy was bought as a runaround suitable for a girl of ... well, my proportions and sensibilities. Yes, a 250cc BSA C15, which is basically the same bike, is a little gentler on the nervous system than the 250cc Trophy, and a little easier on the ankle too. But you can wind the Trophy up a lot tighter and, when the big spring unwinds, get more out of it. And therein lies the fun.


My TR25W had a rockerbox oil leak which, it seems, is fairly common to BSA and Triumph unit singles. Why? Top end pressure, apparently which usually finds an escape route twixt the cylinder head and rockerbox joint. But in this instance, there was something else at play that wasn't understood until the rockerbox was removed.


What I found was stripped thread at the top of the rearmost, nearside rockerbox stud. That meant that the securing nut couldn't apply sufficient torque/pressure to keep that corner of the rockerbox sealed. But finding a replacement rockerbox stud was a little trickier than I'd expected.


My bike is a 1969 machine. It was built shortly before BSA-Triumph switched from one cylinder head thread form to another. However, not realising this, I sent off for a new stud, and (naturally) the wrong one arrived. That went back in the next post, and nothing else was forthcoming, except an apology that that was all that was available. The later stud.


I tried a few other classic bike dealers, and I got the same response. Plenty of studs. All wrong.


It was then that I came across an advert online for an Allen screw kit for BSA B25 rockerboxes, which are the same as TR25W Triumph rockerboxes. This kit supposedly had a number of advantages over the standard stud arrangement. The first was that the Allen screws were stainless steel. So no rust. The second was that, without having the studs poking up in the way, the rockerbox can be more easily removed from the frame. The third was that Allen screws look tidier.


Trouble was, as I later discovered to my cost, the Allen screws were a complete mistake and should NEVER be used in this sort of application. Why not? Because BSA-Triumph, in its wisdom, understood that steel studs are far less likely to damage the cylinder head threads than any kind of bolt or screw.


The reason is that studs are first screwed into the relatively soft aluminium metal of the cylinder head and are brought up snugly. Then the rockerbox c/w gasket is replaced, and then the secured nuts (c/w washers) are tightened at the top of the rockerbox to apply downward/clamping pressure. And importantly, that downward pressure doesn't add a significant lateral force to the threads in the cylinder head. Instead, the nuts are applying a torque force only to the steel threads on the (replaceable) studs.


In other words, a bolt or a screw TWISTS into the aluminium as it CLAMPS, and that twisting motion eats at the threads. It cuts into them. It weakens them. But a nut on the end of a steel stud CLAMPS only. And that's important if you want to preserve the cylinder head threads.


The result was that most of the rockerbox Allen screws came up snug and felt secure. But the final two stripped. Actually, one completely stripped. The other was merely suspect.


Yes, it's possible that my own carelessness caused the thread damage. It's also possible that the threads were simply an accident waiting to happen. It's also possible, even likely, that the Allen screws added unnecessary stress to weak threads. And there's no doubt that an Allen key can easily apply excessive torque and doesn't leave much margin for error.


Either way, two threads died a miserable death in my garage and it was helicoil time. I've got a kit from Chronos Engineering Supplies. We've used this company before, and they've got pretty much everything we need. Here's a link:




▲ Chronos helicoil kit. This carries a 5% discount if you buy online and key in the code Sump100. Make sure you specify whether you want BSF, UNF, Metric, etc.




The Solution


I knew exactly what a helicoil was, of course, but I'd never repaired a thread before. It was always something that I'd had sorted by the local engineering shop or bike repair shop, and they often charged a lot of money for it. This time, I'd determined to get to grips with helicoiling and deal with it myself.


The first job was to get the correct replacement studs. There are seven 1/4-inch outer studs, and two 5/16th inch inner studs (see reader feedback note below).


But these early studs were manufactured with a different thread form on each end of the stud; a coarse thread on the end that goes into the cylinder head (to prevent stripping out of the aluminium), and a finer thread at the other end of the stud (to reduce the risk of the nut coming loose).


On my bike (pre-1971) the form is 1/4-inch BSF at the cylinder head end of the stud, and 1/4-inch Cycle thread at the other. The post-1971 bikes, note, are 1/4-inch UNC going into the head, and UNF for the securing nuts.


Naturally, people tend to tighten these nuts pretty firmly to keep the oil on the right side of the engine. And with the right studs and good threads, the system works fine. But 40-odd years have taken their toll.


I was unwilling to remove the head to fix these threads. Removing a cylinder head that was working fine invited other problems, and it was simpler to mask the rocker box with duct tape, plug any holes, tape some plastic bags everywhere, and keep a vacuum cleaner handy.



▲ Thinking of fitting a top-end Allen screw kit to your TR25W Triumph or BSA B25 or similar? Well check out this workshop feature. Allen screws seriously risk damaging the threads in your cylinder head. Strongly not advised.



That done, I grabbed my helicoil kit and found the appropriate drill bit. It's hard to get that wrong. The details of this BSF kit are here:


1/4" x 26

5/16 x 22

3/8" x 20

7/16" x 18

1/2"" x 18


At a glance, you can see what you need, which was in any case marked 1/4 BSF. At first I was going to use an old fashioned hand drill to keep the speed down and gently remove aluminium. But I soon found that that wasn't going to work. The speed was two slow and the bit kept nipping up. So I used an electric drill.


I measured the depth (this was a blind hole) and rehearsed it all in my head. Then I had a friend help check the drill was square. Then I put on a pair of protective glasses. Then I prayed. Then I pulled the trigger.



▲ Drilling out the old thread. It can be done in situ, but you need to mask the rockerbox and plug the carburettor with a rag. After, you'll reach for a powerful vacuum cleaner.



The aluminium came out freely and neatly. I was very careful to avoid going too deep. But on reflection, I should have wrapped a piece of tape around the drill bit as a marker. You don't want to make the hole any large than it needs to be; not if you want a good thread.


A 1/4-inch BSF tap (from the kit) cut a good thread (I used a little WD40 as cutting oil; not ideal, but better than nothing). Keeping the tap square is important (hence the friend). And turning the tap clockwise half a revolution or so, and then turning it back gently to cut the swarf, kept the cutting neat.




▲ The tap handle isn't included in the Chronos kit. But everything else you need is there. But you need to drill square, and tap the thread square, and to that end, you're best advised to get an extra pair of eyes to help.



The helicoil went in easily with the (supplied) handle device. The coil was positioned slightly below the level of the cylinder head joint. Then a punch (also in the kit), was used to knock out the little driving-tang at the end of the helicoil. Get one in your hand and you'll figure it out. The whole thing took about five minutes for each thread, with another five to Hoover up and check that no swarf found its way into anywhere it ought not to have been, including the carburettor mouth (which was stuffed with a piece of rag).



▲ The first new rockerbox stud. It needed to be seated a little deeper, but it's now threaded into a helicoil and has a good grip. It's a pity that replacement gaskets are so badly cut. But that can be tidied up. Note that the gasket needs to go the right way up. Study it. The second stud was easier, and it fitted well.



So my first helicoil, expertly done. And I'm just a girl. It's been said before here on these pages, but having this kit at hand is very liberating. You know that when you start a mechanical job, a damaged thread is less likely to stop you in your tracks; and damaged threads are the rule rather than the exception on old bikes. Am I right?


The new studs went in easily. The rockerbox went back easily. The rockerbox nuts were fitted easily. As far as I can tell, it all tightened perfectly. The bike is running fine again, and all the better for some TLC. Whether or not it stops the leak remains to be seen. But if the black stuff does pour out, at least it won't be for want of a decent set of rockerbox studs.


Note that I've presumed that you've got a manual of some kind and know how to remove the rockerbox and take the load off the push rods first. And I'm presuming that you know how to re-adjust the tappets after and have got the pushrods pointing at the right rocker arms. If not, you're going to need that thread repair kit more than you realise.



Sump visitor feedback


"Good workshop article. Just one point: The later bikes did not use 1/4 UNC for the studs, they went up a size to make all nine 5/16". Here is my rockerbox video."
- Mike Rothwell


"Excellent article on doing this job. Gives me confidence to tackle mine which I must do at some point. Thank you. I would like to add something for you and readers. In the BSA  B25 1970 workshop manual it states that; 'Ideally, there should be at least one thread of the spindle visible past the locknut. If this is not the case the spindle is not correctly positioned in the rocker box and must be turned clockwise with using a screwdriver at the right side.'  This can only be done with the rockerbox removed [Damn it !]. What in fact this is referring to is the fact that the cam of the spindle is not central to the rocker. [BSA B25 1970  workshop manual page B13 ]"
- Chris Murray






Girl Happy



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