Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board B44 rocker box

So ….. according to the Rupert, the B25 and B50 post '71 had 5/16 UNC / UNF studs to hold the rocker box down. Been thinking, always a dangerous thing and often an expensive one as well, 1) why use studs and not simply bolts? and 2) why not drill and tap the B44 or indeed the pre'71 B25 / 50 heads to accept 5/16 UNC bolts?
Apart from the squeal of the rivet counters and I missing something here?

One of the best mods you can make on a unit single is to unscrew the rocker studs and Helicoil the holes (I've used metric M6 on my B44).Then use socket caps instead of the studs. It's worth counterboring the rocker cover stud holes so that the socket cap sits just proud of the rocker box. Also, use fibre washers under the socket cap to stop leaks.

Doing all of the above means it's much easier to remove the rocker box & head without removing the engine. Additionally, the Helicoil inserts mean you can apply more pressure to the rocker without fear of stripping.

The B44 rockerbox/head top face is not cast to take 5/16" studs or bolts, so you are stuck with 1/4" but with the thread length being so short in the head you have to use a fine thread so enough threads engage and as in UK BSF is still common I use that.

It's a common mod on Gold Stars to remove the studs and fit bolts. It enables rocker box removal without taking the head off

Could understand Kommando's point BUT it is the coarse thread that goes in the head (?)

Usually a coarse thread goes into aluminium (or a casting like the cylinder) there’s less chance of a coarse thread pulling out as there is more meat between the threads. the fine thread is then better for torquing down. More turns on thread for distance travelled. If you helicoil the casting you can fit a fine thread in there, this will be plenty strong enough then.

Course thread is better in alloy, but it's already 26 tpi BSF, the casting is thin and the depth of hole is short so you only get one chance to Helicoil so stick with BSF, the Helicoil is stainless steel so fine thread works.

I used 5/16 cap screws (cut to length) for the rocker box on my B50. It was sort of a necessity as I had to take the rocker box off several times as I tried to cure the leaks. Got most of the leaks cured by lapping. It is not perfect. I have one small leak and it is not worth the effort to fix. It is a a rider.

Mr Mike

As mentioned above, I used Helicoil inserts in all the rocker box stud holes on my B44. I don't think makes much difference which thread form you choose, just make sure its similar in diameter. I used M6 inserts because I had a spare kit hanging around in the garage, M6 being not much different to 1/4 inch.

Before adding the thread inserts I made sure that both the cylinder head to rocker cover gasket surfaces were in as best as possible condition. Initially, this involved slightly countersinking the rocker-box stud holes and then running a large flat file over the gasket surfaces to remove any high spots and burrs.

I added all the inserts with the head in place, carefully drilling out the old threads using the drill provided in the kit though there wasn't much metal to drill out.

I retapped the threads using the Helicoil tap from the kit ensuring any swarf was vacuumed away and then added the thread insert using the tool provided in the kit. I was careful to ensure the insert sat just below the gasket surface.

The rocker cover stud holes were counterbored so that the socket caps could sit just proud with fibre washers underneath to prevent leaks.

After cleaning the rocker cover gasket surfaces I sealed the rocker cover using a tiny smear of silicone and lightly tightened the socket caps. After a few hours, the sockets caps were tightened a bit more.

After well over 7 years I still have no leaks from this area and feel that using helicoils has added strength to this area, I have no fear of stripping one of the socket caps as the helicoils are much much stronger in alloy and are recommended.

All this was precipitated by me forgetting to order nuts (1/4 and 5/16th BSF) for a head I bought off Ebay. What I cannot get my head around is why bother with the Helicoils if you want to go to using bolts? Thinking about it, you have to drill out the hole, cut a larger thread, screw in the helicoil then the bolt screws into the helicoil. Why not just cut out the middleman, drill out the hole, cut a larger thread, put a bloody bolt in it! Adding a helicoil into the mix does not add strength. Sure, bolt holds well into the coil but the coil only holds into the casting as well as a bolt does.
I have ordered the nuts.

You want to go back 50 years and tell rolls Royce then.

The strength it adds is every time you remove the screw your removing it from the steel coil and not risking damaging the aluminium each time it’s fitted and removed.

BSA also did it with the rocker box cover on the oif. I have one head which takes a UNC and has no helicoils. Every other one is helicoiled and takes UNF. They did the same in the rocker box. The rocker box is the engine steady. Must have done it for a reason as these didn’t spend money on something if it didn’t need it.

The other thing is if you just tap out to a bigger size then you will need to open up the rocker box stud hole also.

I get your points, I just don't buy in to them!
BSAs are many things but Rolls Royces they are not, and as for the concept of BSA not wasting money on things they didn't need to do ……….. !

I can see that if you wanted to put a fine thread into an ally block, then sure, you need a steel insert, but it all comes down to application, why do you want to use a fine thread in this situation? Indeed BSA didn't! the stud as it goes into the head does not have a fine thread.

Now the argument on taking the bolt in and out of the ally …… I am an inveterate fiddler and even I don't exactly do this and a daily basis, and given that when you undo the nut on the stud the bloody stud comes out half the time anyway ……..

As far as going bigger and up to 5/6 UNC, this is exactly what BSA did for the B50 (and B25?).

All academic and rather pointless as I have actually ordered the right nuts!

When the coarse thread stud goes into the alloy, it’s not the stud you remove when you put a spanner to the nut but the nut itself, this is usually fine thread. If your putting a torque wrench to anything it’s usually against a fine thread not a coarse thread.

Maybe you will buy into that a bolt would have been easier and cheaper to fit than a nut and a stud.


Regarding the B50, I’m not saying they didn’t go to 5/16, but as I said you would have to alter the rocker box as a 5/16 peg won’t pass through a 1/4 hole.


Your bike and do as you wish

Don't be daft, of course a 5/16th peg will pass through a 1/4 hole, you just have to hit it hard enough! (preferably with a 14lb sledge hammer, anything less and the results can be a tad questionable.)

As I said , the "proper" nuts are on order, I was just questioning BSA's original reasoning.

Bolts are easier and cheaper which I suppose is one reason those chaps from the orient went that route.

I did not use helicoils when I went to bolts on my b50, but I believe that is still an option if one of the bolts strips its thread.....so why go there now if it is a later option? I haven't measured to see if there is enough materia to go to helicoils, but I think there is.

Mr Mike

BSA recommended between 5 to 7 ft/lbs for the torque value on 1/4" rocker studs and 10 ft/lbs for the 5/16 studs. This would also apply if you replaced the studs with cap crews or bolts. There are only two reasons you would want to replace the studs and re-coil for bolts. Stripped threads in the head or to make maintenance easier when the motor is in the frame. It is NOT so you can tighten the stud nuts or bolts tighter. If you have to tighten the nuts beyond spec to stop an oil leak then your problem in not the ft/lbs of pressure used but the sealing surfaces.

The hundreds of stripped out threads in the unit single heads I have seen can mostly be attributed to folks that tighten the stud nuts until their veins are popping out of their forearms. 5 ft/lbs of pressure is not a lot of force with a wrench so use a torque wrench to be safe. And of course BSA should have engineered the whole assembly much better, but then it wouldn't be a BSA would it?