Tech Tip: Mainshaft High Gear & Countershaft Sprocket Sealing

Tech Tip: Mainshaft High Gear & Countershaft Sprocket Sealing

Have you ever heard someone make the statement that "all British bikes leak oil"?

If so, that person does have a valid point!

Since vintage British motorcycles are more prone to leak oil (at some point) there are a few tricks that you can perform to try to make your vintage machine "oil tight" as possible.

Of course making a machine "oil tight" takes a lot of time and patients getting everything just right.

Today's Tech Tip post I would like to take the time to go over an original Triumph Service bulletin that explains mainshaft high gear and countershaft sprocket leaks.

If you are experiencing a problem as such, I will also be going over 3 simple steps that can be used to obtain an "oil tight" sprocket further below in this post.

Without further ado, lets get to it...


The symptoms

The usual tell-tale signs of a countershaft sprocket leak is typically evident on the floor when your motorcycle is on the sidestand during storage.

As your motorcycle is leaning on the sidestand, the oil in the gearbox shifts over to the drive side and will eventually work its way out of the gearbox through the high gear splines.

This type of leak is very common and should not be confused with a bad high gear sprocket seal or a worn high gear bushing although both should not be over looked.

What causes this type of oil leak?

Machining back in the 1960's and 1970's is much different today in terms of tolerances. The age of "swapping" parts also does not help with trying to achieve an oil tight seal.

A countershaft sprocket is typically a light or tight tolerance fit over the high gear (4th or 5th gear). This type of fit will prevent oil from escaping the gearbox between the splines.

However, when a countershaft sprocket features more of a "slip fit", it opens the door for oil leaks to sneak past the sprocket splines.

 Triumph Service Bulletin - Countershaft Sprocket Spline Leaks

Click on the Triumph Service bulletin above to download, print and enlarge


The Triumph Service bulletin shown above is dated 10-23-1973 and was sent to all Triumph dealers at that time.

Notice that the date is 1973 which one can assume they are referring to the new Triumph T140, T150 & TR7 "5-speed" models although Triumph had listed "all models" for that given year. 

Now based off of my own personal experince, I can say that mainshaft and countershaft sprocket oil leaks can and do happen on other models than what is described in this Triumph Service bulletin.

In fact, you can apply this bulletin and "the fix" below to your specific application.


The solution

Now that you are aware of the problems that can arise from poor tolerances on countershaft sprockets and high gear splines, lets go over the fix.

If you courageous enough and perhaps "fed-up" with oil leaks, I do recommend taking the day to remove your clutch and dive in head first to fix this annoying problem.

Assuming that all parts are in good shape and you have the necessary tools I would recommend having the following parts on hand so you don't have to wait or create another problem.

The following parts should be suitable for your year and model motorcycle.

We happen to stock all parts below on our site for most models and applications should you need replacements...

  • "Yamabond" - silicone
  • Aceton / carb cleaner
  • Primary cover gasket
  • Trap door gasket
  • Trap door seal
  • High gear seal
  • High gear nut tab washer

For sealing I do recommend using Yamabond as the sealer Triumph is recommending in this service bulletin is long gone...

Yamabond is a non-setting silicone sealer that seals well and is also easy to remove. If you have a preferred brand, you can use what you like.

Click here to purchase your own tube of Yamabond

Tackling the leak

*At the time of publishing this Tech Post, I did not have an engine that was "complete". However, I have compiled various photos with detailed instructions to aide you with a fix - everything shown below must be done on the engine - you are not removing the entire gearbox.

As stated further above, you can apply the info below on just about all Triumph, BSA and Norton applications both 4 and 5-speed applications from singles, twins and triples.

Step #1 - Dismantle the clutch assembly

To get to the countershaft sprocket on all vintage British motorcycles you will have to remove the entire clutch assembly.

Primary chain, basket, rotor, etc - it all has to come out.

Triumph 650 Clutch Removed For Rebuild

Once your basket is removed, you will be faced with the trap door cover being retained by (6) small countersunk screws. Remove all (6) screws and the trap door cover until the mainshaft and countershaft sprocket is evident.

Bend the tab washer back (if you have one) and remove the high gear nut and proceed to remove the sprocket. If your sprocket is tight, you may need a puller to extract it.

Once all parts have been removed each component should then be cleaned thoroughly and inspected as you will be putting everything right back together!

I recommend removing the gearbox sprocket to properly clean the high gear splines to allow the Yamabond to seal properly.  If you do not have the tools to remove the sprocket, clean the closest area to the sprocket using Aceton or carb cleaner...

Step #2 - Seal the sprocket & high gear splines

Now that all your parts are clean it is equally important that your mainshaft and high gear has no oil residue left behind.

Slide your sprocket over the high gear until the sprocket bottoms out on the high gear bearing.

Triumph Sprocket Over High Gear (5th)

 Sprocket shown is bottomed out on the high gear, apply sealer around the bead

Once the sprocket is driven "home" it is now time to apply Yamabond (or equivalent) around the splines and inside the sprocket groove.

The area where the sprocket slides over the high gear is where leaks are prone to happen.


Yamabond On Sprocket Splines

Yamabond is applied around the splines - nut and tab washer ready to be installed

Once you have applied your sealer around the sprocket and high gear splines you can then install your tab washer (if fitted) and thread your high gear nut on - make sure its tight!

Keep in mind that when used properly (for the right application), silicone can be very beneficial. Apply a liberal amount of sealer as shown above. You can always "tidy" the sealer up when your sprocket nut is torqued and tab washer bent.


Step #3 - install your clutch

Triumph Clutch Assembly

Now that your sprocket is sealed it is time to install your clutch assembly.

Be sure to replace all seals and gaskets to ensure an oil tight fit.

Once installed, you are done.


Pay attention to cleanliness... it is vital to allow the silicone sealer to "set" properly. 

Do not be afraid of using too much sealer as it can be wiped away when it is still tacky.

If done properly, you should be able to obtain a leak-proof gearbox sprocket.

Thanks for reading 

If you have any questions please feel free to leave us a comment below and we will assist you.

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Ride safe - CBS


  • anthony turmel

    started to do this job to-day..after years of leaks…excellent article.

  • neil loveridge

    Finally found the right help, been trying to find an o ring to fit. Many thanks, this has saved me a shit load of time as I am ready to reassemble these exact parts. Pity that I’ve just refilled the gearbox and noticed the leak before I found your site.cheers,neil.

  • David

    Great article. I’ll remember all this if I ever find a new sprocket for my early T150 5 speed.
    I’ve ordered an oil seal, but I can’t be sure it’s correct. For some reason Triumph used the same part number for both early and late gearboxes.

  • Jay Kleinjan

    I have also had trouble over the years removing the 6 small trap door screws as well. Some are loctited on there. Gently heat the screws helps in getting them out and I always use new screws when reinstalling them as well as a small amount of Loctite.

  • Harold Moore

    How about and tech tip on push rod tube sealing. Especially on earlier models.
    But thanks for this tip I’ll definitely do this next time I’m in there. there.

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