nr's blog

Lovely Head 11 October, 2016

Filed under: Motorcycling — nr @ 11:44 am

One of the things that I knew about the TRX when I picked it up was that it needed the gearbox output shaft replacing. Now, on pretty much every other bike I’ve owned, you can get to this by simply turning the engine upside down, and removing the lower engine case. I say ‘simply’, but obviously that’s a bit involved, as it means dropping the engine out of the frame, and all that entails. But still, it’s a reasonably straightforward process. So, I picked up a second hand set of gearbox internals, and read the manual. To get to the gearbox, I needed to remove:

  1. Valve cover
  2. Cylinder head
  3. Cylinder block
  4. Pistons
  5. Clutch
  6. Starter clutch and idle reduction gear
  7. Gearchange mechanism external components
  8. Oil pumps
  9. Cam chain and guides
  10. Oil tank
  11. Oil sump
  12. Balancer shafts
  13. Water pump
  14. Starter motor

Hmm. That’s quite a list. And to get at some of those things, I need to remove other things first. So, it’s a Big Job. And while I’m OK with Big Jobs for the most part, I don’t have the necessary tools for a lot of this. So I made a decision to strip the motor down to the bottom end, and drop that off with the super chaps at Cambridge Motorcycles to swap the gearbox out, while I take a look at the top end.

So, off came the head. That’s a very glib statement for something that took several really good cups of tea and quite a lot of grunting. But there were no stuck fasteners, and other than it being a bit of a squeeze to lift the head off with the motor still in the frame, I soon had it sitting on my bench. You may ask why did I pull the head while the motor is still in the frame, given that I’m going to be dropping the motor anyway. I’m sure I’ll think of a good reason soon enough. It soon became obvious that all was not well with the head. Firstly, the clearances on the inlet for cylinder #2 were, well, basically not there. So new shims on order to fix that. More worrying though, was the fact that the inlet cam was very tight to turn. I popped the caps off, and it was soon obvious why: the cap for the #1 cylinder was binding on the shaft. Now, the preferred way to sort this is to send the whole lot away to a specialist for re-boring and honing and all sorts of expensive stuff. I just got out a scraper and some wet and dry paper. Seems to be OK now. Now, the obvious question is how the thing was still running like this – and I’m not entirely sure. There’s obvious signs that the aluminium in the seat has picked up in a couple of places and burred a little. So I’m being optimistic, and thinking that it had only just started binding when the bike was taken off the road. The alternative is that the head is so warped that when it gets pulled down onto the block it was bringing things back into alignment. I don’t really want to consider that option right now. Anyway, even if that was the case, the seat and cap still needed deburring, so I’m not desperately unhappy.

This is the first time I’ve looked inside a Yamaha five valve head. And while there’s no doubt that it’s all very clever and everything, is there really any advantage over a four valve setup? I have my doubts. Still. It’s pretty, and I took a photo, so there you are.

I know that while I’m in here I should probably replace the valves, but that’s out of budget right now, so again, I’ll clean up what’s there, put new gaskets in, and put it all back together. Now I know how simple it is to pop the head off, this is something I can work on in future.

I’d still rather be working on the TZR250 though.


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