DISCLAIMER: The modifications described in the following text are for educational purposes only. In no way do I recommend that you apply these modifications to your own motorcycle. If you do choose to go ahead and modify your motorcycle based on the information in this document you will accept all responsibility for your own actions. The author(s) of this document, and host(s) providing it for you, accept no responsibility whatsoever. If you are unqualified to make any of the changes described herein but are bent on doing the modification, seek out a knowledgeable friend or professional mechanic for assistance.
Fitting a new timing chain (cam chain) to a Yamaha TTR250
Over time the cam chain on TTR250s will wear or stretch.
They are relatively cheap to buy and easy to fit.
As usual when going inside the engine it is essential that you have the workshop manual to hand for full details of the procedures, torque settings, etc.
This pictorial guide is just to help you along the way. The work was done on a TTR that I have just refurbished and was exhibiting some cam chain noise.
Tools needed:- a socket set, torque wrench and hex-sockets.
Parts/supplies: new timing chain Yamaha part # 94591-43122 (see here) plus maybe a new clutch cover gasket on standby just in case the original gets damaged.
Doing the job.
Remove the seat, petrol tank and exhaust header pipe.
Wash the area above the engine and around the clutch cover. You do not want mud and grit falling into your precious engine when the covers are off!
Set engine to TDC (the single timing mark NOT the double one!).
Remove footpeg and support bracket, brake pedal pivot bolt, oil pipe banjo bolt and washers, cam cover and kickstart lever as shown in the photo below:
Make sure your working area is clean. I tend to rob the recycling bin of old newspapers and lay them around to give a clean absorbent surface on which to put down any parts removed. You are definitely going to lose a little oil when you take off the clutch cover.
Undo the shock reservoir Jubilee clips and move it out of the way.
Your engine should then look a bit like this.
The indicator that the chain was slightly worn was that timing mark line on the left-hand sprocket wasn't parallel with the head. Not a long way out but enough to make it worthwhile for me to change the chain. See photo below:
Ideally the timing lines on the cam sprockets should be parallel with the head casting as shown in the photo below. The blue lines indicate how the lines appear when the chain is worn
Remove the casing bolts and keep them safely in order as they come out as they are different lengths. I mark up a bit of cardboard and punch holes to put the bolts in.
Remove the cover carefully to keep its gasket intact.
Next job is to remove the clutch cover bolts and springs gradually and evenly and remove the plates. Try and keep them in order.
Knock back the tab washer flat and, using whatever form of clutch holder or gear jammer you have, remove the clutch basket retaining nut (27mm). I use this tool.
The internal basket can sometimes stick but you can easily put the tool back on to give something to pull against.
You are now able to remove the timing chain slipper retaining bolts to give access to the chain:
I recommend loosening the sealing bolt before removing the tensioner as it can be tight and difficult to get off afterwards.
Remove the circlip holding the oil pump drive gear on and remove the gear.
Your parts box should now look a bit like this:
Lift the old chain off the sprockets and drop it down through and remove it.
It is then easy to wiggle out the slipper to check it for wear and any possible damage. On rare occasions they can crack around the point that the tensioner pin pushes against it.
A simple check of the old chain is to hook it over the round part of a screwdriver blade alongside the new chain and, if it has stretched, it will be obvious. My old chain wasn't too bad but still worth replacing having got this far! See photo below:
Hook the new chain over its bottom sprocket and pull it up to the top of the cylinder head using a bit of plastic-sheathed wire or string,
Bolt the slipper back on:
At this stage it is worth checking your valve clearances and adjusting if necessary as, to fit the new chain over the top sprockets, you may need to take off the caps from the rear (inlet) camshaft and tip it forward to allow the chain to be seated on the sprockets and get everything lined up properly. See here for a "how to" guide. If you are lucky, the chain can be fitted without disturbing the camshafts!
At this stage double check that the crankshaft hasn't moved and the timing mark is properly aligned so that the engine is at TDC.
It is then a case of putting everything back on in reverse order.
Before refitting the clutch cover, I always turn the engine over a couple of times using a socket on the flywheel bolt to make sure nothing is binding and everything has gone back together properly.