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.
TTR250 bottom end engine strip and rebuild
NOTE: This is just a guide; it does not replace a service manual, general mechanical knowledge, specific motorcycle repair experience and good old common sense. With the proper tools, some general experience and this guide most users will be able to successfully service, replace or repair their starter motor . If you have any problems then stop and ask questions on the forum, take pictures of your point of confusion and get your answers before you start.
I am pretty sure that there isn't an existing pictorial guide to stripping down and rebuilding the TTR250 bottom end.
With the current interest in big bore kits that require the crankcases to be stripped for boring to take oversize sleeves I thought this would be a good time to create one.
The guide is created from a couple of threads that I created on the TTR250 forum so you may spot that the backgrounds of some photos differ but hopefully that isn't a distraction!
As always with my guides, feedback is appreciated.
To be determined as the rebuild progresses.
The starting point for the guide is with the engine removed from the frame and the head, cylinder barrel and piston removed. The order of work to remove the engine from the frame is:
side covers, seat, fuel tank
carburettor and crankcase breather hose
engine guard - bashplate and frame guards if fitted
drain oil - a lot easier to do now than with the engine out!
exhaust header pipe
disconnect magneto and neutral switch leads coming from the stator and the spark plug HT lead
gear shift pedal, sprocket cover and front sprocket
RH footrest and brake pedal
mounting bolts and brackets
remove engine from RH side of the TTR
turn the engine to TDC and remove the camshafts, cylinder head, cylinder barrel and piston
Pictorial guides are available showing some of the work needed to get to this point as follows:
Removing the carburettor here
Replacing the carburettor here
Removing the cylinder head here
Adjusting valve clearances here
But first make sure you start with the engine as clean as you can get it. Once the complete engine is out of the frame it is a lot easier to clean up. See here for some hints and tips.
This is the starting point for this guide. The photos show the engine bottom end complete with clutch and generator covers still in place.
1. Remove the clutch cover and internals
Remove the oil filter cover and clutch cover by undoing the 5mm Allen-headed bolts in a criss-cross fashion to prevent distorting the cover.
Place the bolts in a piece of marked cardboard so that you can easily get them back in their correct position.
Try not to lose the two small oil filter O rings!
You may have to give the cover a bang with a hide hammer or similar to get it loose and, if you are lucky, the big gasket will remain in one piece stuck to the cover.
Next remove the clutch pressure plate by undoing the 5 bolts in a criss-cross pattern.
To stop the clutch assembly turning either use a gear jammer or a thick aluminium or copper washer jammed between the gears to stop them turning.
Remove the 5 bolts (10mm heads) and their springs along with the pressure plate followed by the friction and metal clutch plates.
Next remove the clutch push rod and its ball bearing - don't lose the ball bearing so maybe stick it to the rod with some masking tape!
Knock back the tab washer on the centre nut with a hammer and old screwdriver, hold the clutch centre (I am using a holding tool from Totally TTRs - see here) and undo the 27mm nut.
Remove the nut, tab washer, centre plate and the thick washer underneath as shown in the photo below.
Leave the clutch centre basket on so that you can use its gear wheel to jam against the crankshaft gear and allow the 24mm nut to be removed after knocking back its tab. There is a small key that locates the gear - keep it safe!
Next remove the oil pump and oil strainer. Prise the oil pump cover off being careful not to distort it. Later TTRs may not have this cover. Then remove the circlip holding the gear on and remove the gear to expose the three chromed retaining bolts. I recommend using an impact driver to remove these to prevent damaging their heads as they can be in very tight!
If the oil pump hasn't been taken off before then it might need a bit of force to lever or knock it off.
Once removed, I put all the bits back together ready for the rebuild.
(i) the shift shaft (drive axle) circlips and collar or, if fitted, the kickstart idler gear,
(ii) the timing chain guide by undoing the two 10mm bolts and
(iii) the gear shift lever and the washer behind it (important not to lose it!) by prising off the E-clip and sliding it off.
Remove the stopper lever and torsion spring and the RH crankcase is now empty!
You should have a box of parts looking something like this:
2. Remove the generator cover and internals
Remove the starter gear cover by undoing the 3 bolts. These can be tricky if they haven't been taken off recently. Best to start using an impact driver. If that rounds out the heads then you will have to resort to a hammer and chisel to get them turning.
Take off the cover and remove the starter idler gear, its shaft and bearing. This exposes a bolt which helps hold the generator cover on.
Remove the generator cover by undoing the 5mm Allen-headed bolts in a criss-cross pattern starting with only a quarter turn until they undo freely.
The cover needs to be pulled off firmly as the flywheel magnet will try to hold it on!
Put the bolts on a marked piece of card as you remove them so that you know what order they go back in.
Next remove the 14mm flywheel bolt using something to stop the flywheel turning.
Then use a flywheel puller to pop the flywheel off its taper.
(i) the flywheel (and its Woodruff key),
(ii) idler gear 2 with its two washers and
(iii) the large starter gear 3 and its bearing and washer.
It is easy to lose washers by accidentally leaving them in place and for them then to fall off and disappear into a dark corner of your workshop!
The LH crankcase is now empty and you have a box of parts looking a bit like this:
3. Splitting the crankcases
Loosen the 5mm Allen-headed bolts a quarter of a turn in a criss-cross fashion to prevent distorting the crankcases.
Once the bolts turn easily they can be spun out one at a time and pushed into a cardboard holder such that you know where each one will fit back in place.
A couple of little jobs left to do before splitting the crankcases:
(a) remove the neutral switch lead if still fitted
(b) clean the sprocket and gear change shafts and lube them with WD40, or similar, to prevent possible damage to their oil seals as you pull the crankcase off, and
(c) make sure the shift cam is positioned such that it will pass through the case without snagging.
Then tap around where the cases join with a hide hammer to help them part. You will hear the hollow sound change as the joint seal is broken.
There is a safe spot that you can get a plain screwdriver in to get the process started.
It's important to make sure that the cases split in parallel, i.e. evenly all the way around, and that you make every effort to prevent any damage to the mating surfaces of the crankcases.
IMPORTANT: The crankshaft is a press fit in the LH case so you are lifting off the RH case leaving the gears, etc., in the LH side as shown in the photo below.
4. Remove crankshaft and gears
Try and keep the gears and selector shaft and arms together when removing the cluster if possible. The shift shafts pull out easily but you will probably need to put some persuasion on the crankshaft to get it out of its bearing. I rest the crankcase on a flat surface with the crankshaft over the edge. Place a piece of hard plastic over the flywheel shaft end and tap the crankshaft out with a heavyish hammer. I put my hand under the crankshaft to catch it when it pops out.
The alternative is to use a crankcase separating tool which bolts into the case and a centre spindle screws in to push on the end of the crankshaft to free it. However, such a tool costs over £60 and might only be used this once. You decide!
IGNORE THE TEXT FROM HERE ON AS IT IS ONLY A DRAFT AND IS ALSO INCOMPLETE SO IS POSSIBLY MISLEADING!
5. Check internal bearings and replace if worn or damaged
Tools needed: hot air gun, rubber and metal hammers, an old main bearing, and a large punch.
The bearings had been in the freezer for a few days so were as "contracted" as they ever would be. I heated up the bearing housings (one at a time) well with the hot air gun and then gently started to tap in the bearings and get them level with the rubber hammer. I used the old main bearing as an interface between the new bearing and the metal hammer to get the new main bearings in without damage. The transmission bearings were started the same way and then tapped home with the punch.
DO NOT ever hit or use the centre of the bearing to drive it home as this will ruin the bearing!
6. Fit crankshaft and gears in LH casing
I am fitting a crankshaft that I had reconditioned with a new big end and conrod. This is to give the new engine the best possible chance of longevity.
I used the large starter gear, 3/4" socket and the flywheel bolt over the LH side of the crankshaft to pull it through the main bearing as it is an interference fit.
7. Clean the edges of both crankcases and fit together
It is now a case of replacing parts, cleaning and checking them as you go.
If you are being fastidious, now is the time to replace the gear change and transmission shaft oil seals.
The installation of the gears was quite straightforward but a bit fiddly to get the selectors in place.
Make absolutely sure that you have the dots on the crankshaft/balancer cogs and gear change shafts are lined up. Also, make sure that the two locating casing dowels are in place.
Give the crankcase mating faces a final clean up along with the crankcase bolts and then bolt them together after covering both sides with some non-hardening flange sealant. I use Loctite 574. Take your time as this is a critical part of the build and shouldn't be rushed.
Then, with the LH crankcase side containing the crankshaft and gears in at the bottom, fit the cases together. A bit of wiggling may be required but no force should be needed.
If the sealant was spread correctly then there should be a bead of it squeezed out around the whole of the mating faces.
The crankcases are sold as matched pairs and are designed by Yamaha to bolt together without a gasket but using sealant instead. It's a bit scary as you can't really test the seal is oil tight until everything is put back together and it would then be a lot of work to strip it all apart again. Thankfully I haven't had a problem so far and not heard of anyone else having experienced a leak.
Turn the gearbox over and fit the crankcase bolts and torque them up to 10Nm.
I then use some blue workshop paper soaked in clutch/brake cleaner to wipe off the surplus sealant.
8. Replace clutch internals and cover
Fit the primary drive gear, and the timing chain guide (slipper).
I always fit new timing chains to my rebuilds as they work hard and can be the cause of noise.
New chain, slipper and primary gear in place ready for the clutch basket to be fitted.
Using the gear jammer, the clutch and primary drive gear nuts should be torqued up to 75Nm and 80Nm respectively.
Next up is to bend over the tabs on the tab washers.
Bend the tabs on the tab washers over on the clutch nut and primary drive gear. The clutch one is always difficult because of access.
Replace the clutch push rod and ball bearing.
After fitting the clutch basket, adjust the clutch actuating arm so that the pointer lines up with the casting mark on the casing. Difficult to see when the engine is in the frame.
Fit the cleaned gauze oil filter - only goes in one way so you can't get it wrong unless you force it.
Now replace the cover.
The filter cover O rings looked a bit tired so I replaced them with new.
I fitted the large starter gear and flywheel and it seems that the most technical part of the rebuild is over.
9. Replace generator internals and cover
As I am using parts from a donor engine, I checked that the small starter gear had the correct number of teeth for the new large idler gear I was fitting - and it did - 16T/14T - see here to understand why it was important!
The generator cover gasket was in good shape so usable. I fitted the cover making sure the stator wires were correctly located before tightening it up. I couldn't find a used 72T/19T idler gear so had to break out a new one.
The starter gear cover was then fitted (with some decent bolts to replace the originals which invariably round out) and that's another stage completed.
Make sure all the threaded bolt holes in the crankcase for the cylinder bolts are clean and that the bolts easily thread in. The front left bolt and thread in particular can get rusty and tight so needs to be cleaned so that the bolt tightens up against the cylinder head rather than jams in a dirty thread and give a false torque setting. I learnt this the hard way and the result was a blown head gasket.