Engine Strip


Though the engine in the original bike (engine #1) seemed to run ok, decided to strip it, check the pistons and change the main bearings and crankcase seals. Also, have a quick look at the head, barrel and piston on engine #2 (on the spares bike).

A complete strip it is then, and the engine is something different from the Rotax disc valve engines on the SWMs.

Head, Barrel and Piston

On engine #1 the head was easily removed and the piston and barrel looked in good condition with no scoring. The piston is 0.25mm / 1st oversize and very usable.

Piston in engine #1

Removing the barrel on engine #2 wasn’t quite so easy and a hour or more of heaving finally got the seized barrel away, but only after breaking the bottom fin (grrrr). This piston was also 0.25mm / 1st oversize.
Piston in engine #2

Based on the condition of the second piston, decided that new piston kit and rebore wasn’t a bad option.


The flywheel / rotor bolt is a normal RH thread and holding the conrod with a bar made undoing straightforward.

The flywheel on the MX250 strikes you as being rather small, and it has the CDI sensor on the outside. The main issue for me was getting the right puller, which is an M20 x 1.5mm RH external thread and is hollow to fit over the crankshaft end. Its an MP7 puller by code, but wasn’t one I had in my expanding collection of pullers (sigh).

Gavin turned a brass version of a puller on his lathe, whilst the one I ordered from the US (not available in the UK it seems) was winging its way across the Atlantic. But alas on a bit tight and the puller threads broke before releasing the flywheel.

When the right part appeared from the US, then it took < 5 minutes to remove the flywheel. Right tool and its easy.


Removing the clutch cover wasn’t straightforward and it needed some attention from the rubber mallet.

Clutch revealed

The clutch is pretty standard and relatively straightforward to remove, though some form of tool to hold it in place is very useful.

Holding the clutch in place

I used a flywheel holding tool (acquired for less than a fiver from ebay) and it worked. Took back the lock washer and removing the nut (again normal RH thread) was relatively straightforward.

Clutch removed

Need to check the plate thickness and to see how serviceable it is.


From the Clymer manual it says you reach the kickstart spring and then remove the whole assembly, though I wasn’t sure if I could leave the whole assembly in place as I split the cases. In the end went for a compromise; half took the assembly apart, removed the circlip etc, but stopped when taking off the plate holding it against the spring.

Kickstart assembly

More later.

Primary Drive

Held the conrod in place , whilst putting a 22mm socket on the primary drive nut. Its been punched to hold it in place, though came off relatively easily. My concern is over nut itself and may need replacing with a new one.

Primary Drive

Remove the sprocket, remembering also to retrieve the o-ring thats positioned on the shaft behind the sprocket, as its omission will lead to a loss of performance and lots of smoke.

Gear Selector

The Clymer manual is a bit confusing, as there are three main types of selector layouts. The instructions are also a bit brief and its worth taking a couple of photos before disassembly.

Gear selector shaft

Looking at the RH / Clutch side, removed the clip from the end of the shaft on which the gear lever is attached (on the other LH side) and then pulled the shaft through on the LH side.

Gear selector

Noting the way the selector mechanism is meshed together, then removed the clip from the front-most shaft and then removed both sides of the mechanism, including the blade from the selector drum which lies beneath. Maybe simpler than it looks (will let you know when I try to put it back together)

Splitting the Crankcases

Splitting the cases

The Clymer manual cites the official Yamaha crankcase splitting tool, but you can rig something up with a normal puller and a couple of long M8 bolts. The bolts go into the threads vacated by the clips used to hold the crankcase seal in place.

Position the engine on its LH side, with the puller attached to the RH crankcase, with the puller tip on the split end of the crankshaft from where you’ve recently removed the primary drive.

You probably need to double check that you’ve removed all the crankcase bolts, there should be 12 in total. I’d missed one of the four which sit inside the rotor housing, as it was covered in crap. Gently applying pressure to the puller, it was quickly clear that the bottom of the cases weren’t separating so I backed it off and then noticed the bolt.

We have separation !

Once the ‘hidden’ bolt was removed, the cases came apart really easily and slowly turning the puller meant the cases came apart in parallel. The only thing to remember was to tap the gear shafts so as they stayed in the LH (lower) crankcase.

LH Crancase with gearbox

You should be able to split the cases in such a way as to leave the transmission, gearbox and crankshaft all still in the left-hand case. This makes things a lot easier, especially for re-assembly.

Kickstart spring

I’d decided that I didn’t need to remove the kickstart shaft prior to splitting the crankcases, but the spring for the kickstart (and its cylindrical housing) are infact held in by the LH crankcase. It all got a bit twisted up as the case halves came apart, so then took off the plates holding in the kickstart and removed the shaft, which freed up the spring. You can’t seem to take the tension off the spring before removing the shaft, which isn’t what the manual says.

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