KT250 engine rebuild story

[David Lahey]

A friend's KT250 motor was sounding worse and worse during 2012 and it was running crook too, behaving like there was a crankcase air leak that got worse as the motor got hot. Over the summer non-riding season, the motor had a rebuild and there were a few interesting things discovered along the way. I plan on putting captions saying what each photo is, and plan on writing the story that links them together separately. I know that pictures are what people love so here they are in sequential order:

[David Lahey]

Ok here are the things of note found during the rebuild:
There was no head gasket, and the head surfaces had not been made flat enough for this to work OK, so it had been passing combustion gases and air, giving the impression of a crankcase air leak when it got hot. The head studs had been damaged by being over-tightened.
The main bearings were very sad with bad pitting in the races, likely caused by the presence of water inside the crankcase over an extended period. This was what had been causing the terrible engine noise.
The big end felt fine and was within specs for play, but a close look at the rollers showed that the rollers were pitted, likely caused by water sitting inside the crankcase over a long period
The rings had very poor seal, yet measured up fine for end gap. I suspect that new rings had previously been fitted but without the bore being deglazed. It is still on the standard bore.
Both ball-type gearbox shaft bearings were fine but were replaced because the owner wanted it done
One of the shift fork guide rods was a very tight fit in the hole on one end. Once I got it out I could see it was not round. Never seen that in a Jap motor before.
Standard conrods are not available. A popular fix is to use a KX250 conrod kit which is 5mm longer than the original. The owner sourced a Wiseco rod kit which, because of being a slightly different shape around the big end area required that the recesses in the crankwheels be enlarged, and the crankcase mouth also be relieved a little bit front and back. The 5mm spacer kits are available from Fred and Deb's KT Parts in NZ.
This model KT250 has removable spacers at the front motor mounts which makes it a breeze to get the motor in and out.
The original carby to airbox hose was rigid and shrunken and with new replacements not available, I fitted a hose made from the bend in a Toyota Land Cruiser radiator hose.
While the head was off I compared it with a couple of others here and found that there are two different batch numbers for KT heads, but was not able to see any other physical differences.
Probably no surprise to owners of KT250s, but after all that, the motor started first kick

[Takhli44]

Hi David, What preformance diffrence does the longer dwell at the top of the stroke and better crank/rod angle provided by the 5mm longer rod make in the trials application? Can you feel it when riding?
Looks like a 1st class job. Like the photos.

[David Lahey]

Hi David, What preformance diffrence does the longer dwell at the top of the stroke and better crank/rod angle provided by the 5mm longer rod make in the trials application? Can you feel it when riding?
Looks like a 1st class job. Like the photos.

Thanks Takhli44, I couldn't feel any difference to riding my KT250 which has a standard length rod. I'm sure there is a difference in the motor performance because of the longer rod, but without a dyno and a direct before/after comparison it is pretty hard to quantify. As well as the crank/rod angle geometry changes, there should also be a difference due to the increased volume under the piston.

[Julz]

I fitted a hose made from the bend in a Toyota Land Cruiser radiator hose.

Keen to see a pic of this please, as my airbox and carb connectors are perished.

Probably no surprise to owners of KT250s, but after all that, the motor started first kick

Nope - no surprises here - they always seem to have good spark and start easy... although, I must say that my 'new' Fantic 301 seems to start just as easy

[David Lahey]

The KT250 is back with my friend but I can take a photo of the rest of the Toyota radiator hose. I think it had two bends of the same shape which makes it a good choice for TY250s and KT250s

[Takhli44]

Hi David, What preformance diffrence does the longer dwell at the top of the stroke and better crank/rod angle provided by the 5mm longer rod make in the trials application? Can you feel it when riding?
Looks like a 1st class job. Like the photos.

Thanks Takhli44, I couldn't feel any difference to riding my KT250 which has a standard length rod. I'm sure there is a difference in the motor performance because of the longer rod, but without a dyno and a direct before/after comparison it is pretty hard to quantify. As well as the crank/rod angle geometry changes, there should also be a difference due to the increased volume under the piston.

Thanks David, that's about as I suspected. Not really worth it as a stand alone project but perhaps OK if replacing rod bearings anyway.