Building a 91 KX500 engine

[Sandblaster]

This is a customers engine that was sent to us for a full rebuild.

91 KX500 ENGINE BUILD 10002.JPG (57.58 KiB) Viewed 15534 times

On a rebuild the first thing you must do is take it apart and inspect it.
Since this engine was nice and clean we were able to dig right in.
As you disassemble you will notice various problems that cause you to look at other items more carefully for damage as well..
This engine had a new top end installed fairly recently as far as hours go.
Unfortunately it was not done correctly and there are many problems with this engine.
Note on the pic below how the base gasket was sucked in.
Instead of removing the cylinder and replacing the base gasket the previous builder filled in the gap with some orange sealant

91 KX500 ENGINE BUILD 10000.JPG (136.59 KiB) Viewed 15536 times

So you disrupt the smooth flow of the fuel air mixture with a gasket flapping around in there and probably had a air leak making it run lean.
Watch this video and see if you see a problem.

https://www.youtube.com/watch?v=qsK1umAFyUY

Yup.... All bad....
Now, take a look at the pic of the piston below:

91 KX500 ENGINE BUILD 10001.JPG (43.22 KiB) Viewed 15536 times

What caused that on a recently installed piston?
First off note that the scoring is on the intake side of the piston.
And, there are actually two scoring marks spaced evenly and symmetrically on the piston.

[Sandblaster]

Cracked intake bridges were the cause of the piston destruction.
Some people say that they have ran their bike for years with cracked intake bridges and that theirs never had a problem.
And that is possible.
However, some people experience this exact same problem when their intake bridges crack.
Why?
Believe it or not their are internal stresses on the cylinder casting.
This became apparent when I was cutting a KX500 cylinder for display purposes.
You can read more on that here:

viewtopic.php?f=5&t=481

In any case, when the cylinder heats up and vibrates it can cause the intake bridges to extend into the cylinder bore and start scuffing up the sides of the piston.
Of course a good plating company such as Millennium Technologies can weld up those cracks and much worse damage as well.

Another area that is typically damaged on a KX500 is the clutch cover.
When a guy falls on his right side the rear brake lever pushes into the clutch cover and cracks it.
This one looks like it is not leaking but why take a chance?
We have some special brazing rod that can fix it right up.

91 KX500 ENGINE BUILD 10003.JPG (52.39 KiB) Viewed 15532 times

Here is another piece of damage that we found that is a little more unusual.

91 KX500 ENGINE BUILD 10004.JPG (68.22 KiB) Viewed 15532 times

It seems the drum valves were actually wearing on the cases.. We will need to figure out why they are so badly worn and fix the issue as well as fix these cases.

[Sandblaster]

Here is another area of concern.
The crank seal was leaking so bad that it was letting a lot of fuel into the magneto area and actually smelled burnt when we removed the cover.
Of course we will need to check the OHM specs on the stator to be sure that it is in good shape before we install it.

91 KX500 ENGINE BUILD 10005.JPG (69.46 KiB) Viewed 15532 times

[Sandblaster]

Here is another area of concern.
The crank journal was so worn out that the bearing literally falls onto the shaft...
This should be a light press... No way should you be able to even push it on with your hands let alone letting it fall on..

https://www.youtube.com/watch?v=JgaHUiXfydQ

[Sandblaster]

Here is a related area of concern...
The crank bearings literally fell out of the case..
Again, these are a light press fit.

https://www.youtube.com/watch?v=0oN3sAk_CPo

Check out these pics..

91 KX500 ENGINE BUILD 10006.JPG (70.64 KiB) Viewed 15531 times

91 KX500 ENGINE BUILD 10007.JPG (71.53 KiB) Viewed 15531 times

The bearing was so worn out that it started chattering inside the engine case and severely wearing out the case

This is a good argument for balancing your crank and replacing your main bearings on a regular basis..
Still more to come

[Kawboy]

Here is a related area of concern...
The crank bearings literally fell out of the case..
Again, these are a light press fit.

https://www.youtube.com/watch?v=0oN3sAk_CPo

Check out these pics..

91 KX500 ENGINE BUILD 10006.JPG

91 KX500 ENGINE BUILD 10007.JPG

The bearing was so worn out that it started chattering inside the engine case and severely wearing out the case

This is a good argument for balancing your crank and replacing your main bearings on a regular basis..
Still more to come

I have a '92 engine with the same chatter marks in one side of the case . will you try a repair or scrap it ?

[Sandblaster]

Well.... I know we can clean it up and do the JB weld repair.
They will hold for some time.
I'd rather bore it out and put a sleeve in it.
But I don't have a mill to bore it or a lathe to make the sleeve
I do know several guys with the equipment to do it but it's time consuming running parts around town.
After all you can't just drop stuff off and say bye... Guys always want to talk about what my latest plans are
Probably the best option for this customer is to get a new/used case.
But the school is still out on that as I have to discuss the options with the customer before we go much further...

[Sandblaster]

It just occurred to me today that I could have the case journal plated for the bearing to fit correctly.
Or, if it's not feasible I could have the OD of the main bearings plated oversize but I would need to use a different process the a tank plate job.
It would be done on a lathe

[81cr450]

Awesome to see you building up another one. Think about plating, it gets on everything, you gotta do the bore & sleeve. OS outer diameter bearing would work, but there's nothing that fits the bill. Keep on with the updates, I wanna see how this turns out

If you have a lathe with a a huge throw you could build a center that rides in the seal area & then turn it out from there. There's a product called a speedy sleeve, might could get one made for the bearing OD so your case cut is minimal

[Sandblaster]

With plating, they have a special tape that you use to tape off the area's you don't want to plate
But, we'll see... The school is still out on the method we will use...