300ex engine rebuild ?'s

[cbrman77]

Hello all,

I recently picked up a 1995 300ex as a beater for friends when they come over. I was told it was running before it was parked.

I cracked the top end and found out that it must have jumped timing. The two exhaust valves hit the piston bending the valves slightly and damaging the piston.

My buddy decided he wanted the quad and we traded some work for the quad. When we found out what the problem was, he wanted to go the cheapest route possible to fix it: Hone it, put oversize piston rings in, replace damaged valves and timing chain.

1. Where is a good place online to order aftermarket parts for quads? I have always ridden dirtbikes and streetbikes and the places i buy parts from don't sell quad parts.

What is your experience with honing the cylinder, putting slightly oversized rings in? The cylinder wasn't damaged, he honed it out already and wanted to get some rings once I had access to a micrometer to check the bore size.

For the timing chain, I was planning on changing the chain and both sprockets. Is that usually what is recommended is replacing all 3?

The gears don't look extremely worn, but i am not sure what they look like new so they could be shot. (I could snap a few pics if that might be helpful).

Also what about the piston. it is damaged from where the valves hit, but there are no holes. is it ok to use or should i get a new piston as well?

I purchased the honda service manual and read through it but it doesn't have much on changing the timing chain. Is there any how-to's around for the 300ex?

Where can i get new timing gears? I am easily able to find the upper and chain but no lower other than from honda for 100 bucks or so.

Thanks,

Steven

[roadthrills]

Usually people just change the chain. Sometimes the upper gear. The lower gear is press fit on the crank and the whole engine must be torn down to r&r corectly. I have bought several parts from a guy that sells on Ebay called sledpartsguy.

[cbrman77]

Thanks for the response....

After no responses for so long, I got in touch with my uncle who has been working on dirtbikes/quads and custom harley's since he was a kid.

He told me pretty much the same thing.

I guess there isn't much load put onto the cam sprockets like the drive sprockets and was told unless the teeth look damaged to just replace the stretched chain.

As for the damage to the cylinder walls, we took it to a machine shop and had them mic the head/cylinder. I guess the head and cylinder was salvageable and is still the standard bore.

While we were waiting for the machine shop to get back to us, I stumbled across a fully rebuilt engine for 300 bucks which had a brand new cylinder, je piston and cam chain.

So we opted for that for the rebuilt engine ;)

More than likely we will grab another standard bore piston/ring, new valves to replace the 2 intake valves that hit the piston, new valve seats and just reassemble it in the future as a spare for this quad.

With the wealth of knowledge on here and the rest of the internet, I am surprised more people haven't rebuild the engines themselves or can/are willing to share their information.

Me being a hands on type of guy, I like to fix things myself but also like to know what i am getting into before i start. Since all i could find on here were how to replace pistons, etc that is why i asked.

I bought the honda service manual before even thinking about working on the quad in pdf form and found that it is pretty much worthless, unless you want a diagram with numbers showing the order you take things apart/put them back together but no torque specs.... ?!?!?

Since we didn't buy the quads new, we don't have the owner's manuals for the 1995 300ex, the 1997 300ex donor bike/golf cart monstrosity, or my 2005 400ex for that matter.

From what I understand the cam chain replacement is listed in there so I am guessing that's why it is not in the service manual :(

Thanks

Steven

[roadthrills]

I totally overhauled mine myself with a Clymer repair manual. It is very in depth, and they do a good job of explaining things. You should get one.