Jump to content
Sign in to follow this  

How hard is it to change the piston?

Recommended Posts

I bought a used 04 kxf250 and it has low compression. I had a leak down done and they said piston, blah, blah. The thing runs fine, starts one one kick, but I'd rather not risk it.

Anyhow, I have all the parts and I've looked at the manual and I am wondering how hard it is to do. I've done valve adjustments fine on my yz250f, crf450 & now the kxf. Is there anything super tricky about this that a fairly competent and patient home wrench can't do? Special Tools? etc?

tia

Share this post


Link to post
Share on other sites

You probably cracked ur piston if u have unusual low compression( no resistance at all)

But if u have looked at the service manual and have all the tools to do it go ahead it not hard at all. Just dont think the piston rings compression themselves like on the 2 stroke top ends.

Share this post


Link to post
Share on other sites

It is fairly easy to do, the hardest thing is actually resetting the timing after you put everything together. By all means get a service manual (I have the link if you want to get one in pdf) and follow it closely. Have you rebuilt a top end on any bike before or at least some kind of engine? If not there are some basics you should know, I'd be happy to go into detail if you want more info. Finally, make sure you are very gentle with the rings, especially the oil ring (it is the one that is bent into a crinkly shape) as they are quite easily bent.

Share this post


Link to post
Share on other sites

calidunes, i'd appreciate any info/knowledge your willing to share. I have the service manual, and the only the most I've gone into an engine was removing the cams to put in shims. Do I really need a Piston Pin Puller? Is there a homeade solution that works?

Share this post


Link to post
Share on other sites

If you can shim the valves, you can do a top end. Dont drop the circlip into the cases, no need for a pin puller, just use a socket and extension and support the back of the piston with your hand. Torques are critical and so are clean gasket surfaces. The bike is very easy to do a top end on if you can time the cams properly.

Share this post


Link to post
Share on other sites

Make sure the oil ring expander ends stay butted when installing the oil rings over it. I found it easier to put the piston in the jug on the work bench, (easier to see what is happening with the rings) leave just enough piston sticking out the bottom of the jug to get the wrist pin in. Pack the area around the rod with rags so you don't drop the clip in the crankcase when installing in the piston. Use a piece of safety wire around the timing chain so you can pull it back up thru the head to install. Make sure the chain guide toward the front of the bike is in proper position (in the notches) before you attach the head or it will not go in place later. Look at it closely when taking it apart. Oh. and for some reason the manual says do not use a ball type hone. This seems a left over from a 2 stroke manual. Also make sure to keep an eye on all the dowel pins as they don't stay in place very well during disassembly. Some stay in head, others in cam caps, others in the jug. Easy to lose one from the bike to the bench. Happy wrenching!

Share this post


Link to post
Share on other sites

Just wondering, how do you or they know from a leak-down test that it's the piston and not valves?? A bent valve or bad seats will pass air (or nitrogen) just like anything else.

All you know right now is that you have low compression... not the reason.

Share this post


Link to post
Share on other sites
Just wondering, how do you or they know from a leak-down test that it's the piston and not valves?? A bent valve or bad seats will pass air (or nitrogen) just like anything else.

All you know right now is that you have low compression... not the reason.

I thought that was the reason for a leak down? From what I understand a compression check checks the compression (and it was low), so I took it in for a leak down and supposedly you can hear where the air is passing through and pinpoint the valves or piston. I found a good explanation here http://ca.autos.yahoo.com/maintain/cylinder_head_answer5.html

Im gonna try tonight, but the valves are definately going to get a look regardless, if not replaced with SS just to help me sleep better.

Share this post


Link to post
Share on other sites

If the valves are shot you will have low compression too, regardless if the piston is good or not (if the piston is bad as well, then the comp. will be even lower). You may need a piston pin puller, but you may not. It'll depend how badly it is stuck in there, but you can pretty much go with the wooden dowel and hammer method because you don't care about beating up the old piston. 4strokesrule covered a lot of the things I was going to mention. But I have a few more to add; make sure you put oil on the piston pin, make sure the cylinder is still in spec (it may need to be honed as well). Make sure everything (ie, the area around the cylinder head, etc.) is spotless, I recommend giving a thourough wash of the whole bike before you start working. Honestly, the hardest part of the bike is setting the timing back up. When you put everything back together. Turn the bike over with your hand a few times slowly, before you fire it up. You want to listen for any kind of noise that suggests the valves are hitting the piston or that something is loose, was dropped in the engine, etc.

Share this post


Link to post
Share on other sites

A question on piston change. If you remove cam chain and all of top end does it matter if you turn engine over. I know how to time up the cam chain with cam shafts as i have done a shim changees, but i never turned the engine over so i new that i was putting it back how it came apart. ie does it matter if the engine is on its compresion stroke or exhaust because it seems that you can line the dots up in either. So does it spark on compression and exhaust thanks Clifford

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...