400 toasted need help and advice please

1998 xr400 bought 4 yrs agao in rough shape but still running strong with a valve clearence check oil change and some upgrades...

so new in last 2yrs. mikuni pumper, new CR Hi bars, Q4 exhaust, tires, bark busters, front wheel etc. also fully plated in Colorado
with all the new additions i cant bring myself to part it out.
So after all these years....  while in a rush I did the fatal rookie mistake! oil filter in backwards!!!!!
one long 3 hour ride and after that it was all i could do to start it in order to move the bike a couple blocks to my new place.
never having torn into an engine i pulled it apart last night after trying to adjust the valves with hopes that i could do valves oil change and fire it up but... 3 valves were way looseand adjusted fineBUT the passenger side exhaust valve i couldnt get the feeler gauge under no matter how much i turned out the screw!

to me...

being un-knowledgeable about inside of engines i notice the cam looks rough...where it sits looks worn and some of the rocker arms look worn and 2 feel sharp on the edges (closest to spark plug)... piston allthough not removed yet looks ok but would like to replace since Im in that deep along with CRF450 cam chain... cylinder/sleeve looks ok to me?


and not sure ...what am i looking at needing done given the limited view of pics and knowledge of my fatal mistake??? and if you know a reliable shop that can help me out for services in the Denver area????


Bummer, but if you do decide to part it out, DIBS on the carby!

It needs a full evaluation by someone qualified. I'd guess you're looking at minimum of, bore and hone, oversized piston , new cam, new valves, seats machined. Roughly $600 in parts and machine work +++

Really a bummer, which could happen to any of us for various reasons.


You need to get the opinion on how much damage might have been done to the lower end.


This might get you back to having a damanged engine, but I would look for an XR400 with a totalled frame.



For sure the cam and rockers are shot, maybe the oil pump. The metal ground off the cam and rockers is now in the oil and has come into contact with all the engine components.  Completely disassemble the engine and check everything for wear/damage. Have the cylinder measured to see if an overbore is needed. If major damage is throughout the engine, a good used engine would be a cheaper route.

That's a slower and more complete death than losing the drain plug.

yeah upon looking closer after a good degreasing its obvious the the cam and rocker arms are toast


when i changed the oils i found no metal shaving...perhaps too fine for me to see?


any idea how to check oil pump while torn down? and what i would be looking at  for  bottom end damaged? seems fine metal shavings arent likely to create really obvious damage.

or am i wrong?  guess this will be a much bigger project for this winter! was hoping to not have a project this winter... and just ski and work! also not having the $ to do top and bottom engine... crazy ...frustrating that rushing an oil change and complacency killed my ride! take your time everyone..... 


 We've all done stupid things. Sometimes we get away with it, sometimes we don't. 

At least it was toward the end of the riding season?   ;)

If you rebuild it, go to 10:1 compression, pump gas but much more alive.  

If you need some help, I have been in a couple Xr's in my time. Broomfield area. There's a couple folks here that could vouch for my handiwork. In fact, I am beginning an XR250 rebuild this weekend.

thanks for the advice and offering of help... i may need some for sure i have to figure something out... while taking it apart I never saw the little spring near the cam ! and have not found it yet... I've been huntingelk  the last 4 days and season is over so i will look into "finding" the spring this week and investigate the bottom end a bit...

looking at pistons 85mm is standard bore correct? many pistons are coming up "5.25 and 5.50 standard bore" since my cylinder appears to be in good shape should be able to do the 85mm 10:1 compression without bore work correct.... remember i have never cracked into an engine before,

any alternate pistons suggestions.... thoughts on going larger? since i live at 8500ft. and ride mostly above10,000 i lose a bit of power compared to you guys in the low country! thanks again for any help or advice offered.... i also found a Weisco 85mm 11:1 for 135$... thinking about pulling the trigger on that one!

Edited by brettb34

You have to measure the cylinder with special tools. Dollars to doughnuts it's worn beyond limits.

I have bore gauge tools, top end kits are all over the web. pull what you have, measure, then decide if you want to go one over or go big bore. bigger you go, the more money you'll throw at it. Not necessarily just the piston kit, but bore work, hotter cam, jetting, mikuni pumper, etc.


as for the altitude, the general rule is you lose 3% HP from sea level for every 1000' of altitude. So your stock XR400 at 31hp measured at seal level is pumping out a staggering 21hp at 10,000ft. Depressing isn't it?


SO yes, you can improve upon those numbers, with displacement and with compression increases. the 10.5/1 compression on aftermarket pistons works well, as do the stage 1 cams, along with other random mods like gordons, etc.I would personally steer clear of higher compression, with the crap premium gas we have here you'll not yield the benefits unless you want to put race gas in your bike. It's really a matter of how much money you want to throw at it.


the 426 kits are well liked, ask SMV ryder how he likes his. this was the thread on his rebuild:




get me your cylinder, and I can measure for you. I am headed to Acme Machine in Fort Collins this Saturday to drop off our XR250 jug I have getting it bored to 284cc.

 since my cylinder appears to be in good shape should be able to do the 85mm 10:1 compression without bore work correct.... ?


You must hone the cylinder to get new rings to seat.  If you have scoring in the cylinder, you may be beyond spec after honing.


I agree with stock compression. 

+1 on stock compression. (Pemex)

If it were me: replace the cam/rockers and try to fire it up. the piston/cyl doesnt look THAT bad, but will likely burn oil at the very least.I ran my 250 out of oil last year; replaced the cam, rockers, bent exhaust valve and found a used 280 cyl w/ piston. My piston/bore was way more messed up than yours appears. Id suggest pulling the cylinder and checking the piston skirt, thats where itll wear first/most. Maybe toss some rings in if there isnt friction damage. Also check for excessive vertical play (side to side is normal) in the main bearing (where the rod connects to the bottom end).


If itll fire up, loosen an oil fitting to make sure its pumping (also do this after every oil change)!


Of course this is by no means good advice. Everything should be measured with the correct tools, but I did mine poor mans eye style and its been fine (aside from burning/leaking about half a qt every 3-400 miles). TBH im just running mine til it craps out again. Its lasted much longer than I expected (7k miles fingers crossed).

Edited by Coop355

Listen to redhurricane... he's done this before. There is no way to guess if you NEED to bore without checking the clearance with a mic.


My son just toasted his 2002 XR400 (ran out of oil),  To my surprise, the piston was the only real damage. Cylinder was not scared, but had aluminum deposits from the melted piston.  Since it needed a new piston,  I bored it to 416cc (found a deal on a Weisco 87mm @ 10:1 comp.)


Cam was a little rough so I swapped it with my old stock cam. (replaced mine with HotCams stage 2)


It's truly amazing how tough these XR's are... Even when we test how far they'll go without oil !


Son's bike running strong now. "Still in break in mode"



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