new top end on my thumper, how to check the head?

Discussion in 'The Garage' started by skierd, Feb 19, 2013.

  1. skierd

    skierd Wannabe Far-Rider

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    I've got just under 43,000 miles on my WR250R. I decided to take the time to finally tear it down and go over everything this winter since I'm stuck not riding for the first time since I've owned a bike. I noticed my bike felt down a bit on power at the end of the season, so one of my goals was to do a compression check to see how healthy the motor is.

    Spec is 120psi, the gauge showed nothing more than 75psi. The motor cranks and runs fine and has never used a drop of oil in its life, but it has been dirtying the oil faster (in say 1000 miles versus the 2000-2500 it used to take for the shifting to get crunchy). When I added some oil down the plug hole, it spiked up to 100ish psi, so worn rings are likely the culprit. I ordered a new piston, rings, and wrist pin along with the necessary gaskets and a couple dozen other small minor maintenance parts I've been making do with.

    What I want to know is, how do I check the head, specifically the valve seats, valves, and guides for wear and what am I looking for when I have the head off to show that the valves need attention? How do I know if the valves are worn, if the seats need recut, if the guides need replacing, etc? I've only ever taken the head off one motor before (an old 2.5L turbo dodge that blew a piston) but never tried to do any work on one, so any advice is welcome and pictures definitely help.
    #1
  2. Beezer

    Beezer Long timer

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    pour some kerosene in the ports & see if it leaks through the valves.... my guess is it will. if it were me I'd probably get new valves just because it off & you have access. if they look really good then maybe face everything off, lap it, consult your manual... some valves you don't lap because of the special coatings.
    as for the guides... that needs a measurement with the right tools. you can get an idea by rocking the valve in the guide (springs off) & seeing how much play it has. some manuals have this check listed as well as the engineered fit and limit. usually the guides stay pretty good with a bucket design on top of the valve
    #2
  3. thomas.tc.young

    thomas.tc.young Been here awhile

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    +1 on everything beezer said. it's a single so just send it out and have the valve seats recut. then ride for another 40k miles.
    #3
  4. WayneJ

    WayneJ City Slicker

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    Dave, did you remove the head and check the cylinder?

    Before ordering piston & rings it's SOP to remove the head and check the cylinder for wear.

    I'm not familiar with your engine so I don't know whether the cylinder can be bored or if it would need to be replaced, some cylinders are plated and can't be bored without having the cylinder re-plated. If it's plated you can't hone it either.

    With 40,000+ miles on the engine I'd be surprised if the cylinder wasn't worn beyond spec. A new piston and rings would be a waste of time and money if the cylinder is worn or grooved.
    #4
  5. D.T.

    D.T. Difficult but useful

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    Wow, that's lots of riding!

    I would have waited to tear down the head first before ordering. Then just get the worn parts fixed, if your a cheap bastard like me. :lol3

    Use some sort of solvent to check for leaking valves is always good.

    You check valve clearances yet?
    #5
  6. qkenf4u

    qkenf4u Been here awhile

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    nice warm bike riding COTTONWOOD ARIZONA...
    just and FYI.. youre not gonna get much more than 75PSI since your bike has a compression release built in... sounds like its just tired as any other bike with 43k would be...

    did you check valve clearance before tearing down ??? that is a huge indicator if the head/valves are worn out...

    once you get it tore down check a ring end gap with the old cylinder and a old top piston ring... that will tell you a ton about the bikes condition...

    i rebuild these bikes (250/450 yz/wr/rmz/crf/drz etc) bout once a month at the shop i work at....

    its a nikasil cylinder so no boring but make sure the nik isnt worn thru anywhere. you will see ares that are GRAY (like aluminum) if the nik is missing/worn...

    you can check the new piston fit by using the new ring and checking ring end gap (.003-.004 for every 1" of piston diameter 3" piston = .009-.012" ring end gap)
    and
    also piston to cylinder wall clearance (.003" ? your manual will tell you this if its a factory manual/piston)

    yes you can re-cross hatch a nik cylinder with a ball hone .... it takes about 5 swipes with a ball hone and it will be a nice clean bore as long as its not worn thru..

    if you are really worried about it have a machine shop check it for wear from top to botton (dial bore guage).. if its way out of round then youll have to either replace it or have it recoated (lots of places to have it coated but $$$$) we usually have the customer just get a factory cylinder/piston set and save them time and money...

    im gonna say the intake valves are prob. like razor blades on the edges by now (margin is prob gone).. replace them all with KIBBLE WHITE VALVES (stainless) and lap them in,unless the seats are really beat up too, if so you have to have them cut and lapped to new valves. you can check how the new valves seat by using black majic marker and just rolling the new valve against the old seat... it should ride bout the middle of the valve face if correct. this pic is a perfect pic of how much contact you should have and where...

    http://www.google.com/imgres?hl=en&...start=25&ndsp=34&ved=1t:429,i:170&tx=54&ty=89


    i use a BRASS wire cup brush to clean all the old valves if they are good (your exaust are/maybe still good as long as the margin is still good .050")
    http://www.google.com/imgres?hl=en&...&start=0&ndsp=26&ved=1t:429,i:103&tx=70&ty=62

    if you stock valves are not worn bad and have a good margin you can just lap them back in after cleaning all the carbon our of chamber. look at the valves close cause they can have a ok margin but the face will be DISHED (not flat anymore)

    also make sure ya put new valve guide seals in while ya have the head apart.

    i have yet to see a guide worn out..... the bikes i work on are all dune/sand/single track arizona bikes so they get a bit more abuse w/ the sand in the air than most bikes out there...

    i use FINE lapping compound when i lap the valves... make sure its all cleaned out of head and off the valve... once i have the head back together i check them with solvent/carb cleaner/alcohol for leaks.

    good luck
    #6
  7. skierd

    skierd Wannabe Far-Rider

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    I haven't started tearing in to the motor yet except for the compression test. Is there a way to do prevent the decomp from working to get a good test? Have not checked the valves yet even, been waiting to get the swingarm bolt loose first.

    I ordered the piston with the hopes of being able to just put a new one in with fresh rings. If the cylinder is damaged, its getting shipped to Thumper Racing for a 280kit sleeve and I'll return or sell the new OE stuff.
    #7
  8. clintnz

    clintnz Trans-Global Chook Chaser

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    Have you done many WR-R's? They are quite different to the WR-F. A longer lasting motor so there's not so much rebuild knowledge out there yet. Some good advice there though :thumb

    I'm not an expert on them at all but I did have my girlfriend's one apart to replace a failed countershaft bearing a few months back, at about 22k miles. The ring gap was well in spec & the bore, piston & valves all looked like new. Will be interested to see how your's looks Skierd.

    Cheers
    Clint
    #8
  9. anonny

    anonny What could go wrong?

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    With that mileage I'd tear it down and plan on piston and rings and Kibble White valves as already suggested. Another thin you can do to prove you need to do something is put together a simple leak down tester ala

    <iframe width="560" height="315" src="http://www.youtube.com/embed/LeASdDbGI3U" frameborder="0" allowfullscreen></iframe>
    #9
  10. qkenf4u

    qkenf4u Been here awhile

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    nice warm bike riding COTTONWOOD ARIZONA...
    sadly we dont get the WR-R.... so no i cant compare the two... actually the 5 valve heads are stupid dependable... very seldom adjustment and once the valves are done ya just throw KIBBLE WHITE VALVES in and enjoy the motor for a few more years ..(replacing a piston or two along the way) ... :clap

    i do agree with ANONNY that a leak down would tell you a lot about the top end... oh and you can do a comp test , it will end up around 65-75psi if its in good shape...
    #10