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Questions About Rebuilding The Top End of an Engine

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Hello everyone,

I recently disassembled the top end of my engine on my 2004 CRF250 (Following step-by-step directions of a Clymer manual ofcourse)

Before I decided to do work to the bike, it was extremely hard to start. I also felt like I had lost a great deal of horsepower when I was riding.


  • Since I already have the head off, I planned on replacing the Piston, Rings, Pin and Clips. I don't take my engine apart often.

  • I also wanted to replace some parts in the head such as valves, valve springs ect.

  • The bike is 7 years old and I bought it used, so I'm thinking it's time I replace some parts.

  • The last mechanic I took this bike to ripped me off, so this time I'm doing this job myself. I have successfully rebuilt automobile engines and I don't doubt my own work.

  • I created this thread as I'm just looking for advice, tips, knowledge or anything else along those lines.

*KEEP IN MIND, THIS BIKE IS A 2004, and I have never touched the top-end. I bought it USED in 2006.

CRF250R2.jpg

So today, when I went to check the valve clearance of the Intake Valves, the clearance was 0, as I could not fit any size of the feeler gauge in between the cam lobe and the valve lifter (intake side)

The question I have for you all today is this:

  1. Should I replace the valves?
  2. Should I replace the valve springs?
  3. Would you recommend installing the OEM stock piston and parts or going with an aftermarket high compression piston?
  4. If I installed a high compression piston in this bike, what other modifications or adjustments would I have to do?
  5. What other parts in the top end of the engine would you recommend replacing?

Below, I've included pictures to give you more of an idea of what I'm dealing with here.

2i7mkvo.jpg30lor45.jpg

Here is a picture of my piston as it currently sits in the bike.

Thank you all for your input. It's greatly appreciated.

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You can reshim your valves to get there clearances right, but that only gets you by for a bit. I'd go with the new valves, seals, springs, keepers and retainers. I bought kibble white stainless steel valves and they are 100000 times better than stock. You also need to cut your valve seats when installing new valves. For a high compression piston people like JE 13:5:1 and you can run 91 octane. And you are most likely going to need a crank soon considering the fact that it's a 04. But that's up to you so we'll see what others have to say about swapping the crank

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The crank is going to be suspect being an 04, so while it's apart, you should replace it. Wiseco has a nice kit you can find on e-bay for around $225. Comes with the crank, bearings, seals and bottom end gasket set. About the only other thing to consider is the timing chain. They stretch fairly quick, so if it's the original from 2004, I'd be willing to bet that it's shot. Personally, I also would go with an aftermarket piston. JE, Wiseco, Moose (CP). The quality is far superior than OEM. The 13.5:1 will add a little more power, but with what you're doing to it, the tired old engine will come back to life like new again.

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A few things to consider is how much per year do you ride? If you go out often and the are still running the stock crank it would be a good idea to swap it out with a new one of your choice. shimming the valves is a band aid to get you by for the time being , if you can afford it do the intakes. Have you ever shimmed the intakes before ? what was the clerance the last time you checked them? Timing chains are cheap and one less thing to worry about when your out and about also look at the adjuster. I am not saying its a must like others will but it will help put your mind at rest. Again alot of it depends on how much you ride , if you ride twice a month in a field behind your house for half hour you will be good to go with shimming the valves for the time being , if you ride the alot i would go through the whole thing and do a rebuild and slap an hour metter on it when your done and not worry about it for a while. just my two cents worth

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I only have been riding a few times every year. I usually ride the open practices, which are 20 minutes every hour for 8 hours. This year I plan to be on the bike a lot more than I have in the past.

Well today I went to the motocross shop and dropped off my cylinder head.

The services I purchased today were:

  1. Recondition Intake Valve Seats $18.00
  2. Recondition Exhaust Valve Seats $18.00
  3. (2) Kibblewhite Exhaust Valves $79.90
  4. (2) Kibblewhite Intake Valves $79.90

Now I've been reading online in forums and a few guys were saying that you shouldn't run that heavier valve train with a weak stock auto chain adjuster.

They were saying that the chain and or chain tensioner could cause the valve to slam into the piston head causing the engine to completely seize.

Also I want to run a High-Compression piston but some people are saying that I would need to replace the crankshaft before I did that. A crank shaft is very expensive and I don't have experience working on the bottom-end of an engine, so that's a bit scary for me.

Let me know what you guys think.

  1. Should I replace the entire crankshaft if I run a high compression piston in this motor?
  2. Should I replace the chain or chain tensioner on the camshaft due to the new Kibblewhite valves?

Thank you everyone for your feedback so far. I'm trying to act like a sponge and absorb any new information I can. I really appreciate it.

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you had said the intakes zeroed out but you did not metion the exhaust? if the exhaust are in spec i would leave them be. next queston is why the high compression piston ? do you need more power , are you ridding the bike to its full ability and feel the need for a little more? a new crank can cost a little bit of money but its cheaper then having a rod snap and blowing your cases apart or taking your head out. the can chain is just cheap insurance same goes for the tensioner. keep in mind that if you do get into the bottom end more tools are needed to undertake the job. I would also buy the honda shop manual if you do decide to do the crank. again this is just my two cents take it for what its worth and best of luck

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Take it for what it's worth. Replace the intake valves, have the seats cut on the intake side and re-lap the exhaust valves to make sure they have a good seal. Replace the crankshaft. All you need is a flywheel puller. Get a real shop manual, the Clymer is fair at best. If you follow the instructions and have fairly decent mechanical ability, you can replace the crankshaft yourself. It's not that difficult, but requires care, attention and organization. Timing chain is a must and a high compression piston is not. Trust me, you'll be sorry if you don't replace the crankshaft. Re-shimming the valves you'll be lucky if you get 1 maybe 2 rides out of it. Make sure you have a torque wrench or you'll be posting "how do you get the broken bolt out of the _ _ _ _ _ _".

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^^^^why do you think he needs to re-lap the exhaust when he never said they were out of spec? also if you replace the crank you are going to need new bearings which means you will need them pressed in and out, i dont know your machanical ability or budget but these are all things to consider

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^^^^why do you think he needs to re-lap the exhaust when he never said they were out of spec? also if you replace the crank you are going to need new bearings which means you will need them pressed in and out, i dont know your machanical ability or budget but these are all things to consider

I'm pretty new at these CRF250's so if I was out of line with what I mentioned, I apologize.

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Sorry about the late reply guys...

Yes, I checked the exhaust valves and they were in Honda spec.


  • The reason I suggested a high compression piston was just due to the fact that I have the whole head apart.

  • My logic in thinking was that I mine as well replace anything necessary while the head is completely apart... due to the fact that I usually don't have this much time to work on the thing.

This is the first time I've had it apart.

The reason I wanted to replace the piston and rings is because I bought the bike used in 2006 and I don't know if the original owner ever rebuilt the top end.

You can never trust the owner selling a used bike! lol

So everyone's saying that replacing the crankshaft is a must.


  1. If you don't recommend Clymers manual, what is a better manual that you would suggest? I have Haynes too but it's not all that great of a manual.

In college, my automotive classes used expensive software (ALLDATA) that made an engine overhaul seem easier than building a model airplane...

With dirt bikes... I haven't found such luck with manuals and or software.

Thanks for the feedback!

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The Honda shop manual is about the best you can get and for the 06, you can find them real cheap. ALLDATA I use all the time. Have a buddy that ownes a car dealership so he gave me the user name and password to get in it. Its about the best! Regardless if the exhaust valves are in specs, re-lapping the valves insures a good seal between the vavles and the seats. From dirt and dust being ingested into the intake track has to go somewhere so wouldn't you think that some of it might go out through the exhaust valves? Thats why you re-lap the exhaust valves. It removes the fine particles enbedded into the seats and valve seating surface.

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All you need is a flywheel puller.

Charlie's right (as usual :thumbsup:) .... that's all you need.

... also if you replace the crank you are going to need new bearings which means you will need them pressed in and out ...

Heat should be fine to get the bearings out. To put new ones in, just put the bearing in the freezer for 20 min, heat the case ... the bearing will go in as smooth as butter!

Get a real shop manual, the Clymer is fair at best.

I use the Clymer, and I think it is actually pretty good IMO.

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The bearings are simple to replace. Clean the cases and put them in an oven @ 400 degrees for about 15-20 minutes. The bearings fall right out. Be careful, they are hot and all the bearings fall out so be prepared to grab them with a maginet and set them back in. There is no stress on the bearings when you remove them and install them this way. Once the new bearings are in, let the cases sit and cool off. Don't force them.

Clymer isn't bad, but I think the Honda shop manual is more detailed specially for folks that don't know these engines as well as others.

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  • Charlie thanks for that tip my man. I would have never read that in a manual.

I think things are starting to become fun hahah

Well at 20 years old, I'm glad I'm learning.

I'm gonna save tens of thousands over the next 30 years with this bike maintenance if I do it myself.

I gotta try sometime... now is the time.

Well here's some pictures of the progress. Don't mind my garage.. aka my mom's Shed lol

oivpf8.jpg

szd2zn.jpg

3007azl.jpg

Seriously thinking about having my frame and swing arm powder coated black considering I will never have the bike

in this many pieces again lol but I'm afraid of how durable that powder coating would be.

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Powdercoat is pretty durable. If you do the frame and swingarm, don't ride it. Put it in your room and admire it. After a few spills or a few roost's from the 450 that went around you in a corner, you won't be happy you had it powdercoated. You're better off taking some WD-40 and a scotchbrite pad and cleaning the frame to make it look like new again.

Oh, the oven trick for the bearings is not in the manual. Those are the types of tricks that you learn over time from the school of hard knocks!

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^^^^ How do you sugest for him to hold the fly wheel when he needs to put it back on?I understand your theroy on the exhust valves but at the same point i dont think its nessasary as there is nothing wrong with them and they are in spec. The bearing trick is hit and miss IMO i have had it work well and also not work at all. As far as the manual goes I have and always will use the honda factory manual , its the same one that Honda uses in house , just saying for an extra 20 bucks its worth the money. the best way i have found to pull the fly wheel off is with an impact gun and a puller , to put it back on i use the proper tool , some guys use an oil wrench or other methods but the tool works the best IMO, good luck and keep us posted

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To remove the flywheel, the book says you need a "24 mm x 1.5 right-hand internal thread fly-wheel puller." The manual calls for the Motion Pro Fly Wheel Puller. YAY... more tools I have to rent.

So far I have replaced:

  • Intake Valves
  • Exhaust Valves
  • Valve Reconditioning

and so far I'm thinking about replacing...

  • Timing Chain
  • Timing Chain Tensioner
  • All Top End Gaskets (of course)
  • Wiseco Complete Crankshaft Rebuild Kit
  • MAYBE a High Compression Piston

5344__0.jpg

This is the kit and it sells for about $250.

The stock crank alone sells for $300 so its pointless to not buy aftermarket.

I am looking to make the bike faster and at the same

Anyways what do you guys think?

And is a high compression piston worth the money? I'm hearing arguments from both sides...

Thanks Guys!

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Is there any way you would recommend cleaning out the inside of my engine before I replace these parts?

I have a feeling parts of the gasket and dirt particles fell in there during dis-assembly.

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dont switch from titanium intakes to steel intakes unless you run heavier springs....

you will have a kick start time bomb if you dont

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