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If half the lower tip of the valve guide breaks off, does the whole valve guide need replacing?

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This is what it looks like

https://drive.google.com/file/d/12SPWO3qBCmdnkYp81xcasu0oh61iTGPw/view?usp=sharing

 

If anyone is curious about the back story then here goes...

I didn't realise how lose the steering head bearing was until I had it up on a stand. the lower end of the forks could be moved in a circle about 5-10cm in diameter without the rest of the bike moving. I think this excessive play caused me to come off at relatively high speed. I remember the front end feeling really weird and basically folding under the bike as it flicked to full lock. Me and the bike went fyling about 20-30m off the track and the subframe snapped in a few places. I managed to finish the trail ride and rode the bike as is back to the car park. When I got back to the car park I noticed the air box was half removed from the carb. So half the air going into the carb was straight atmosphere dirt, dust, and maybe stones. After welding up the sub frame and taking it to another trail ride, the bike stalled really quickly half way up an 'expert' hill. After that I couldn't get any compression out of it so I had to be towed back to the car park for the first time. Now that it's back home I was hoping to strip it down, sort it out and have it ready for the trail ride the following weekend but my hope is starting to fade. Here's what I think has happened - A stone was sucked into the carb and thrown into the intake manifold and sat there for quite some time. Finally the stone got stuck between the intake valve and the head. This stopped the valve from returning so the piston hit the valve on the next stroke. The valve stem bent which snapped the lower tip of the valve guide as well as the spring retainer up top. After that the bike wouldn't start but it didn't stop me from trying. Unbeknown to me, every time I tried to start the bike I was mashing the broken bits of valve guide into the piston and head. After getting the bike home I began the tear down and first noticed the missing valve spring retainer. As I pulled the spring off, two halves of the broken spring retainer promptly fell down the cam chain well. So I have removed the clutch cover and recovered one broken pieces of metal. The other one is somewhere behind the oil pump below the main gear. I need to figure out how to remove the clutch basket and then see if I'm any closer. Lets just say I'll be making sure there is no sign of play in the steering head from now on!

 

I'm hoping to leave the existing broken valve guide where it is and just drop a new valve in. Does anyone know if that would have much if any affect on longevity?

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From the picture it isn't real clear to me how much material is left. One of the jobs of the valve guide is to transfer heat from the valve stem to the head. With part of it missing it will not do that as effectively. 

Will it run if you put it back together as is? Probably...for a little while. But there is risk that the new valve you are putting in will be damaged or wear out prematurely. 

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6 minutes ago, Bad Moon said:

Will it run if you put it back together as is? Probably...for a little while. But there is risk that the new valve you are putting in will be damaged or wear out prematurely. 

Not to mention the rest of the broken valve guide breaking up, coming out and trashing things again. You're having to replace the valve. Why you wouldn't buy a new guide and install it at the same time is a mystery.

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I would be concerned with additional cracks forming or already being hidden in the guide that you can't see. Cracks form at sharp edges usually, so the chance of it getting worse is probably good. I'd replace it because next time it breaks, it might be a lot more expensive. 

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Haha I like the idea of a half valve guide. Maybe I could JB weld the fragments back onto the guide and drill a hole through the middle for the valve to fit.

 

Half of the tip broke flush with the top of the intake manifold.

 

There are a few reasons why I didn't want to replace the valve guide.

1. The intake valves were replaced a couple of years ago and I assume the guides at the same time. So it's not flogged out or anything.

2. Time is of the essence. Would like to get it ready for the enduro next weekend and replacing valve guides looks like one of those pita jobs that could take me weeks.

3. The cam chain will probably need to be replaced within the next few years anyway. The valve just needs to last about that long and I can change it at the same time as the cam chain.

4. If the valve were to bend again I assume it would bend in the same direction. With that in mind I was hoping it wouldn't brake the valve guide any further because the bit that normally breaks is already gone.

5. I didn't want to do all that work and then somebody says why would you bother?

But I didn't consider heat transfer and I wasn't going to worry about micro fractures if it feels solid on my finger. You guys have bought up some good points so I think I will replace it if time permits.

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 If you already have the tools for replacing a valve, then you probably do for replacing the guide too. It's just the 1 guide. If you have the tools and parts, it's an afternoon job.

Maybe it's just me, but there's no way I'd risk and assume that if the engine bent another valve during the race that it wouldn't also take out the piston crown (that's been subjected to impact once already), resulting in trashing the whole top and bottom end. Over not wanting to miss an enduro.

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Ah I was wondering what would make this more expensive - Ruining the piston and bottom end would certainly do that.

The clutch basket has been removed and the last piece of the spring retainer and half of what I assume is the circlip has been recovered. I ended up just torquing the staked nut off with a breaker bar. Probably not the best for the thread but everything else I did just seemed to push the stake in further.

I have also been reading up a bit more on valve bending, guide replacement, and cam chains.

Apparently it is possible for valves to bend when the cam chain becomes too lose and skips a tooth. I thought one tooth would be enough to make it run terribly but not enough to make the valves and piston attack each other. That's why I was planning on just running the cam chain until it runs rough and then replace it. Unfortunately I didn't check the cam sprocket alignment before I removed the head to see if it had skipped a tooth or not. But I guess it's entirely possible a tooth was skipped which caused the valve with the tightest clearance to bend on the piston. The tensioner is at it's most extended point (11mm measured from the top of the tensioner to the top of the piece of metal that sticks out of the tensioner) and there is room to move it back and forward without removing the cam chain or sprockets. The other point to note is the engine has been making a ticking noise for a couple of years as if the valve clearance is too lose. I assumed it was bad valve clearance. Then I adjusted it and it didn't go away so I assumed I did it wrong. Then recently I measured and adjusted the valve clearance carefully but the ticking still didn't go away. Hindsight suggested it was the stone making the noise but it seems other people on this forum have heard a valve tick when the cam chain is worn. So maybe there was no stone after all.

I have decided to replace the cam chain, valve, valve guide, and valve spring retainer. Then hopefully I won't need to look at the engine until the piston rings, bore, or gaskets need doing again.

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13 hours ago, Sharft 6 said:

I have decided to replace the cam chain, valve, valve guide, and valve spring retainer. Then hopefully I won't need to look at the engine until the piston rings, bore, or gaskets need doing again.

I think that's wise.

The valves making a ticking sound when adjusted properly is normal. The cam chain when it's worn out however, can make a terrible sound when it has a lot of slop in it. Which sounds like it is, if the tensioner is fully extended. Pull the tensioner itself, use a screwdriver to retract it, to make double sure its internal torsion spring is intact and makes the plunger extend out on its own when you release the screwdriver.

When removing a staked nut, the proper procedure is to carefully file or grind away the stake.

You haven't said yet what model and year your XR is.

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It's the 2001 model

I had to wait a couple of weeks for the valve spring retainer to arrive from japan so I missed all the enduros this year anyway. Changed my mind about replacing the valve guide because I'm kind of over the bike and just want to run it into the ground then part it out and get a new one. I also didn't lap the valve in, stake the clutch nut properly, or replace any of the gaskets so something's bound to go - will post back here when it does.

The valves are not even making the slightest ticking nose now it sounds good. Interestingly it doesn't seem to have lost any power after neglecting to lap the new valve. Also It only takes a very light kick to start now. It hasn't been this easy to start since I drowned it in the river a few years ago and welded near right angles in the exhaust. Shows how much difference a new cam chain can make!

Edited by Sharft 6

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