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Edited Date/Time
11/26/2017 2:52pm
On a stock 2002 YZ250F that I'm rebuilding, I am wondering if anyone can tell me if the stock titanium valve and stock valve guide both expand at the same rate with heat? I'm basically wondering if the stem to guide tolerance will get looser or tighter when the engine is warm?
It wouldnt make sense to have a thou of clearance when the valve expands a thou. Otherwise it would seize in the guide. Make sense?
I don't have any way to hone the guide, and I think it'll be fine. But I was just wondering because if the clearance gets looser with heat, then I feel fine about it. If it gets tighter with heat...I don't know. Then again, now that I think about it, if the intake valves get a steady shot of fuel on them, it should help alleviate my concerns about.
The Shop
I'm pretty sure this is an iron guide too....right outta the Yamaha parts bag. All of the guides in this head look iron to me.
Paw Paw
The coefficient of thermal expansion for titanium is 8.5x10^-6 per degree Celsius at room temp up to 100C (temp of boiling water.) This appears to be the best temperature range to use.
Brass has a coefficient of 19x10^-6 for the same temperature range.
This means that a rod of Ti will grow by .0000085, or .00085% of its length for every 1 degree celsius it gets hotter.
Brass will do the same by .000019, or .0019%.
It doesn't seem like much, but multiply those values by a 100-degree change in temp and it gets significant. Like Acidreamer said above, however, the engineers have already thought about this for you.
I found out that 5/32 is nearly identical to 4mm. And I happened to have a harbor freight drill bit in that size that I simply could not believe how well it fit into the guide. I could also feel the snag in the guide that I installed, but the drill bit passed through the other two intake guides. So a friend held the head, and I chucked the bit into my hand drill and gave it a spin. After checking with the gauge, the snag remained, so I hit it again, and again, and again with some oil. After a few passes, I was able to remove that snag about 80% and the intake valve is now much happier in that guide.
However, I still had a problem from before and after my awesome harbor freight drill bit clearancing, and that was the intake valve wasn't hitting the seat square. You could clearly see where it was hitting 1/4 of the seat first. So I grabbed an old intake valve and lapped it into the seat. I know this is something you usually do to match a valve to a seat, but I was trying to match the direction of the valve to the seat. After several passes with a hand drill hooked up to it, I was able to refinish the offending part of the seat and now my "good" valve sits nice and square in there. I installed the spring and shined a light in the intake port and it looks to be sealing as good, if not better than the other valves.
I know none of this is the right way to do this, but I'm trying to do this restoration (http://www.vitalmx.com/forums/Bike-Builds,46/2002-YZ250F-Restoration,13…) on a budget and I don't need it to be perfect, I just need it to be good.
P.S. The Yamaha manual does say to ream, and it offers a Yamaha part number for said ream. But the manual also says to lap the valves into the seats, so I'm taking their instructions with a grain of salt.
Micahdogg, Yes the factory manual says to lap the valves. However, I'm certain it also says to grind or recut the valve seats, then lap the valves. After replacing valve guides and reaming to the correct ID, you would then recut the seats to make the valve contact patch concentric with the guide, set the contact patch to the correct width (typically .5mm-1mm), and set the contact patch to the correct area on the valve face. Lapping can be done as you did, to correct alignment issues, but the contact area will be too wide. This can result in removing too much material from the valve seat, and moves the valve too far back in the head, limiting your ability to adjust valve clearances. The factory manual specifies lapping as the final step to obtain a perfect seal.
If you find your valve clearance is too tight for the factory adjustment shims, Kibblewhite makes them down to 1.2mm thick
Pit Row
I do agree though that the hand drill/harbor freight drill bit sounds like a bad idea, but everything I've been doing to this head sounds like a bad idea.
And I am half expecting my clearance to be too tight. Good to know on the kibblewhite shims, but I was kinda thinking of removing a little material at the end of the valve. My neighbor has some tools that I think will work for getting a square cut.
As for the "hardened finish" on Ti valves, it's about .0002" thick. They're not telling you to lap the valve for gross reshaping, and the lapping compound you'd use isn't what you can buy at autozone. Clover (now owned by Loctite) makes it up to 1200 grit. I use the fine stuff when I have to.
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