Motor builders i need your help - NorCalMotocross.com - Your Source for Motocross in Northern California

punkinrider55 · #1 (**permalink**) 05-06-2008, 10:53 PM

Anthony just left my house and left his 2006 yamaha yz250f in my garage. He had 15 hours on his valves so they were time to have them checked again. Backtracking, at Hollister the center intake valve broke and took the piston with it. We replaced all 5 valves,springs, clips, ect.. It didn't do anything to the cylinder but did put a nick in the piston..the piston was replaced along with rings,wrist pin, clips and a fresh hone. The crank was also checked and appeared to have taken no dmg.. The bike ran great and we haven't had any problem with it. mind you with is also a valve recall bike. So back to tonight. I checked the exhaust valves and they seem to be right at .17 for both...The intake on the other hand has 0 clearance and we already have a 150 155 and 145 for the current shims.

Getting to my ? is that is this wearing a little fast? to be at such thin shims already. I know what im doing when it comes to the work but i am not perfect. I guess thats why im going to MMI in august to learn all this and be perfect at it. If you guys have any info that would be great. Maybe it is normal and im just worrying about it. thanks again.

countrykid631 · #2 (**permalink**) 05-06-2008, 11:02 PM

allan thank you so much for the speedy work.
bike runs great after every time he checks it.

just these gay valves are pissin me off

CaliMX · #3 (**permalink**) 05-06-2008, 11:17 PM

Where do I start...
First of all you need to try and make an educated guess on why the valve broke to begin with, excessive valve guide wear, valve was shimmed to many times, valve float, etc. Check the valve to valve guide clearance carefully.
When you do a complete head rebuild with all new valves, springs, etc. you have to cut the valve seats to match the seating surface of the valve perfectly, if you match new valves with worn beat up seats the new valves will not last very long at all, the seats may look OK to the naked eye but they will not be perfectly round and the seating surface angle changes over time from the old valves moving and being reshimmed, not to mention when you use new valves without cutting the seats you will not get a good seal and you'll loose some compression. The issue is not with the valves it's with the valve seats and/or the valve guides. You will learn this more in depth at MMI.

punkinrider55 · #4 (**permalink**) 05-06-2008, 11:33 PM

When we did put all 5 new valves in I did use lapping compound and cut new seats for the new valves...after lapping them u can see where the compound was hitting the seat and it has a perfect ring all the way around and was in thickness per the manual spec (cant think of it off the top of my head). It didnt get thinner at one part of the valve and then thicker at the other. It was an even ring all the way around.

This bike was a valve recall bike and the center valve broke in a very odd place. it broke about .5CM off the stem of the valve where the keepers ride and hold the valve up. (let me know if you understand that kinda hard to explain) maybe Anthony can chime in but there wasn't but 20hrs on the bike when that valve broke so i figured the seats were ok and it was just a bad valve.

countrykid631 · #5 (**permalink**) 05-06-2008, 11:41 PM

yeah i dont even think there was mor than 20 hrs on that thing.

but anyways. allan knows what he is doing
cuz if he didnt.
and my **** didnt work.
id have a nice lifted dodge in my driveway!!

CaliMX · #6 (**permalink**) 05-06-2008, 11:48 PM

Quote:

Originally Posted by punkinrider55

When we did put all 5 new valves in I did use lapping compound and cut new seats for the new valves...after lapping them u can see where the compound was hitting the seat and it has a perfect ring all the way around and was in thickness per the manual spec (cant think of it off the top of my head). It didnt get thinner at one part of the valve and then thicker at the other. It was an even ring all the way around.

This bike was a valve recall bike and the center valve broke in a very odd place. it broke about .5CM off the stem of the valve where the keepers ride and hold the valve up. (let me know if you understand that kinda hard to explain) maybe Anthony can chime in but there wasn't but 20hrs on the bike when that valve broke so i figured the seats were ok and it was just a bad valve.

Lapping and cutting the seat are 2 totally different things, when you lap in valves it will make a ring all around the seat but that is not the measurement you take. The valve seating surface area is generally .040", the first angle is usually 30 to 32 degrees, the second angle (Seating surface) is 45 degrees and should only be 040" wide, and the third angle is 60 degrees, you need to use valve seat cutters with the correct angles or take the head to a machine shop that really knows what they're doing and have the seats cut. Also, I know the Yamaha manual says to lap the valves but it's the worst thing you can do to ti valves, all ti valves have a coating on them that dissipates heat, when this coating wears off the valves will start to move, the lapping compound take this coating off in the lapping process. You need to cut the seats correctly and do not lap in the valves, if the seats are cut correctly there is no need to lap in the valves anyway.

CaliMX · #7 (**permalink**) 05-06-2008, 11:54 PM

Hey Allen,
Check your PM's

RCR YZ125 · #8 (**permalink**) 05-07-2008, 06:54 AM

Quote:

Originally Posted by punkinrider55

When we did put all 5 new valves in I did use lapping compound and cut new seats for the new valves...after lapping them u can see where the compound was hitting the seat and it has a perfect ring all the way around and was in thickness per the manual spec (cant think of it off the top of my head). It didnt get thinner at one part of the valve and then thicker at the other. It was an even ring all the way around.

that's your problem, never lap ti valves. i know it sounds crazy but ti doesn't need it.

sean 252 · #9 (**permalink**) 05-07-2008, 06:59 AM

Sounds like BVAL is a machinist...My dad is a retired machinist and has buillt lots of race motors...He says exactly what BVAL has said...Sounds like Allan if you keep learning you might be the next Al Posey...Keep on getting educated and talking with real motor guys...

CaliMX · #10 (**permalink**) 05-07-2008, 10:52 AM

Quote:

Originally Posted by sean 252

Sounds like BVAL is a machinist...My dad is a retired machinist and has buillt lots of race motors...He says exactly what BVAL has said...Sounds like Allan if you keep learning you might be the next Al Posey...Keep on getting educated and talking with real motor guys...

Yeah, I've been building MX motors for years now, I'm in SoCal now but I used to live in Folsom (and might move back soon), I have a lot of customers in NorCal and my daughter still lives up there so I'm actually in NorCal a lot and go to some of the GFI races.
A lot of engine builder went through similar situations as Allens in the earlier days ('03/'04), MMI will teach you a lot but there's a lot to be said for forums like this one, ThumperTalk, etc. you can get valuable information from experienced builders.