Intake wearing into head? What now?
Posted January 08, 2006 - 09:53 PM
I think I figured out why all of my intake valves are tight. Check out how the cam has worn down into the head and it's mushrooming over the buckets. I can't get them out now.
One of a few causes I can think of. First, I recently had someone rebuild the top end and put in a 13.5:1 piston - which blew the head gasket and it got a little hot. I've been having problems with it since then.
Second, I just recently sucked in a bit of sand after the plastic decompression plug blew out. (Thanks Yamaha). It wasn't very much though, 1/2 cc at most, and the exhaust valves are perfect.
So do I:
1. Whip out the trusty dremel and try to clean up the mushrooming so I can at least get the bucket off, put smaller shims in and go from there? With as much wear as I see, it's going to be off more than the thinnest shim.
Order a new head from the TT store and figure out how to swap that out?
The shop that did the origional work went under and the best shop in town wants _Way_ too much money for something I'm sure I can do myself.
Has anyone seen cams do this before and is my head toast?
Posted January 08, 2006 - 10:52 PM
new head.....which stinks...
also, new valves/springs and everything else...
i wonder if head "assembly" includes the valves and everything else?
Posted January 08, 2006 - 10:54 PM
I saw someone parting out a YZ here a couple weeks ago. Does anybody remember who?
Posted January 08, 2006 - 10:55 PM
Posted January 09, 2006 - 12:49 AM
Posted January 09, 2006 - 03:01 AM
Posted January 09, 2006 - 10:26 PM
That kind of damage is often caused by several problems or a combination of two or more. Oil starvation to the cam journals is the most obvious as is oil contamination. Either problem coupled with the cam caps being overtightened could lead to this result. Since you mentioned overheating, that coupled with any one of the other conditions I noted could be catastrophic. When oil overheats the microscopic film between surfaces disappears causing metal to metal contact. Add in an overtorqued cam cap bolt or two and voila! A seized cam journal, which is what I see in the photo. As someone else said, once the damage is done head replacement is the only alternative. Unlike a car that has replaceable cam bearings, the Yamaha head does not. Minor galling can be cleaned up, but your pic shows distortion of the metal. Time for a new head.
Any idea what the cause is?
Posted January 10, 2006 - 05:49 PM
Posted January 10, 2006 - 07:38 PM
With a true OHC engine there is only so much material that can be removed before the head is junk. Once the journal support has been damaged to the point of mushrooming, it's toast.
That type of damage can usually be fixed by cleaning it up and line boring the cam journal. (Where they take a skim off the cap and head surfaces then bore the standard size journal through. You end up with only a slightly shorter distance between the cam and valve buckets, plus your cam sprockets and drive sprocket. I'd try a good machine shop first before buying a new head.
Posted January 10, 2006 - 08:05 PM
That's cheap, actually. I had to replace a TTR250 head and it cost more than that for a BARE head. Make sure you find out the cause of the damage before you toast another head.
That's what I figured. I ordered a new head from the TT store... ~$450, along with new valves & all.
Posted January 10, 2006 - 08:35 PM
They only want about 70-75 ft/lbs or so... that spec is critical.
Posted January 10, 2006 - 09:24 PM
Posted January 10, 2006 - 09:26 PM
That type of damage can usually be fixed by cleaning it up and line boring the cam journal. (Where they take a skim off the cap and head surfaces then bore the standard size journal through. You end up with only a slightly shorter distance between the cam and valve buckets, plus your cam sprockets and drive sprocket...
The other drawback to this is it reduces clearance and the shims only come "so thin" so if this is done, you will need to have the valve stem tips knocked down a smidge with a valve grinder.