problem on valve guides

[superbike]

Hi all

Having almost finished a job i found a problem.
I did my normal porting and roughing out to true valve diameter using the original guide. I then removed the guides to fit new and found one particular seat or guide is miles out. If you put the valve in you can see 0.5-0.8mm of light in one corner.

The guide went in nicely so why is it so far out. Im a little afraid to recut the seat with this problem as it looks well out to me and the others were all pretty close.

[Guy Croft]

Unfortunately you've made the fatal error of doing full spec new seats with old guides and this can happen.
The reason for the loss of alignment is most likely be that the OD of the new guide and its bore are not accurately concentric. If you have a very accurately made guide you can sometimes instal them and find a total seal between valve and seat even without recutting.

There is nothing you can do except first - verify what I have said about the new guides - and hope that if you fit them you can recut the seats and still have enough metal there. Otherwise you'll have to put new seat inserts in.

GC

[superbike]

I can get away with sinking in the valves another 0.3mm before i get to the point where larger than normal shims will be needed so it could be ok.

Or i could press it back out and try another guide and see if its closer.

[Guy Croft]

I cannot advise really on what best to do, seeing that dropping the valves deeper into the head could cause shimming problems especially as the thing has desmo valve gear (which is presumably makes valve tip height rather critical?). I suppose grinding down the valve tip to compensate for cutting the contact face deeper to true-up is a 'no-no'?

My concern might be that pressing guides in and out can make the alignment problem worse, due to scuffing/bore damage.

You better do a careful dimensional survey (incl concentricity of bore to OD) before deciding whether to go with these guides. Are they OE Ducati?

GC

[superbike]

I tried another guide and it was basically the same.
I saved it though there was enougth metal and it didnt end up any deeper so im happy.

I did just find from the customer that it bent a valve so i guess i roughed the seat out with the original guide wich had previously taken an impact.