what are the reasons of Valve Touch in an engine

[vinaykulkarni]

what are the key factors affeting valve touch in a diesel engine.
Problem:
Exhaust valves are touching the piston. The bumping clearance is quite high in the range of 1.5mm to 1.7mm on the exhaust valve side. The cam shaft being checked seems to be all right, but when the valve timing is measured, it come to around 6 degrees BTDC for exhaust valve opening(specified is 9 deg BTDC). the remaining values of valve timing are well within limits.
If the camshaft is right and bumping clearance quite right, then what else could be the other factors influencing the valve touch? and also why is it that the valve timing is not coming right?

 

[patdaly]

Without knowing how your cam is ground, I can only speculate, however I would lay money the problem is in the cam phasing. ( you said the Exhaust is aorund 3 degrees late ) You need to know that valve operation is not timed with Piston TDC in mind, therefore if the cam is out of phase, things like this can occour. Advance the cam 3 degrees and test it again. You may also want to check the Intake C/L to see if it is also 3 degrees late, if not, a new cam may be in order.

 

[ElwoodGreen]

You could also be experiencing valve float--the valves are not closing properly. On the exhaust side, the valve needs to be closing as the piston approaches TDC at the end of the exhaust stroke. If the valve springs are weak and/or the valve stem is sticking in the guides, then the valve may get assistance in closing from the piston.

Our exhaust valves on engines in landfill gas application experience excessive silica deposits from fuel contaminates. The deposits act as insulation and the valve doesn't dissipate heat through the seat to the coolant. The valve to guide clearance starts shrinking and the valves start galling. These large bore engines don't have a problem with valves hitting the piston, but the valves do burn or "gutter" if we run too long prior to head overhaul.

In a Diesel application, oxidized oil and other combustion products can create similar valve sticking issues.

Hope this helps.