3 replies to this topic

[coopertaz]

i'm currently rebuilding an engine that only smoked on start up but not on overrun what amount of play, if any, is permissible in the valve guides? none are sticking, all valve stems are good with no wear marks but on  2 you can feel a tiny amount of lateral movement. New guide or will stem oil seals suffice? seals on there at moment are later type and not hardened or split 

[Spider]

The running clearance on the valve guides to valve stems is 0.0015 to 0.0025" for the Inlets and 0.002 to 0.003" for the Exhaust Valves, the latter having a higher clearance as they run hotter and expand more.

 

Because of the small diameters here and that they don't wear round but tend to 'slot', they are difficult to measure. You can get an idea by using a DTI on the head or stem and rocking the valve.

With fresh engine builds, I have always overhauled the head, for it to properly run in, you really want to eliminate oil getting in to the cylinders and let that be controlled by the rings and cylinder honing, you also want to have the valve seats re-cut so they seal properly

[pete l]

Can valve guides be replaced as is, or does everything else have to be realigned once they've been changed ?

[Spider]

When the guides are replaced, the valve seats in the head do need to be re-cut. The centres of the new vs old guides are not always in exactly the same place and you can be sure, the seats will be out of shape and at the very least, need a dressing anyway.