Piston Contacting Exhaust Valve

[LaceyDucati]

Hello
Sorry for the late intervention here. As your bike is a 250 Mach 1 and the the piston we supply is identical to an original Mach 1 piston in crown/valve pocket detail there should be no issue with valves hitting the piston. There are only a few likely reasons for valve to piston contact:

1) The valve timing is incorrect, check and rectify if incorrect.
2) The valve is sticking (unlikely), correct insufficient clearance....I think at early start up this is very unlikely.
3) The valve is out of line to the valve pocket. This is usually if the guide has been replaced many times and maybe not very well at some point, leading to the valve being swung out relative to the seat and pocket. A poorly bored cylinder (off centre and out of true) will add a little to the issue, but on it's own will not be enough to take up the valve to pocket radial clearance. Checking The clearance with modelling clay will show up any issues and the pocket will need machining to correct if necessary. Ideally you should have around 1mm (40 thou) radial clearance to the valve around the pocket, certainly not less than 0.5mm (20 thou). Clearance to the valve face should be around 2mm inlet and 2.5mm exhaust for comfortable safe operation.

Hope this helps

Nigel

[Geordie]

Checking The clearance with modelling clay will show up any issues and the pocket will need machining to correct if necessary. Ideally you should have around 1mm (40 thou) radial clearance to the valve around the pocket, certainly not less than 0.5mm (20 thou). Clearance to the valve face should be around 2mm inlet and 2.5mm exhaust for comfortable safe operation.

Apologies for jumping in here. I have rebuilt my Mototrans Elite engine and am waiting on delivery of parts to complete before starting. I know all timing marks were lined up simultaneously before torquing the head bolts. The head has, for an unknown reason, large valves. The cam and piston are replacements said to be the factory spec. The engine was turned over slowly with no problems then kicked over to prove the oil pump - all good and with the plug in, so is the compression.

But what are the clearances that I hadn't checked and how to

1mm solder wire, blu-tak and cling film, two revolutions of crank for each valve.

blutak.jpg

All I could think of to avoid removing the head. Result: 5.3mm Ex and 3.5mm In. No wire contact at valve pocket.

Thank you Nigel

[blaat!]

Sorry for the delay. I got pulled away from this for a bit. Finally getting back to it.

Well this explains some things...

IMG_3080.jpg

IMG_3082.jpg

[Bevel bob]

I think we can safely assume that the cam drive bearings will be shot from the debris that must have gone through.

[Duccout]

Wow! Never seen that before.