measuring piston to valve clearance

[fergus]

Conceptually would the process below work?

1) fit cylinder head with 1 exhaust and 1 inlet valve on weak springs, and set valve clearances per spec
2) install cams and caps
3) bring block to TDC for #1 piston (dial gauge)
4) install cylinder head on a used head gasket & torque up
5) advance (inlet) and retard (exhaust) cam until their respective valves are physically touching the cut outs in the pistons (do this one cam at a time obviously)
6) using a dial gauge in the same plane as the cam follower, retard the inlet cam to give 1mm clearance and advance the exhaust cam to give c. 2mm clearance. Mark the cam caps for this "no go" zone prior to setting correct LCs (or as close as possible to the danger zone found above)?

Thoughts?

I've heard various mentions for use of using zip ties to check clearance at TDC, plasticine, etc.

Edited by fergus on Monday 13th March 14:14

[227bhp]

Yes it'll work, what it won't show you is radial clearance if its tight and the valve sits in a pocket in the piston, Plasticine does.

[DVandrews]

Depending on valve acceleration and the dwell time at TDC/Piston acceleration you may find that the point where valve clearance is tightest is not at TDC.

Dave

[Inline__engine]

DVandrews said:

Depending on valve acceleration and the dwell time at TDC/Piston acceleration you may find that the point where valve clearance is tightest is not at TDC.

Dave

it almost always isnt at TDC due to reasons you provide you need to check upto atleast 10* either side of TDC depending if you are doing inlet or exhaust valve. off the top of my head you typically lose about another 1.00-1.50mm with mild performance cam at point of closest position compared to TDC.

[99hjhm]

Helps if you say type of engine but 1.0mm is a little close for many applications. Usually around 8 degrees after TDC for CPA depending on cam acceleration rod ratio etc. The clearance reduction varies, most don't bother checking.

[Inline__engine]

if you have light checking springs on some engines you can push the valve open by hand or with small lever at each crank angle to see the clearance

none of this helps with radial clearance though

[Little Pete]

The method you've suggested will give you a ball park figure but you really need to have the engine assembled and timed up to get an accurate figure, particularly either side of TDC.
A fast opening inlet valve can easily out accelerate the piston.
As for radial clearance, I use an old valve stem ground to a point and with the piston at TDC, put some engineers blue on the point and slide it down a valve guide to mark the piston crown. Once you know where the centre of the valve is you can accurately measure radial clearance. I realise this seems a lot of work but it is what dry builds are for. Hope this helps.

Edited by Little Pete on Tuesday 14th March 15:32

[fergus]

Thanks all.

The engine is a 2 litre Alfa Nord twin cam.

[DVandrews]

I have used plasticine in the piston reliefs in the past and rotated the engine after setting LATDC, if the valve face is lubricated then it wont stick. You can then get a good idea of clearance both vertical and lateral.

Dave