Maximum valve shim thickness

[tom3c75]

I have just had the valves done and the seats replaced on my 1980 Jota cylinder head.   

The specs I gave the machine shop for the measurement from the bottom of the spring seat to the top of the valves were 36.6mm for the inlet and 36.0mm for the exhaust valve.

The head guy, in his wisdom, worked on 36.0mm for both valves.  He has asked me to check the valve clearances and, if needed, will adjust the seat depths.

I?ve now measured the clearances and, if I stay with the current seat cuts, would need the following shims:

Inlet        ? Approx 3.25mm
Exhaust  ? Approx 2.90mm

So, it seems that the inlet seat could do with sizing down by an extra 0.5mm. 

But, is really necessary?     

I like the idea of starting with thickish shims, but two things worry me. 
Firstly inlet (39.5mm) to exhaust valve (36.4mm) contact and, secondly, inlet valve to piston contact.    Pistons will be Mondial high comps, but are not yet fitted.

Inputs would be appreciated.

Tom 

 

[sfcpiet]

Hi Tom,

The factory used shims up to a thickness of 3mm.  Depending on the cams fitted, these may cause problems at valve overlap.  Piston clearance is not so crucial, if you stick to stock valve lift figures.

Thicker shims are probably available somewhere, but adjusting in the range over 3mm might cause delays while trying to source the correct shims  Up to 3mm should be available from most Laverda specialist suppliers.

With valves of 40 and 35mm respectively paired with C4 cams, I aim for a shim thickness of 2.7-2.8mm, larger than that and valve-to-valve clearance at overlap becomes quite tight.  Might tangle if a gear shift is messed up.  C7 or P1 cams can pose real problems with big valves and thick shims.  These pairings seldom retain decent flow characteristics at the valve seats once the shims are adjusted towards safe valve-valve clearance.

HTH,
piet

 

[redax5]

Tom,
Wolfgang has valve shims through to 3.5mm in 0.10mm steps.
I do heads on a regular basis and have used the larger shims in the past with no issues, even when using the F1 full lift cams that Keith has.
The F1's have "thicker" ramps then a 4C and higher lift on the Exhaust.
Not checked on the 7C cams, only have two sets of those.
On the engines with F1 cams, big valves and new seats, hence large shims we set the cams at 100/100 degrees, never had any issues yet with valve contact.

You will never hit the top of a Mondial tractor piston unless something goes wrong.

HTH
Red

 

[CLEMTOG]

Mondial ?
high comp?
maybe not
CLEM

 

[tom3c75]

Piet & Red:  Many thanks for the info.

Red, yes, I guess the Mondials are a bit old school.
I agree that Ross is the way to go, and probably less expensive, as they come with rings & pins.
Thing is, I already had the Mondials, bought a few years ago for my 3C, but never fitted.
And, I can use the funds for the ignition upgrade.  Expect a PM in the near future.

Clem, herewith a pic of the HC Mondial, and an old 9:1 flat top.
The HC Mondials were fitted to Jota?s destined for South Africa and, I believe, Australia.

Regards
Tom