rocker arm geometry

[vrod]

does any one know how to check the rocker arm geometry on a tr250 inline six. I have had the head skimmed from what I measured .110 thousands and the car is not running now because of a valve dropping down into number 3 cylinder caused from a broken valve spring. I think the engine needs shorter push rods to lesson the stress on the valve springs

 

[piman]

Hello Vrod,
firstly there are different length pushrod made by Triumph, with the 2.5 injection engine having shorter pushrods due to the thinner head. The geometry is fixed by the height of the rocker pedestals and for standard valves in a good head (i.e. not having the seats excessively recessed or cut back which would give a higher valve tip to head distance) should be correct if you are able to get the standard valve clearance. The thinner head, given the correct clearance, just means that the adjusting screw is higher in the rocker. With a standard cam the geometry should be unaffected. If you have a much higher lift cam than standard then you ideally need to machine the pedestals down, which then would almost certainly need shorter pushrods. Also you would need to be sure that the springs do not become coil bound. This is a common cause of breakage. If, as I outlined earlier, the clearances are correct then it is very unlikely that the spring breakage is due to the pushrod length.

Mechanically, the ideal geometry is with the rocker horizontal at 50% lift.

Alec

 

[Dave Russell]

I don't think that push rod lengths have any direct bearing on the situation.

With the lash adjusters on the PUSHROD end of the arms, the adjuster settings have no bearing on the rocker arm geometry. If the adjuster lengths are still within the needed adjustment range no change is needed. Obviously, if the adjusters are backed all of the way out & you still do not have enough lash clearance, shorter pushrods are needed. If the adjusters were on the VALVE STEM end of the rockers, you would definitely need shorter pushrods to maintain correct geometry.

The rocker arm to valve stem geometry is determined by valve stem length & or valve seating depth & rocker pedestal height. The geometry should be such that the rocker tip contacts the valve stem at a little past center of of the stem when the valve is at 1/2 lift & the rocker is at 90 degrees to the stem. The object is to minimize the lateral (side to side) scrubbing of the rocker contact on the valve stem. Minor variations of this are usually the case & will likely be OK.

Ideally, the rocker tip radius is such that it matches the rocker arm arc of motion so that there is a rolling motion, rather than sliding, of the rocker tip across the valve stem. This usually is not the case & is why roller rocker arm tips are sometimes used. Especially with high lift cams which make the total rocker to stem contact lateral motion greater.

An easy check is to insert a thin piece of paper between the rocker tip & valve stem. With ideal geometry, the paper will not be cut by the valve stem as the rocker is cycled.

None of this should affect the valve springs unless the springs are coil binding when the valves are fully open. This can be checked by accurately measuring the length of a fully compressed spring (in a vice) & comparing it to the spring length when the installed spring is at the valve fully open position. The springs should be at longer than the fully compressed length by at least .100" with the valve fully open.

Careful measuring between an installed valve retainer (but minus the spring) & the spring seat will give the available spring length. This measurment minus the valve lift, must be .100" longer than the compressed spring length.

Corrections to the geometry can be made by changing valve stem lengths, valve seating depths, & rocker arm pedestal height. Some engines have shims under the rocker pedestals so that height may be adjusted.

Most production engines do not meet the ideal conditions & still work OK. Likely, you will not want to get involved with all of this. Spring coil bind is the most important consideration in the equation.
D