Finding top dead centre
QI base my Achilles 9m
Lotna in Faro, Portugal, and suffered a blown head gasket on its Perkins Perama M30 engine just before the Coronavirus pandemic. I’ve been in lockdown in South Wales ever since but plan to go out to tackle the job as soon as I can safely.
I’ve never taken on such a large job before, but was inspired by Geoff Higginbottom’s excellent step by step guide for the same engine in PBO July and Summer 2020.
However, in the reassembly section there’s no mention of how to adjust the valve clearances, ie how to get the valves in position for measuring with the feeler gauge/and what sequence.
This was one of the procedures that was concerning me and is not clear in the workshop manual.
I understand the term top dead centre, but not how to get there. Could you advise me on how to do this?
STU REPLIES:
On the VP 2030 it is quite simple to check the tappet clearances. Top dead centre (TDC) of No1 cylinder needs to be found. The flywheel is marked but TDC can be found by turning the engine over and observing the valves.
First of all tie the engine stop lever in the stop position – otherwise the engine could fire up when you’re turning it over!
Take off the rocker cover so you can observe the valves and access them to adjust them. Then get a socket on the crankshaft pulley and start turning the engine over.
The front cylinder is No1. Observing it you can see the valves moving. There are two TDCs on a cylinder, one when it is ‘TDC firing’ and both the valves are closed, and one when it is on ‘TDC not firing’ and the two valves are ‘on the rock’.
On the rock means that one valve is closing and the other is opening. It’s quite easy to see as you turn the engine over very slowly – one closes, the other opens.
The manual says set No1 on TDC with the valves closed using the mark on the pulley and timing case. The inlet and exhaust can then be set on No1 at 0.008in or 0.20mm and also the exhaust of No2 at the same time. The manual then goes on to say to turn the engine backwards by 240° and set No3 inlet and exhaust and No2 inlet… a bit complicated!
However if you turn the engine over slowly clockwise and watch each cylinder in turn, you can see the valves of each cylinder go from both closed when it is on the firing stroke to going ‘on the rock’ as the piston comes up after the firing stroke, then down before coming up to TDC again for the firing stroke, it is then that the valve clearance should be set.
So the choice is yours, set by the book using a protractor or set by turning the engine over three times and using your eyes!