If testing an engine with a leak down set...NEVER try to hold the engine stationary with anything other than a flywheel lock.
Using your hands on the belt will tear your skin off or worse if the engine is just a little off tdc.
Using a wedge under a belt will not stop it turning either, nor would putting it in gear if it has a manual transmission.
When the gauge is set up you'll be applying 90 to 100 psi to the cylinder if it has 2% or so % of leak down.
Even as stated, a pressure of 50 psi...which is 50% leak down ( Engine would not run at this kind of leak.) will generate an angular force to the crank that would be equal to cranking the starter.
At 100psi, there is an adverage total pressure on the piston at tdc equal to 1000 lbs.
The way I get tdc is, to balance the engine where it seems to like tdc and apply air...if the egine rotates, reset and try again...usually you'll hit the tdc mark for that piston in a couple of tries.
Having read and recorded tdc leak, test at bdc and see if there is a better reading...if so, this indicates tappered bore or wear at tdc allowing the ring set to increase it's end gaps......the difference is the total amount of wear.
The difference for a good engine would be 2% to 8% with 12% as a maximum.
All used engines I get to install are first tested this way to make sure I have an engine that is healthy.
Using the TDC O/T mark on a 240 will get is right most times.
Practice a little...it is suprising how it gets easy to do.
What ever way you work on your vehicle...stay safe.
Which means having jack stands to support it, not just a hydrulic jack, fire extiguisher....within reach ( even if your under the vehicle.) tell someone where you are, wear face protection when grinding/welding/cutting/heating or hammering . NEVER use an incandesent drop light.
PS.........For those of you who think you could 'hold' your engine at tdc, try this = apply shop air to cylinder at bdc....with a crank bolt socket and long breaker bar, try to turn the engine back to tdc.....let me know how you do !
ATTENTION : For the above test of strength....Make absolutely sure the cylinder is at BDC......you'll not get a second chance.
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