Leak down test - block engine

[mopetta]

I try to do a leak down test, but the engine always turns, no matter whether it is exactly on tdc. What is the best way to block the engine?

[Peter Clayton]

One way is to lock the flywheel by making a finger of steel that can engage with the ring gear and be retained with one of the flywheel cover retaining bolts.  Imagine a letter "L" with a hole at the top and the base bent down slightly to lock into the teeth. (HOPE THIS MAKES SENSE).

[A G Wood]

A valve in the air supply to the leakdown tester helps as well, you can fit the adaptor to the spark plug hole with it off, and then you can ease the air on which sometimes helps. I'm assuming the engine is still in the car? You can put in top gear to lock the crankshaft but make sure you chock the wheels if you do.

[mopetta]

I put a gear in, locked the wheels and handbrake but the engine still turns. I have a buget air compressor where i cannot swt the pressure. I tought of loking the flywheel as mentioned by Peter, but was wondering of another ways.

[A G Wood]

Locking the flywheel is the ideal way, but I'm a bit puzzled as to why the engine rotates if its in gear and the rear wheels are locked?
It may move slightly but shouldn't be more than a few degrees, definitely not 180 deg.  
Is there no way you can incorporate one of these [with suitable hose tails] in the air line between your compressor air connector and the leakdown tester ? This is an expensive-ish one, there are much cheaper ones which will work just fine for your application.
Back in the day we used to TDC automatic transmission cars [which you obviously can't lock the crank by putting them in gear] by the bubble method and easing the shop air on to the tester with one of these, 99% of the time the crankshaft stayed where you'd put it.
(Sorry I can seem to be able to reduce the image to sensible dimensions)

[mopetta]

The piston just moves enough to have a false test

[A G Wood]

Ah, I see, enough backlash in the driveline to mess you up. In that case you'll have to lock the flywheel. I'm not really an A7 guy, is the starter motor easily removed? If so you can jam the ring gear with a scrap piece of ring gear [any old ring gear will work] meshed with the Austin ring gear and placed across the hole where the starter motor mounts. I recommend tying a piece of wire on the piece of ring gear if you try this, they have a tendency to fall into the bell housing [don't ask...]

[Colin Wilks]

I haven't tried this idea, but it's much easier to get at the timing gear than the flywheel with the engine in the car. A bit of fiddling to accommodate the backlash to stop it dead on TDC, but I could see this working, and if you made up a bespoke lump to bolt to the timing gear cover all four cylinders would be tested at exactly the same point in relation to TDC.

[Bob Culver]

Will a leak down test reveal anything the crank handlle will not? Can hear leaks at valve ports or oi filler tube. Should be distinctly bouncy. As I recall a long screw for the flywheel cover plate locks the flywheel on later cars.

[Dave Mann]

We used to do this test on big 18 cylinder 8000 bhp engines and always ended up with an argument as to which were the worst cylinders.