I can turn over my engine by hand without de-compressing

[crown22]

Hello
Was surprised to find that I can turn over my Lombardini 702M by hand using a spanner on the flywheel boss.I expected to get only so far before
arriving at a stop due to compression.The engine starts and runs perfectly and is stopped not by pulling a cable but just by turning the key off as
you would do on a car.I'm guessing that turning the key to off uses a solenoid or similar to operate a decompressor but can't find any reference to
such a device in the manuals.I'm probably missing something obvious!Thanks

 

[Old Harry]

Hello
Was surprised to find that I can turn over my Lombardini 702M by hand using a spanner on the flywheel boss.I expected to get only so far before
arriving at a stop due to compression.The engine starts and runs perfectly and is stopped not by pulling a cable but just by turning the key off as
you would do on a car.I'm guessing that turning the key to off uses a solenoid or similar to operate a decompressor but can't find any reference to
such a device in the manuals.I'm probably missing something obvious!Thanks

All quite normal
stop will be an electric shut off fuel valve. NEVER use a decompression lever, it can damage valve or piston

 

[sarabande]

PH, can you explain that reasons for last statement please ?

 

[AntarcticPilot]

All quite normal
stop will be an electric shut off fuel valve. NEVER use a decompression lever, it can damage valve or piston

Some older diesels (e.g. the Sabb range) can ONLY be stopped using the decompressor.

To answer the OP's point, the compression ratio on marine diesels isn't all that high, and I can turn my engine over in the same way. It gets stiffer over the compression, but it's still easy enough to turn it over. The test would be to turn it until you can feel the compression and hold it at that point. If the force required to hold it remains constant, then there's no loss of compression.

 

[crown22]

Thanks for the info guys.I was mulling over how I could hand-start the engine if I could safely attach some winding device to the crankshaft pulley.An assistant would need to turn the key to allow fuel to flow into the cylinders.I hope never to find myself with flat batteries or a faulty starter motor but a Plan B would be nice to have.

 

[DownWest]

I turned a VP three banger over the other day with a spanner and thought it was a bit low on compression. But, starts and runs OK with no smoke.

Engines with decompressors have to allow you to turn them without valve/piston contact, or why bother with the decompressor?
I used to deal with loads of Listers and other diesels, non had problems with contact and several had to be hand started, which requires getting it up to speed before flicking the decompressor lever to get a bang. On multis, one flicked one off then the next after the first fired. On the bigger ones, quite a few Wheatabix were needed before trying.

 

[solent clown]

when you turn an engine over slowly by hand there is more opportunity for compression leakage past the rings and anywhere else that might not be 100%. Faster rotation is a whole different story

 

[DownWest]

Thanks for the info guys.I was mulling over how I could hand-start the engine if I could safely attach some winding device to the crankshaft pulley.An assistant would need to turn the key to allow fuel to flow into the cylinders.I hope never to find myself with flat batteries or a faulty starter motor but a Plan B would be nice to have.

The crank handle socket is often into the end of the camshaft, so one gets twice the rpm on the crankshaft. The VP 2003 I was looking at had the socket for the handle and the decompressors. But no handle in the kit and the owner had no idea it could be hand started.

 

[Iain C]

I could turn over my old Lombardini 602 too. You may find on the injector pump there is a short lever with a cable terminal ready to go that you can attach a pull stop too if you want to...mine had one. But it just cuts the fuel, it does not decompress the engine.

 

[scottie]

Volvo stated that the handle was only for turning the engine over and was not for starting!

It is not uncommon to break either the end of 5he camshaft or more often the drive pin of which there were several generations to get the ratio of strength as to stop breaking the camshaft ends.
They also had a decompression lever and were often hand startable especially on a bench but once installed not always accessible enough to do so

 

[davidej]

I used to regularly start an old single cylinder Lombardini by using a cord wrapped round the flywheel.

 

[vyv_cox]

What the decompressor achieves is to depress a valve. This increases the tappet clearance enormously. It may be that some engines support the push rods in some way so that no damage occurs. However not all engines do this and stopping them with the decompressor may allow the push rods to be displaced, which can lead to all sorts of problems.

Turning the engine over with a spanner is a good test of compression. There should be significant resistance. Adding oil to the bores can give lots of useful info. Details on the website.

 

[TQA]

There was a comment made about compression ratios on diesels. They are significantly higher than petrol engines which max out at about 10 to 1.

The lowest diesel I know of was the early Gardner at 14 to 1 the highest I know of was the BMC 1.5l diesel at 23 to 1.

 

[penberth3]

What the decompressor achieves is to depress a valve. This increases the tappet clearance enormously. It may be that some engines support the push rods in some way so that no damage occurs. However not all engines do this and stopping them with the decompressor may allow the push rods to be displaced, which can lead to all sorts of problems...

Another problem is the unburnt diesel which can remove the cylinder lubrication and find its way into the sump and dilute the oil. So always shut off fuel to stop a diesel.

 

[AntarcticPilot]

Another problem is the unburnt diesel which can remove the cylinder lubrication and find its way into the sump and dilute the oil. So always shut off fuel to stop a diesel.

As noted above, in 4, some older diesels can only be stopped with the decompressor. Sabb diesels are built like that, to my certain knowledge (we had one in the 60s!)