valve timing/beb failure

[frank rabbets]

Is there valve overlap on the 12 or 24valve engines? There does not seem to be much room with the piston on TDC on the exhaust/induction part of the cycle for the valves to be open much. Just thinking of the tapping noise in the other post. Presumably if there is overlap and the bebs start to let the piston go further and further up the valves on overlap would be the first to be hit. If that were the case the valves would be against the cams, not so on the compression/ignition part of the cycle, and this would cause damage to the valve gear as well as the other parts of the engine.

Anyone know if damaged valve gear is associated with beb failure?

Just find it all very interesting.

 

[Towpack]

I would expect very little overlap on an engine like the 1H. The pistons actually protrude from the block slightly with the head off, hence the several different gasket sizes available to select the correct squish clearance. Apparently the piston hits the head due to the excess travel caused by worn BEB's, generating the ticking sound.

http://www.lcool.org/technical/80_series/td_bearings.html

 

[Dave2000]

I think Towpack has this right, the piston do hit the head when the BEB's start to fail. The normal result is a 'leg out of bed' or if you like a conrod through the block.regardsDave

 

[clivehorridge]

I seem to remember that someone said it's an interference engine, presumably meaning that valve contact will occur if the timing belt was to snap. A different scenario I know, but indicative that there's not much clearance in there.

Slightly on the same theme, I was surprised to see this, when browsing overland's Farcebook page...

 

[Towpack]

Most modern engines are of the interference type. As you say, different scenario, bad BEB's won't affect the valve timing but with the squish clearance being so critical on these engines the 'piston/head' scenario sounds the most plausible IMO.

 

[frank rabbets]

If the valves are even slightly open on TDC then these would be hit by the piston with play in the beb. This would put impact stress on the rockers etc as they would be locked against the cams? But the noise seems equal on every stroke so there may be no overlap. I'll be doing valve clearances soon so I'll see if there is clearance on both 1 and 6 at TDC.

 

[byrnieman]

I went in to see my mechanic today to see my engine in about 7000 pieces. The first thing he showed me was the head & the pistons. Beb number 2 was completely disintegrated causing a fair bit of wear to the crankshaft which is being sent off to be regrinded. Looking at the top of piston 2 - you can actually see the two round circles where it had been slamming into the valves - no cracked piston, no "leg out of bed" conrod through the block, but if I hadn't shut her down and not started her again until I got her to the mechanic then anything could have happened I guess. All caused by the number 2 BEB being obliterated. For interest sake I went and had a look at the sump and it had heaps of iron filings in it which means they are throughout.

 

[frank rabbets]

I should ask him to check any damage to the valve gear. If the valves are slightly open on top dead centre on every other piston stroke the rocker arms etc may be damaged. There have been pictures on this site of broken valve gear I think due to broken cam belt.

I think the term "interference" refers to piston hitting valve with the valve fully open. Obviously this is an interference engine even though the valves may be closed on every cycle.

 

[Dave2000]

I have no doubt the pistons would hit the valves if the belt snapped and the valves stopped in the right (wrong) place. The big ends obviously allow the piston to travel x mount further up the bore, anyone who continues to drive and ignoring the clattering will eventually find a hole in the block. Valves tend to bend/break when hit by the piston but, if the piston hits the deck of the cylinder head (immoveable object) something has to 'give' so the rod or piston or both will fail.

regards

Dave

 

[frank rabbets]

Yes, I've had plenty of big ends fail in the past but not on a diesel so hearing that tapping noise of a piston hitting the top has taught me something new. Well done Karl !!

My big ends used to start with a ticking which developed in to a loud tapping/knocking noise just as you started to release the throttle but silent on idle. Eventually the knocking would be all the time by which time the crank was knackered. Continued use would cause the rod to break.