Aurelia B12 Timing Chain Tensioner Pressure Limiter

[Brian Long]

The Aurelia parts book TAV 3 #18 doesn't state when this item was discontinued for the B12 engine..
Does anyone know?

Happy motoring.

Brian

[Geoff Goldberg]

Brian -

This little part is a peculiar one, and mysterious. It isn't clear how it works - as a shuttle valve, one would expect a spring on one end. Maybe its for lessening oil pressure to the tensioner.

The book states it ends with the s. 4 B20, and that seems right - as 6th s. B20 engines don't have it. As to B12 engines - can't help you there. Sorry.

Geoff

[Brian Long]

Geoff,
Thanks for input. It does seem strange as depicted in the parts book There is a hole in the tensioner yoke behind the tensioner sprocket which serves to lubricate the chain and sprocket rollers. Maybe the factory thought that there was enough pressure drop in the gap between the engine mount and the crankcase face due to the thin gasket to dispense with the little jigger.

Brian.

[Geoffrey Goldberg]

It gets more curious: the tensioner sprocket itself runs on needle bearings, which need very little lubrication. So what is the purpose of this valve? It might have had to do with the casting of the block, and the need to drill larger oil passages, and then this is a way to lessen the oil flow to the chain tensioner. Maybe in the later blocks they found another way to solve this.

By the way - the fuel pump and its spacer nearby to this area are also critical: we found that by just some paper gaskets between the fuel pump and the block (and varying the length off the actuating rod) the pressure could go from 0 to 7psi. Big range, yes? So it pays to check fuel pump pressure and shim thickness together. Hope this helps!

Geoff

[Brian Long]

Geoff,

Yes, I was aware of this too. When I reassembled the motor, I set the camshaft to the lowest lift point of the camshaft lobe which drives the fuel pump pushrod. Offering up the pump and spacer block showed that the slack in the pump mechanism was just taken up.
I surmise that when the pump is operating it 'freewheels' once pressure is built up, removing the load on the pushrod until the diaphragm is able to displace the petrol. It would get to a sort of equilibrium state where its discharge rate would equate to the amount of fuel let through by the carburettor needle valve.
I have fitted a later type of FISPA (I think) filter with integral regulator. Fuel pressure is about 2.5 PSI

Cheers, Brian

[Brian Long]

Problem solved!
The limiter had fallen out unnoticed at some stage during dismantling. Have obtained a replacement; trick now to install it without removing engine!

Cheers, Brian.

[Steve Katzman]

I remember discussing this a number of years ago. It was then my conjecture that it was not a pressure limiter, but perhaps a check valve, to maintain tensioner pressure during drops in RPM. I can't remember exactly how it's all set up, but it's not a limiter, that would only need be a metered orfice. But as episodic low tension, coupled with quick acceleration/deceleration, could have such disastrous results, it would be more than sufficient to send the engineers into paroxysms of joy at having such a problem to solve in their inimittable style. I have a motor apart right now, but it's a late 6S. Not much help.
Steve

[Steve Katzman]

But if my original contention were true, why would the shuttle later be deleted? I shouldn't be writing at this hour. I could look in the parts book and wonder exactly what "pistoncino regolazione olio tendicateno..." means. I now believe it means that it was pressure regulation, not retention, that made such sport for the engineers.
Enough.
Steve

[Geoffrey Goldberg]

oh this lovely crew of self-critical Lanciste! We actually correct our own posts. Wonderful. Good work, Steve. Lets have more. Truly.

[Brian Long]

Well, I have been able to replace said regulator/piston/check valve and have noted the following.
There is an angled drilling which connects from the reduced diameter section of the 'piston' to the outboard end of the chamber that the tensioner plunger slides in.
The piston is a free-running fit in the block drilling and would act as a sort of concentric oil restrictor. The replacement piston showed signs of moving in the block i.e. shiny wear spots. So it possibly gets shunted 'into' the block should the slack side of the timing chain tighten up. If this happened, it would close off the supply side of the angled drilling and the oil trapped in the plunger would be forced out through the small hole near the yoke which carries the tensioner spocket thereby limiting its 'bounce'.

Meanwhile, I have discovered the cause of the oil leak (now) to be the plug in the crankshaft which was not sealed properly and oil was getting past it and the crankhandle dog (being a B12) and the being spread far and wide by the fanbelt pulley. Tomorrow's job is to Loctite the plug back in place.

What a deep and meaningful dissertation!

Cheers, Brian