Low compression

[Malcolm Towrie]

An update on the running problems: the distributor shaft was quite badly bent. It had 0.010" runout at the cam. Since the points gap is only 0.014-0.016" this seriously messed up the dwell and timing with big changes on a cylinder-to-cylinder basis as the shaft did a full rotation. The points being set too wide didn't help.

With a good, used shaft with 0.002" runout, and the points set at 0.015", dwell was right on the money and she's purring now.

Malcolm

[rob love]

Good for you.

When you say "distributor shaft" I take it you are talking about an 8BA engine with the distributor up top? Seems like it would take a lot of force to bend the shaft on either a crab or a divers helmet distributor mounted on the front of the engine.

[Malcolm Towrie]

Originally Posted by rob love

When you say "distributor shaft" I take it you are talking about an 8BA engine with the distributor up top? Seems like it would take a lot of force to bend the shaft on either a crab or a divers helmet distributor mounted on the front of the engine.

No, this is a divers helmet distributor. And it doesn't take much force at all to bend it when all the parts are stripped off. I chucked the small end in a lathe and measured 0.060" runout at the journal on the other end. It only took a few taps with a soft hammer to get it straight to within 0.002". It's pretty spindly in the middle as you can see but I was surprised how easy it was to straighten. I cleaned the shaft up thinking maybe it was cracked but I couldn't see any.

Some rough handling would have bent it easily.

I bet the last person to set the points must have struggled.

Malcolm

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[Lynn Eades]

I think somewhere back there Malcolm confirmed it was a divers helmet type.
How to bend a shaft in one of those, I don't know.
As it was the centre cylinders and one half of the firing order rotation, I think I was headed in the right direction as far as a misfire goes, but can't see really why it had matching low compressions, when comp test pointed toward valves.

Good news that it is sorted!

[Malcolm Towrie]

Lynn, I should have got into the ignition problem in a separate thread to avoid confusion as it was a totally separate problem. The low compression hasn't gone away!

That shaft wouldn't bend in service but throwing it in a box and then throwing other parts on top of it would bend it easily, based on how easily I straightened it.

Malcolm

[Malcolm Towrie]

An update: due to the low compression test pressures reported at the beginning of this thread, we just pulled the engine to find the cause. I've only got the heads and intake manifold off so far, but checking each valve when fully open, I can see damage to the exhaust valve seats of the 4 low pressure cylinders. That seems to be the culprit. The other exhaust valve seats look ok, so I'm not sure why these particular seats went bad. Tappet clearance too tight and holding the valves off the seat perhaps?
The cylinders are 0.015" over and a bore gauge check of the bores just above the pistons at BDC and at the TDC wear "ridge" shows wear in the 0.002 to 0.003" range for all cylinders, so acceptable, IMO. But even the 4 "good" cylinders only had 85 to 95 psi results instead of 110 psi, so I think the bores need honed and new rings installed.
The valley shows a depressing amount of sludge on all surfaces. And the oil pan has a good layer of black sludge on the bottom. We use a 15W40 diesel oil in most of our vehicles. This is a high detergent oil and I suspect it is bad news in this type of application as the detergent dissolves the stable sludge deposits and circulates it around the engine. We should be using a non-detergent oil on these old engines.
I also found a water pump with the bearings almost seized, front crank seal rope packing installed so tight, it had burnt the paint on the timing cover, causing a severely scored front pulley sleeve, and a crack in the block around the rear boss for the oil cooler supply, due to overtightening the fitting. I'm not sure how to repair that. All in all, a typical 75 year old engine.
Malcolm

[Lynn Eades]

Malcolm, What model is the engine? What casting numbers if any? Does it have domed pistons? What cylinder heads does it have? What bore size? (just checking)

[Malcolm Towrie]

Originally Posted by Lynn Eades

Malcolm, What model is the engine? What casting numbers if any? Does it have domed pistons? What cylinder heads does it have? What bore size? (just checking)

Lynn, yes, it has domed pistons. CR is 6.12:1.
Stock bore size is 3 1/16", 3.0625". This one is 0.015" over so 3.0775".
It's the 85 HP, 221 cu. in engine. I'll get some casting numbers.

Malcolm

[Richard Farrant]

Originally Posted by Malcolm Towrie

The valley shows a depressing amount of sludge on all surfaces. And the oil pan has a good layer of black sludge on the bottom. We use a 15W40 diesel oil in most of our vehicles. This is a high detergent oil and I suspect it is bad news in this type of application as the detergent dissolves the stable sludge deposits and circulates it around the engine. We should be using a non-detergent oil on these old engines.

Hi Malcolm,
A mistake people often make is getting an old vehicle of unknown history, draining the oil and refilling with a detergent multigrade oil. Most of these old WW2 era engines (and earlier) had either no filter at all or a bypass filter, which was tapped of the oil gallery and returned to sump. Detergent oils keep dirt and carbon in suspension so with a full flow filter system it is quickly cleaned. The older mono-grade oils had no detergent in them and the sludge formed in the sump mainly, so it was flushed out on oil changes. If you get hold of an unknown engine and it has a bypass filter, or none at all, go for a non-detergent mono-grade as blended for vintage vehicles. Actually the Classic / Vintage mono-grades in the UK do now have a very slight detergency in them as I am informed by the oil companies, but they do not scour out all the old sludge like a multigrade does.
Hope I am not teaching you to suck eggs, but I am sure there will be readers who are not aware.
regards,
Richard