During normal running, crankcase pressure varies as the pistons move. This pressure should be vented through the breathing system, so even a loose dipstick should only weep a little.

The breathing system should be checked to ensure it is all as it needs to be, not restricted. If the system breathes into the engine intake, there is usually some system to condense the oil and prevent it being sucked through and burnt. Might be a little gauze filter or similar. If blocked, crankcase pressure may not vent properly and push past the dipstick.

An update on the oil issue.

No problems from the overflow pipe on the rocker cover.

Dipstick secured in place (cable tie, so temporary) and no further problems with oil explosions. (Slight concern that if there is high oil pressure, now there is no vent.)

Oil pressure gauge maxing out, but seems to be an earthing issue with the sensor, so unrelated, though curiously coincidental?!

Last thing to check is piston rings. She has been running rich and I'm told this can wear the rings? I've had the carb set up so hopefully she's not running rich now. She's going in for a compression test soon which will tell me how the rings are. I suspect a large bill if they need replacing.

There seems to be a bit of confusion regarding high oil pressure and crankcase pressure.

High oil pressure will cause leaks from joints and seals. It will not cause the dipstick to pop out.
High oil pressure is caused by pressure relief valve issues.

High or unvented crankcase pressure can cause leaks from joints and seals, and may eject the dipstick.
High crankcase pressure is caused by excessive piston blow-by, or blocked crankcase ventilation systems.

timgarman said:

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Last thing to check is piston rings. She has been running rich and I'm told this can wear the rings? I've had the carb set up so hopefully she's not running rich now. She's going in for a compression test soon which will tell me how the rings are. I suspect a large bill if they need replacing.

Click to expand...

They used to say that running 'on the choke' (which would be running very rich) too much would wash the oil film from the cylinder walls and could cause ring/bore/piston wear, but it would take many thousands of miles to become noticeable.

I doubt your bit of running 'rich' (you don't say how many miles you've covered but iirc it's on a recently rebuilt, modified engine?) would have had much negative effect so soon. A compression check is a good idea if you have any concerns. I'd wait for the results and the prognosis of the Mechanic before starting to worry.

Al.

Thanks Al.

Engine has been re-bored and reconditioned. Travelled around 2,500kms since.

Portland-Bill – I'm leaning towards the crankcase but we'll see. Compression test next week.

timgarman said:

Thanks Al.

Engine has been re-bored and reconditioned. Travelled around 2,500kms since.

Portland-Bill – I'm leaning towards the crankcase but we'll see. Compression test next week.

Click to expand...

Could be that new rings are not yet fully bedded into the new bores, so not sealing properly yet. OR! Not run in properly, and bores are glazed (polished) so rings won't seal. When running in, it is important to give the engine some work to do, not run it too lightly. If you've run it too lightly, the rings have not bedded into the bores, so the bores become polished, preventing the rings from sealing. May need teh bores honed lightly to unglaze them.

Just out of interest what oil are you using?

Oil is a mineral based 20-60.

Results of the compression test are marginally over 7-1 (7.2-1). Engine is an original re-bored to 594.

I think this is normal and should rule out the rings.

Dip stick is cable tied in place and I haven't had any further problems. Whilst this points to the oil pump valve, I'm thinking to leave it alone.

Thoughts?

timgarman said:

Results of the compression test are marginally over 7-1 (7.2-1). Engine is an original re-bored to 594.

Click to expand...

Normally a compression test gives results as pressure achieved during the test. This involves a pressure gauge in the spark plug hole, throttle wide, and crank several times for pressure to rise and stabilise. Test done twice, second time with small amount of oil addd to bore to see if any difference noted. If adding oil increases pressure, this suggests rings not sealing, as the oil helps seal for the test.

A compression ratio is normally measured by comparing swept volume with combustion chamber volume. This involves head removal, and measured amounts of usually paraffin. That will not diagnose piston sealing issues.

timgarman said:

Dip stick is cable tied in place and I haven't had any further problems. Whilst this points to the oil pump valve, I'm thinking to leave it alone.

Thoughts?

Click to expand...

Crankcase pressure and oil pressure are not related.

portland_bill said:

There seems to be a bit of confusion regarding high oil pressure and crankcase pressure.

High oil pressure will cause leaks from joints and seals. It will not cause the dipstick to pop out.
High oil pressure is caused by pressure relief valve issues.

High or unvented crankcase pressure can cause leaks from joints and seals, and may eject the dipstick.
High crankcase pressure is caused by excessive piston blow-by, or blocked crankcase ventilation systems.

Click to expand...

Evening Tim;
As "Portland Bill" has pointed out, the result of a compression test usially is shown as PSI, not as a compression ratio. (plugs out, foot fully down on throttle, one person holds the gauge in the plug hole, the other person cranks the engine). As an aside, I would be VERY surprised if the compression ratio of your engine is only 7.2:1. The original 479cc engine was 7:1, all the 499cc engines were 7.1:1, the 500 'sport' was 8.6:1 and the Mk1 (594cc) 126 engines had a 7.5:1 compression ratio. If your engine started as a 499cc engine and was then bored to 594cc, I would expect a compression ratio nearer 8:1.

Ok, so maybe not comprehensive enough. We only removed one spark plug. Didn't go to the lengths that Portland- Bill suggests.