Engine question

[npanne]

Quick question, if I may.

The GT6 engine that I have to drop into my Spit came with the head detached. I didn't want to take it much further apart, largely because I suspect I'll never get round to reconstructing it, so I checked a few bearings and ring, and made the executive decision to just bolt it all together and drop it in "as is".

I've torqued down the head (new gasket, obviously), but not yet replaced the rocker gear, so all valves are shut - and the other day I thought it would be prudent to stick some old plugs in the holes to prevent any damp getting in - now here's the thing...

With plugs in, and with all valves shut, when I turn the crank by hand, obviously there's much more resistance (as you'd expect, with the compression), but there is a distinct sound of the compressed air "hissing" in or out somewhere - sounds like it's coming from the inlet/exhaust ports to me.

Now I don't know whether this is normal or not - it's not exactly a normal circumstance, so it's not in any books or anything, but I kind of assumed that the chambers (with the valves shut) should be air tight (ish).

Any idea if this is normal?

[bodders1]

npanne wrote:

With plugs in, and with all valves shut, when I turn the crank by hand, obviously there's much more resistance (as you'd expect, with the compression), but there is a distinct sound of the compressed air "hissing" in or out somewhere - sounds like it's coming from the inlet/exhaust ports to me.

Now I don't know whether this is normal or not - it's not exactly a normal circumstance, so it's not in any books or anything, but I kind of assumed that the chambers (with the valves shut) should be air tight (ish).

Any idea if this is normal?

I think this is normal.  When turning it over by hand, there inevitably some leakage of air through the gaps in the piston rings.  It is not usually enough to affect compression at running speeds, but as you push a piston through a compression stroke by hand, you will hear some air hissing thorugh the ring gaps.

[npanne]

Thanks - that's reassuring.

Had the idea that I might fit the rocker gear and a starter motor, and spin it up to do a compression test whilst it's on the engine stand - this should be okay provided that the flywheel is clear of obstructions right?

[Aussie Mk1]

You will get noises turning the engine by hand, no problem there, there is no problem doing a comp test on the stand, ideally comp tests are done with the engine hot, but a cold test will show up any problems, do the test with the throttle open, and if you have a lowish comp you can do a wet test, that is squirt a little engine oil into the cylinder and re test, if the comps come up it will be a ring problem if not a valve issue, ideally you want no less than 10% variation between readings, i hope that this helps.

[Aussie Mk1]

Just to clarify a point, the noise you are hearing while turning the engine by hand is probably compression leaking past the rings, under slow cranking conditions, by hand and also to a degree on a starter compression will leak past the rings as the piston speed and cylinder pressures increase (engine running) the much greater compression pressures developed with in the engine act on the back face of the rings increasing the ring pressure on the bore and creating a far better gas seal.

[Gt6s]

Give the engine a chance take the head off and at the very least grind the valves in, as you have not run it up you should still be able to reuse the head gasket.

[SpitNoir]

Hi npanne.

I'm going to mildly disagree with those that say this is normal and ok. Normal for an old tired engine? Yep. Ok for one just being put back together? Not necessarily.
I would suggest at least verifying where you're hearing the hissing sound come from before you decide what to do.
I agree with the others that during a slow hand-cranking you're going to hear blowby getting into the crankcase, but you should not be hearing any getting past the closed valves and into the ports.
Instead, you should hear it coming from the valve cover or any open vents into the crankcase, such as the distributor/oil pump port or oil filter fittings.

That's my take on it anyway. I've played with both old and newly reground heads, and there is a HUGE difference. Once, when cleaning a set of heads with kerosene, I noticed that some little bit of fluid was dribbling past the valves and seats. Was told that was normal and not to worry. Car ran ok, but was never quite as powerful or as smooth as I expected an otherwise new engine to be. Took the heads off and had a full valve-job done.
Can you say, night-and-day? I can now.

Thinking about it too (which I didn't know about at the time), how do they "CC" a set of combustion chambers if the precisely measured liquid was able to leak past the valve seats? Same for compression.  A well ground valve will hold things tightly inside and any compression will leak past the gaps in the rings only.
Yes, an engine will run (maybe even very well) with imperfectly sealing valves, but it'll never be able to achieve it's potential.

And just so we're clear, I'm certainly no professional engine builder. But I've played with, and rebuilt enough different engines over time to have come across this before. Granted, never a Spitfire engine, and there are differences, but all combustion chambers work off the same principle. Better sealing is, well, better.

Bottom line for me though, would be just how much is leaking is actually occuring, and how much of a hurry I was in. If it's just a tiny amount, fine, live with the imperfect sealing in the chamber and deal with it next time the head is off. But if it's quite noticeable, and more than is leaking down into the crankase through the rings, I'd pull it back off and have it refurbed.
Of course, it wouldn't hurt to simply tap each valve tip gently to snap them open and closed to make sure that no little bit of debris or soft buildup is causing the leakage. If they're worn, or had not been properly machined or lapped, this won't fix anything. But it would at least eliminate one possibility, and possibly make you feel better!

Good luck!

Paul

[piman]

Hello Paul,

I agree with you about valve seats, If I'm building a good engine, I fill the chambers with paraffin (kerosene) and leave overnight, if there's any loss of level them a bit more lapping until it is leakproof.

Alec

[josok]

Sorry to jump in with a new question, but i am lapping the valves of a spare engine,
One exhaust valve did leak on the liquid test, and the seat was really bad looking.
Now, after about 15 mins of hand lapping the valve looks nice, but the seat still has spots.
And the question is how far can i go? Would pictures help?

[piman]

Hello Josok,

ideally you need to take it to an engine reconditioner to recut the valve seat(s). You can keep on grinding if you wish but it is better to have it machined as it will be more accurate.

Alec

[josok]

Thanks Alec, i will grind a bit more, but i´probably end up going for an exchange later.

Terve, Jos