leaky valve seal

[dave friday]

Hi all,so how perfect does the seal have to be? when I pour petrol into the ports I get varying amounts of seepage...
Ta.

[California Dreamin]

I take it you are talking about how perfect the seat to valve needs to be?

Without significant pitting and a continuos unbroken band of seal after you have 'lapped' the valves in using carbarundum paste. The correct way to check for this 'continuos band' is to apply a very thin film of 'engineers blue' to the seat, drop it into its guide and applying pressure with a valve grinding stick, push and turn through one full turn. The 'blue' should now reveal a perfect unbroken 1-2mm band of 'contact' of seal, if it doesn't you lap the valve some more until it does.

I suggest you use white spirit instead of petrol if you are checking by pouring into the ports. Petrol evaporates for starters, so is not ideally suited to this task.

Martin

[itchyfeet]

Surely a jx with it's 23:1 compression ratio needs perfect valve seals, even white spirt is no guarantee of a good seal because it does not apply any pressure under gravity alone.

[dave friday]

Thanks both,the check was done with the valve springs in place [the valves were under normal spring tension].It's 50years since I last had a go at lapping valves in!
The reason for pulling the head;
Compression check.
No 1,370psi
No 2,370psi
No 3,260psi
No 4,370psi
I think that the top ring on number 3 piston is broken!
I did a leak test on no3 cylinder [at 50 psi] no hiss from the inlet or exhaust but hissing from the sump and cam breather.

[kevtherev]

Thanks both,the check was done with the valve springs in place [the valves were under normal spring tension].It's 50years since I last had a go at lapping valves in!
The reason for pulling the head;
Compression check.
No 1,370psi
No 2,370psi
No 3,260psi
No 4,370psi
I think that the top ring on number 3 piston is broken!
I did a leak test on no3 cylinder [at 50 psi] no hiss from the inlet or exhaust but hissing from the sump and cam breather.

That's pretty conclusive then

[dave friday]

Well! the rings on no 3 piston are all ok! but all 4 pre-cups in the cylinder head have cracks and there is a small crack between the pre-cup and the inlet seat on no 2.
What to do?
Any ideas/suggestions?

[kevtherev]

You say the rings are ok
Can we assume you have gauged the ring gap and bore and it's within spec?
Was No3 subjected to a wet bore compression test.
If so did the sump noise stop?

Small cracks are not necessarily a bad thing.
If the crack is across the seat, then it's not so good.
If the crack is all through the casting then your cooling system would pressurise.

We need a picture really.

To have such low compression and breather noises, I would still be looking at the big volume movers.

[dave friday]

Hi Kev,
Yes im happy with the condition of the rings.
I've checked the top ring against a new ring .
No wet test done.
The sump noise was from an air source at 50 psi !!

The American forum I use say that cracks in the pre-combustion chambers [pre-cups] are a potential problem [they can fall down the bore!!]
For some reason I ignored the cracks in the "fire rings" of the head gasket [looking for problems else where?!]
Ta.