Club 80-90 Forums

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.

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

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.

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.

dave friday wrote: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

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?

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.

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.