Porosity in combustion chamber?

[Ian928]

The two middle combustion chambers on my Stag have what looks to be severe pososity. I am not sure if this is from the casting process or if it may indeed be corrosion? Both exhaust valves protrude higher than the aluminium. The question is, what do I do with this? The car ran with this head before being removed. Among other things I am concerned about detonation because of the uneven surface.

what would you do?

[Motorsport Micky]

Looks like surface corrosion to me, possibly the car has been little used or even left standing with a weeping head gasket or even inlet manifold allowing water to constantly "top up" the chambers. If it were mine I'd leave it.
You could grind the marks out of the chambers back to good aluminium at the cost of messing with the chamber volumes and then having to balance them all back to equalise which would slightly reduce the compression ratio and power. Or if you decided to weld the chambers filling in the voids and then grinding back and equalising the volumes again, and because that welding where the valve seats are would cause a localised heat spike and then you have the possibility of the valve seat dropping out later in use !
With the heads on it looks the same as any other Stag and the corrosion didn't cause detonation before (there's none there) use it as it is and wait for something to force you into addressing it, it will probably outlive you.

Micky

Edit: In your post you say the valves sit above the surface but inspecting the photos again do you mean the exhaust valve seat edges sit above the surface ? If so you could grind or stone a small radii on that lip to help avoid any hotspots.

[jakesmig]

No saying how many years the heads have been like that. If it was mine I would get the valves and seats recut ant put the heads back on.

I suspect getting the holes filled and ground back would be expensive, and as per Micky's post, not without associated risks. Years ago the central core of a spark plug came out into thre engine of my Sprint ant the head looked like the surface of the moon, without any holes though. It was cleaned amd put back on and ran for many miles thereafter.

If it doesn't leak re use it.

John.

[Lingen]

Ian.
I think that type of damage around the inlet valve is caused by a long term leak of the inlet manifold gasket at that cylinder. The pitting is caused by the escaping water atomising in to steam when the fuel ignites. If the valve seat is no longer fully supported by the head casting, it probably ought to be repaired to avoid the risk of the seat falling out, which would do a lot of damage.
Mike.

[rwdstag]

It’s time to call Tony white me thinks

[flying farmer]

I would ignore it.

The corrosion pits will fill with carbon deposits quite rapidly.

The only thing I might do would be to remove the top edge of the exhaust valve seat if it protrudes a lot but it doesn't look too bad. I have seen heads with very deep corrosion between the valve seats (as in several mm) and I wouldn't want to use something like that in case the seat fell out.

Neil

[Ian928]

I was not clear in the first post, it is the valve seat that protrudes above the chamber wall. I do have some corrosion also between the ports on one of the cylinders on the other head.

The pits were filled with carbon before I soda blasted the chambers, but I could see that the edge of the Seat was above the chamber wall even then.

I have a friend that can do the welding with TIG for me, I have still not decided If I am going to leave it as is or try to repair it.

Regarding detonation, I have the County pistons, the block has been decked and the heads have been decked severely in the past. I have two options about how to solve the problem with valves hitting pistons:

1: This was my first plan, I have aquired 0,5mm saver
2: Use Flying Farmers method: Use a modified TR7 exhaust valve inna battery drill and fly cut pistons with the assembled engine as a jig.

I had planned to do go for method 2, but If I keep the heads as is I probably do not want any more compression?

[dasadrew]

....
I have a friend that can do the welding with TIG for me, I have still not decided If I am going to leave it as is or try to repair it.....

Not saying that all Stag heads are the same, nor that your friend couldn't do it, but I had some corrosion around the fire ring area on my head and a breakthrough in a bolthole/water jacket and took it to a local welding company. That company just do welding of every type imaginable and nothing else. They finally did manage the repairs but tore a lot of hair out in the meantime, saying that the alloy casting was such poor quality that every repaired section just opened up another void.

Drew

[jakesmig]

Not running a TV8 I'm being a bit cheeky, but, as I understand it,one of the underlying faults with the TV8 was the poor quality alloy that was used in the casting of the heads. As Drew points out this can cause real difficulties in rectifying the faults. A major issue in aluminium welding is determining the type of alloy used so you can match the appropriate technique. Get it wrong and you create more problems.

How do you know the valves will hit the pistons? If they hadn't before you took the head off why would you expect them to do so when rebuilt?

If you have any real concerns on this you could take the head and valves to a machine shop and see if they could re cut the valve and seat so that the valve sits lower in the seat. Presumably you're only looking for a few thou.

You could try reassembling the valves into the head, install the camshaft turn the engine so the piston is at TDC, place some plasticine over the piston, place a spacer plate of known thickness between the head and block, loosely tighten then turn the camshaft until you have an imprint of the valve in the plasticine. That will give you enough information to work out clearances. At least you would have a known figure to work to.

[Mark Serj]

Definitely do the clearance check as Jakesmig says, but I'm also curious as to why you are worried about valve to piston clearance?

Is there much head material left between the edges of the inlet valves and head face?

[Ian928]

Regarding valve to piston clearance: there were marks in two of the pistons when I dismantled the engine. This was with thicker head gaskets. If I remember correct the head has been skimmed 2mm, and I had to skim the block 0,5mm. The inket valves sit below the head surface when closed, so as long as the pistons stay below the block surface It should be Ok, but I think I would like some more safety if possible.

[flying farmer]

The easy way of checking piston to valve clearance is to set the piston no 2 on TDC.

Then set the camshaft to TDC by the timing marks.

Both inlet and exhaust valves should be open the same amount, but the exhaust is not a problem as it is a smaller diameter.

Carefully rest the head on top of the block with no head gasket. If the valve is missing the piston then the head will sit flat on the block (or the piston if it protrudes above the block deck)

If the head is resting on the block there will certainly be enough clearance even with a standard gasket.

If the head is resting flat on the piston, measure how far the head is sitting above the block, then subtract that from the head gasket thickness.

I have always aimed for 2mm clearance (standard gasket thickness), and notched the pistons if I did not have it. I have never had any problems even with fast road cams and high rpm with 2mm clearance