15 replies to this topic

[JimmyStinkpickle]

Hi all,

I’ve run into a concerning problem with my rebuilt 12G940 cylinder head and was hoping someone might be able to help me out. I ran the engine for the first time without coolant for a few minutes to bed-in the head gasket and then let it cool overnight. My plan was to retorque the head and adjust the valve clearances before the 20 minute cam break-in period. When I went to adjust the number 4 exhaust valve, I noticed that it is stuck open resulting in a considerable gap between the valve and the rocker. All seemed fine during the initial run so I am at a loss. Any advice before I remove the head? Please help! Thanks in advance.

 

 

 

Edited by JimmyStinkpickle, 28 July 2026 - 02:27 AM.

[Spider]

You'll need to remove the head to properly check it out. If it has any warranty on it, contact the supplier before removing it and seek their advice so as to not void your claim.

 

A common cause that I see for this for this on fresh overhauls is not reaming the guides after fitting.

[JimmyStinkpickle]

You'll need to remove the head to properly check it out. If it has any warranty on it, contact the supplier before removing it and seek their advice so as to not void your claim.

 

A common cause that I see for this for this on fresh overhauls is not reaming the guides after fitting.

 

Hey Spider thanks for your advice as always!

 

I think perhaps the new guide may have been a little tight around the valve after my initial start? So I sprayed the valve stem with some penetrating fluid and using a small pry bar, was able to gently lift the valve back up. I turned the engine by hand a few times to get the valve moving and spread the penetrating fluid. It eventually returned to the closed position on its own. I am cautiously optimistic that did the trick.

Edited by JimmyStinkpickle, 28 July 2026 - 05:06 AM.

[Earwax]

DONT take it for a run--- heat will be your enemy-   A tight guide and heat expansion are problematic -    ( repeat Spider's advice here)   You could also check that the oil is getting up top - and through the rocker post hole N2. ( 

[Quinlan minor]

+1 on Spider's advice. I went through a patch, back in the '80s of mismatched guide materials with stainless valves and suffered exhaust valves seizing in the guides.

The consequences, while running can be very expensive.

[Spider]

Now that it's cold, it may well re-close, but you can be sure there is at least a marr on the valve stem, it will most likely - at this stage - clean up OK, but I would suggest not running it any further until it's been stripped and checked. I'm sure something will need to be rectified. I'd suggest contacting the supplier or if you are going it alone, checking all valves and guides as a minimum.

Assume nothing, check everything.

[Cooperman]

I do recall having that problem many years ago with a 970 S engine.
When doing the head I fitted the then-new manganese-bronze guides and reamed them correctly. However, two of the valves stuck when the engine warmed up. I power-reamed them and eventually they worked, but I have used iron guides ever since with no issues.

[JimmyStinkpickle]

Better to be safe than sorry so I will pull the head and have it checked out by the machine shop. The iron guides were reamed after installation according to them. Is it possible for the valve to hit the top of the piston? The head is from an MG Metro and was skimmed. I have a Minispares EVO 001 cam and P21253 pistons installed. CR is 10.5:1. Thanks.

[Cooperman]

The valves cannot hit the pistons as the A-Series is a ‘non interference’ engine.

[imack]

The valves cannot hit the pistons as the A-Series is a ‘non interference’ engine.

A standard A series may be non interference, but, depending upon specification a modified one can certainly suffer valve to piston contact!

[Earwax]

Probably just camera angle , but when reinstalling - look at the alignment of the rockers over the pad - dont want any sideways forces on what looks to be like a very nice build - and again it is probably a bit keyboard warrior of me talking about oil supply to the head - but catching any problem early is the inexpensive option .  

[sonscar]

The rockers are designed like that.Steve..

[Cooperman]

I have never had an ‘interference’ issue despite running a 286 cam with 1.5 rockets and 11:1 CR. My piston tops are 0.006” down the bores at TDC and there is still some nominal clearance.
Badly adjusted valve clearance would reduce it if too tight, but even then it would only be an issue if the timing chain snapped.

[Spider]

There's been a few engines that I've built that were 'interference' for sure, but these had high lift cams, high ratio rockers, and a fair bit of meat from the heads.

You can always do a quick look up, see what the valve lift would be and then measure in the combustion chamber from the valve faces to the deck, if this comes close and you are running a long duration or a late closing Inlet Valve, then dry assembly checks would be in order.

[Steam]

Just for completeness and to throw in another cause, using old fuel will cause the fuel to turn to varnish on the valve stems causing valves to stick.