Lost valve - Alfa Romeo Forum
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Lost valve

Gents (and Ladies?),

Why would a valve stem snap?

I rebuilt my old CF1 T/Spark 1.8 after a water pump failure wrecked all the valves.

The crank bearing shells were all fine (no marks at all) and the pistons showed only the faintest of marks so it must have been a gentle one.

Head was rebuilt professionally (all new valves and seats) and I also fitted 8 new buckets (all on the Inlet side) since a couple of the buckets (both from the exhaust side) were looking a bit ropey - from wear rather than trauma. I put the best 8 of the remaining 14 buckets on the exhaust side.

When re-assembled, the motor was running sweet. Probably as good as a TS can run, if I say so myself.

Sadly less than 200 miles later the beast just died on me in a cloud of oil smoke.

I have just removed the head. The head of one of the inlet valves (no. 2 cylinder) looks like it snapped off (about 8mm up the shaft) and then got smacked into the piston, stem "stub" first, putting a hole in it. Then it go wedged in the valve port and generally knackered the combustion chamber. The big plug was not a pretty sight.

I'm mystified what went wrong. The valves were all new (they had to have been) and even if they got walloped again, they would surely have bent, rather than snapped off? On the Top side (rockers) everything looks okay. No issues at all.

The Motore somehow did still run on 15 valves and a holed piston... but perhaps "a bit lumpy". The wreckage in the chamber is not pretty..


Ralf S.

No bullets for Chaingun..

Last edited by Ralf S.; 24-04-16 at 21:09.
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Does the crank need to come out to withdraw the piston/con-rod from below or is there a fiendish trick to take the con-rod/piston out without removing the crank?

I presume I can't fill the hole with Araldite Liquid Metal (or weld it )so... where's the best place to get a new piston and rings (and potentially a con-rod)?

Is this a good time to replace the small end/gudgeon pin or is that fairly bullet-proof..?

Any value in putting a whole new set of rings in there while it's in bits? The motore was pretty good before it died... quiet and hardly any oil consumption, so I'd guess the rings are pretty tidy, despite its 150k mileage.


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pistons come out the top, and go back in that way aswell.

Trouble is knowing what size you need. look for markings of any kind on the piston.

Get a piston from another dead engine....

No new rings unless you'll redo the cylinder honing.
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I think the valve dropped: one of the retainers wasn't in properly, or the valve got stuck in it's guide.
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What's the condition of the guide for the valve that hit? Just wondering whether the first valve-bending incident cracked or ovalled the guide and led to this.
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Quote:
Originally Posted by Cuore_Sportivo_155 View Post
pistons come out the top, and go back in that way aswell.

Trouble is knowing what size you need. look for markings of any kind on the piston.

Get a piston from another dead engine....

No new rings unless you'll redo the cylinder honing.

Thanks C_S.

I managed to remove the piston complete with con-rod out through the top of the cylinder .. I didn't know if the con-rod/big-end would fit but it came out very easily.


>> The piston has a number 60624372 but this isn't listed in ePer. Does anyone knoe the new part number?


The big end shells show some flattening damage that wasn't there after the first valve wreck (when the water pump failed) so I'll need at least one new set of shells. Will the other shells be similarly knackered? I will have a look to check.. (at least the bottom ones) but just wondering what the collective experience might suggest.

Con-rod is dead straight and the gudgeon pin looks unmarked. The little-end shell looks either worn or a little marked but I dunno what it looked like before the valve failed. I'll change it if I fit a brand new piston... obviously no need if I can find a whole second-hand piston/rod assembly.

Rings looks okay and no reason to suppose I can't re-fit them.

Thanks again,

Ralf S.
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Quote:
Originally Posted by Cuore_Sportivo_155 View Post
I think the valve dropped: one of the retainers wasn't in properly, or the valve got stuck in it's guide.
The end of the valve broke off with about 8mm of the stem attached. It was this stub of valve stem on the back of the valve that punched a hole through the piston (the valve head was obviously upside down in the cylinder at the time).

The rest of the valve was still attached in the head correctly and came out okay. It's a tiny bit bent which needed a slight effort to pass it through the valve guide past at that spot... but otherwise the valve moved in the guide easily.

The valve stem probably got bent by the loose head of the valve after it had snapped off. I found the head wedged sideways in the valve port, jammed between the wall of the port and the valve stem.

Unfortunately the snapped off ends of the valve stem had been bashed by the piston and the debris, so I can't clearly work out what happened to them but the valve stem looks like it was a clean snap.


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Quote:
Originally Posted by halftone View Post
What's the condition of the guide for the valve that hit? Just wondering whether the first valve-bending incident cracked or ovalled the guide and led to this.
The guide looks and works smoothly. The remaining valve stem was a little bent so I had to tug it a bit to get that through the valve guide (I pulled it out through the top rather than pushing it out through the valve port) but where the valve stem was straight, the valve moved easily in the guide.

The stem looks like it just snapped... but I dunno how it would manage that. Even if it jammed and the piston whacked it, the valve would bend... (I had 16 bent valves last time... not one of them snapped.. though they were Alfa valves and this set is "unknown" brand).


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I'm supposing that the valve seat wasn't recut properly, placing an uneven seating stress on one side of the valve. If the valve is rotating then the valve would be getting stress (bent) by a small amount in every direction... fatiguing the stem.

The other option is that the head was skimmed too far and there was insufficient valve clearance.. but the other valves (in the other cylinders) look clean. No.2 is otherwise a wreck so not much damage can be seen that might have happened before the failure... plenty happened afterwards...



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