Clifford mkIV breaking exhaust valves

[vhgmcbuddy]

Has anyone else had trouble with a JAP engine breaking the heads off exhaust valves?
My dad owns a Clifford mkIV that gets regular use on his allotment and it breaks at least one exhaust valve a year.

[charlie]

Strange. Has it always done this? Where do you get new valves from, could they be poor quality copies?

[trusty220]

The only thing that I can think of is an incorrect setting somewhere- either tappet clearances are too great or the engine may be overspeeding. It certainly is not a common fault and I’ve run JAP engines on my Trusty tractors for years.

I wonder if the clue is in the description- it is only the exhaust valve that is doing it. Exhaust valves heat up considerably more than inlet valves because they are not being cooled by the incoming air, so many manufacturers tend to make the exhaust valve out of a harder material. You may be replacing the broken exhaust valve with an inlet valve (which is the same size and shape), so that may be worth checking out.

Similarly, the exhaust valve spring is normally made of thicker wire so that it conducts heat away from the valve stem more efficiently. If it can’t dissipate the heat then the valve stem may fail.

I hope that helps.

[vhgmcbuddy]

Dad’s had the machine for near enough 50 years and it’s always done it.
He’s been assured that the valves he’s buying are correct and of OE quality – but who knows given the age of any new old stock spares!

Correct me if I’m wrong, but I believe the JAP engine on a clifford is quite different to those fitted to a trusty – perhaps it’s a fault only found on the Clifford engine variant.
He’s talking to someone at the moment about “competition” spec valves as used in the old JAP powered speedway bikes as apparently they’ll fit.

I just wondered if anyone else had ever had the issue.

[charlie]

Would it be worth speaking to someone at Meetens to see if they have come across the problem.

[vhgmcbuddy]

Also worth considering is the engine burning oil? As oil burns hotter than petrol and maybe removing the heat treatment?

[vhgmcbuddy]

I got a JAP 5 sat here needing a home

[vhgmcbuddy]

Not aware of it burning oil excessively.

Thanks for the offer, but it’s not a Model 5, it’s a UCZ.

[vhgmcbuddy]

I’ve run quite a few of these engines on gems, and never had an issue, would it be worth obtaining a second hand valve out of an original engine and see if it lasts any better?

[vhgmcbuddy]

Been in touch with Meetens as Charlie suggested.
Apparently it is quite a problem on the JAP 600, so much so that they’ve run out of replacement valves!
Beginning to look like a custom valve might be the way forward.

[andyfrost]

What grade of oil are you using ??.

Andy.

[will-haggle]

I wonder if it’s something in the valve guide like it was undersize from the factory or it has a burr.

[Author]

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