Pitting on seat of exhaust valves

[Garth]

Having my fairly new Sigma 125 engine (about 6000 miles) rebuilt with upgraded rods, cams, pistons etc. and have discovered pitting on the seats of the exhaust valves - I am told this is probably caused by running a lower octane fuel, resulting in some carbon deposits getting stuck on the valve seats during the exhaust cycle. Evidently exacerbated by short start/stop cycles (this happens living in London). I will speak to Jeff Wilson when I collect it later this month, but was wondering if the POBC could help me understand how this happens in a bit more detail. Engine will be run in on the dyno next week so looking forward to the results (if they are good!!?). Thanks

 

 

Edited by - Garth on 7 Jan 2013 20:24:34

[Dr Slotter]

Pitting usually occurs as a result of cyclic elastic stresses forming cracks at or near the seating surface. These cracks eventually result in chunks of the surface becoming detached. The combustion products that adhere themselves to the surfaces increase the stress in the surface because the reduce the effective area that the forces arising from the combustion pressure act over. It is correct that this is more likely to happen if the exhaust valves do not operate at an elevated temperature for a significant period of time.

 

You can see a different example of this easily by looking at rail surfaces next time you're at a rail/tube station/tram stop.

 

[Garth]

Thanks for your response - are the elastic stresses something that you would expect to happen in a relatively new engine? I assume these two things could happen independently of each other? Although it would seem likely that carbon deposits resulting in higher local stresses would accelerate the 'elastic stress' deterioration? I presume there is not much that can be done about long term elastic stressing, but what is the best approach to limit carbon deposits forming on the valves??

[Dr Slotter]

The elastic stresses are a function of material, geometry and operating conditions, nothing to do with age of engine. Pitting would be accelerated in the scenario you describe. You can't stop deposits forming at all, but all the usual good engine care practices regarding oil use and the like should be followed.

[Stationary M25 Traveller]

Use something like Shell V-Power too, rather than cheap supermarket fuel.

[Dr Slotter]

Aye, crap fuel often equals incomplete combustion which equals crap on your valves.

[Garth]

This may surprise you, but Jeff at HWR who is rebuilding my Sigma engine recommends Tesco's 99 octane petrol!

 

Edited by - Garth on 15 Jan 2013 18:56:50

[Stationary M25 Traveller]

99 better than regular 95

[Garth]

Jeff recommends it above Shell V power (98 octane)

[Dr Slotter]

Yep, the 99 octane Tesco fuel is pretty good.