Hello Mick,

I work in the field of testing and developing prototype engines, both petrol and diesel. These are full size ones though, not model scale!

Exhaust temperatures measured at the header pipe, just after the exhaust port, can reach 900 degrees Celcius for a modern fuel injected petrol engine when at full power. I am not sure how this might relate to a very small cylinder as in a model. The fuel is still the same and if using similar compression ratios then the cylinder pressures and therefore bmep will be similar too. The temperature is not constant of course and so the valves and seats will heat and cool rapidly.

I'll do some further digging on this subject.

Regards,

Mick

Like Mick Dobson, I was involved with I.C. engine manufacture and development for almost all my working life before retiring.

I would be very doubtful if you could find an adhesive/sealant that would withstand the temperatures to which the valves and seats are subjected at full speed, full load, To give you an idea of the temperatures, for many years, valves have used sodium cooled valve stems, and the valve seats are made of materials that cause experienced Methods and Production Engineers heartache to machine.

Despite the exhaust temperature being upwards of 700 C, the valve and seat temperatures will, even instantaneously be well above coolant temperature. And Air Cooled heads tend to run hotter than liquid cooled.

Believe me, if a valve seat comes loose, you will be very likely to find the only repair being to jack up the rocker cover and bolt a new engine under it!

Fit the valve seats by the "Heat and Freeze" technique (In industry the seats are frozen in liquid air or nitrogen before fitting and then pressed in). This assumes that the parent metal is of sufficient thickness to withstand the stresses from the interference.

If possible have the outer edge of the seat inserts overlap the cylinder wall, so that they can't fall into the cylinder. I have seen even the valve inserts (steel in an alloy head) staked, or even screwed in, come loose on air compressors, which run at much lower temperatures, and then they are not a pretty sight.

You are unlikely to run the engine for 50 hours non stop at full load rated speed, but a model aircraft, or powerboat, engine will run long enough to total itself if the seats come loose.

Months to machine and build, but a second to become scrap metal if anything goes badly wrong, so do be careful!

Howard

If the seat is deeper than the valve lift it probably will not drop out but just rotate. I know this from experience having had it happen on a single cylinder motorcycle. The seat rotated and since its outside diameter was slightly eccentric to the valve compression was lost. The engine ran well at high revs but…………. I must admit on a multi-cylinder engine the damage could have been greater.

On the subject of temperature a model engine is likely to run cooler than a large engine simply because the ratio of surface area of the chamber (through which heat flows) is greater that the volume of the chamber (the source of the heat). However I would not trust any adhesive to hold seats in place. For a model (I am building a Whippet side valve engine) I rather like the convenience of an integral valve seat and guide although it appals me as a retired fluid/thermodynamics engine.

JA

(1) Just for interest there are metalurgical bonding processes available which would do the job .

The AlNiCu one is the best known but there are others .

Don’t suggest that you actually use these for a one off job though !

(2) Have to agree with everyone else that mechanical fixing is best .

MikeW