400/4 exhaust valve seals

[Murf]

I am reconditioning a 400/4 cylinder head for a friend of mine and noted that valve seals were only fitted to the inlet valves, which is as the manual. So I am thinking should I fit seals to the exhaust valves when I rebuild it.

I have looked at past threads (2017) about this on this site, there was plenty of info but no definitive conclusion. It seems like early models had 8 seals then this was reduced to 4 seals on later models. Probably as a cost cutting exercise by Honda.

Unlike Honda my friend isn’t too worried about spending an extra £8 or so, (to be fair to Honda he is only paying for 4 extra seals whereas they would have been forking out for 1000s).  In Hondas favour they were dealing with a brand new engine with no valve stem or guide wear.   We have a 40 + year old engine that has done 25K miles then stood for 25 years unused and was seized due to rust in the bores.   So my feeling is to fit seals to the exhaust valves as well, why not.

Is there any real reason why I shouldn’t and what do people rebuilding 400 generally do.

I know the exhaust valves are not subjected to a suck stroke, but I don’t want the oil making its way down the valves while the engine is stopped and having the horrible Smokey start up.  The valve and guide wear isn’t bad but you know how things go.

[McCabe-Thiele (Ted)]

I'm pretty sure most of the  engines I have worked on have not have valve stem seals on the inlets and exhaust so I'm assuming there isn't anything to be gained from fitting them.
If an engine is smoking on start up it's probably has more to do with wear in the guides/valve contact area or poor piston ring seal when the engine is cold.

My 400 has just the standard 4 guides it's had a rebore by Graham - no smoke at all, the guides are original, the valve stem wear was within the limits.

[robvangulik]

Those seals eat (some) horsepower

[Bryanj]

You cant just put seal on the exhaust guides, you have to change the guides for some with the seal retention groove which then means reaming the guides and recutting the seats thereby adding more expense.
Unless he is racing or doing tens of thousands of miles annualy i would just leave it as Honda designed it

[McCabe-Thiele (Ted)]

There is a case for not putting seals on the exhaust guides if the engine was not designed for them.

As previously stated the pressure from the exhaust gases reduces oil flow out of the guides, fitting seals will reduce the lubrication available to the very hot valves so could make them wear out more quickly. Honda would have designed the materials and tollerances accordingly.