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4.6 Rover exhaust valve size
Posted: Wed Jan 14, 2015 10:45 am
by ratwing
I'm rebuilding the heads because the tops of all 16 valve stems (and the rockers) are badly worn.
Its got SU carbs, a P6 cover/distributor/oil pump & a standard 3.9 cam and I was planning on doing a little work on the ports, just smoothing off the casting marks - nothing drastic because I don't really know what I'm doing and gather its quite easy to make it worse by removing metal from the wrong place.
Anyway, here's the question - the standard valves are 34mm, is it worth fitting 35.5mm valves since the seats need re-cutting anyway?
Posted: Wed Jan 14, 2015 10:59 am
by kiwicar
Hi
you will get far more from cleaning up the castings and opening up the seat inserts a couple of mm then giving the heads a 3 angle seat job than fitting big valves. Big valves are not worth it without porting work to match.
beat regards
Mike
Posted: Wed Jan 14, 2015 9:07 pm
by ratwing
Ok, thanks for that.
I didn't think 1.5mm was that a big difference and they're the same price as the standard 34mm valves (at Rimmers) but sounds like I might as well stick to the standard size.
Posted: Sun Jan 18, 2015 5:01 pm
by DaviesDJ
Valve throat blending is possibly the best "free" single thing to do with the standard setup - but a 3 angle valve seat is also just as good but get is done right.
Posted: Sun Jan 18, 2015 8:44 pm
by SimpleSimon
DaviesDJ wrote:Valve throat blending is possibly the best "free" single thing to do with the standard setup - but a 3 angle valve seat is also just as good but get is done right.
And remove the awful valve guide protrusion especially on the inlet ports, shortening in situ is possible
Posted: Tue Jan 20, 2015 8:46 am
by ratwing
Valve throat blending - I wasn't planning on doing this since I'm keeping the standard size valves but is it easy to do? Where would I remove metal and how much?
I've got a die grinder & burrs (a proper one, not a dremel) but as I said, I've no experience of this sort of thing and really don't want to muck up my heads if I do it so any pointers would be useful.
Posted: Tue Jan 20, 2015 1:42 pm
by Denis247
DaviesDJ wrote:Valve throat blending is possibly the best "free" single thing to do with the standard setup - but a 3 angle valve seat is also just as good but get is done right.
I wouldn't recomend a 3-angle seat on exhaust, these seats carry a lot of heat away and a thin contact would soon burn. Completely different on inlets where the incoming charge actually cools the seats and valves.
Also the exhaust airflow is under greater pressure and the effect on airflow would be minimal. Clean up the throats by all means, and any shrouding in the chamber, but it's the inlets that would benefit more. All assume a normally aspirated engine.