Testing new valves & valve guides

[Fingolfin]

Hi all,
You may be aware I've been (slooooowly) rebuilding a Midget 1275 to go in my Mog. I've finished everything but the head work. A few weekends ago I drove the old cast iron valve guides out and drove in fresh manganese-bronze ones, and I bought new valves too. The guides took a little reprofiling to accept the valves, but nothing serious; now the valves drop in under their own weight.

But a question prickles - how do I know they'll still do that when the engine is hot? Since I've done all this work myself, and it's my first time, I'm not awfully confident in it. The last thing I want is a seized valve in a brand-new engine.

So I came up with this idea: it would be relatively simple to place the head in my domestic oven (Better Half doesn't mind), heat the head up, and test the valve motion.

I'm posting here because I want your opinions on two points: 1) is this sensible and worthwhile to test? and 2) if it is sensible, what temperature would give me the most reliable test, without harming the head itself? (My guess is somewhere around 100-120 degC.)

[alanworland]

There is a clearance specified between valve and guide - could you not measure this? Even by 'rocking' the valve?
Warming it in the oven wouldn't do any harm although in running conditions the exhaust valves will be considerably hotter, glowing red under load which wouldn't be simulated in your oven!

Alan

[amgrave]

As Alan said, the head would be cooled by the water circulating around it but the exhaust valve would get considerably hotter as the gasses pass by it. You might be better testing it by leaving the head at room temperature then heating the exhaust valves to around 100c and see if they drop in nicely, it would be nearer a real world test.

[philthehill]

For daily and general use the standard cast iron valve guides are considered best - the cast iron valve guide is also suitable for use in a lightly modified cylinder head. Valve train loading will have an effect on the wear rates of both cast iron & Manganese-Bronze (MB) and should be taken into consideration.
Unless you are building and using the engine for performance there is absolutely no requirement for MB (or similar) valve guides.
As regards clearances - the standard clearances for cast iron valve guides are published in the Morris Minor workshop manual in the general data section.
When fitting MB valve guides the clearances should be increased.

See also the detail in the link below:-

http://www.calverst.com/technical-info/ ... ty-basics/

[oliver90owner]

Buy, beg or borrow an appropriate reamer would be my advice. Job would be then completed reliably.

I've only once had sticking valves on a newly built engine. Simply not enough clearance on over-sized valve stems fitted to a hot 1650 ford head. A pain at the time, but at least I could solve the problem by removing the valve stem seals, as the guides were reamed accurately.

[philthehill]

What ever valve guides are fitted the valve seats must be re-cut to the guides. It is no good fitting new valve guides and not re-cutting the seats as you may be on the road to having burnt out valves/seats as the valves may not sit true on their seats.

As regards reaming the valve guides - best done by a competent engineering firm as unless you have the correct equipment the valve guides may end up being made unusable or at best have excessive clearance.

Only fit the top hat seals to the inlet valves as the exhaust valves stems/guides need the lubrication.

[Fingolfin]

The head is going to be sent to a machine shop for seat recutting. I can have them do a proper reaming at the same time, I'm sure, though by my micrometer I'm pretty sure I have them to the correct 9/32" bore.

My point was that, with such an important and potentially damaging thing, it'd be nice to test before assembly - even if the work has been performed by a competent machine shop - and especially considering the differential thermal expansion rates of manganese-bronze and cast iron. I like Amgrave's idea of heating the exhaust valves with a blowtorch or similar...

Or am I worrying unnecessarily? Should I have the seats recut and then assemble it all and leave well enough alone?

[philthehill]

Worrying unnecessarily? I would say Yes.
So long as you have the right clearances there should be no need to do any testing before assembly.
Only when you are modifying the engine and reducing clearances should a dry assembly be carried out.
What sort of micrometer are you using to measuring the bore of the valve guides.
The use of a precision ground rod for a GO/NO go measurement is the better way of checking the bore of the valve guide.

The valve stem diameter for exhaust is 0.2788 to 0.2793"...............for inlet 0.2793" to 0.2798"

The valve guide diameter..............outside 0.469".............inside ..0.2813 to 0.2818".

Valve stem clearance for ..............inlet 0.0015" to 0.0025"..............exhaust 0.002" to 0.003"

The above data is taken from the BMC workshop manual.

Increasing the valve to guide clearance by 0.0005" should see you right.

[Fingolfin]

Just a cheap electronic micrometer, Phil. Accurate enough down to the thousandth, but no further. I drool over the precision-machined analog dial ones. But haven't got the funds!

Say, I have precision ground rods, don't I? Eight of them, in fact...and those valves all drop in without resistance.

[philthehill]

Thank you for identifying what type of micrometer you are using.
Have you checked the micrometer for accuracy against a test piece at room temperature? The micrometer may be reading plus or minus even after you have zeroed it.
Those precision ground rods as you call them i.e. the valves - whilst they drop through without resistance it does not tell you the clearance between valve stem and valve guide which should be the valve stem diameter plus the clearance dimensions quoted in my post above.
As I have said on numerous occasions once you deviate from stock or original specification parts it develops into a whole new ball game.
Phil