Article on The Internet Mini Encyclopædia
Valve clearances, reasons for
The main reasons for having a valve clearance are:
- To allow for thermal expansion of valves, block, rods, etc.
- To protect the cam lobe (no gap, valve slams shut from explosion, cam
- To allow for unevenness in the cam base circle (non-lobe side)
- To allow oil to penetrate under the rocker tip and pushrod cup.
Some people say that the gap closes up a bit when hot, and hence you should use a smaller gap set when the engine is warm. Others say the block expansion counteracts valve train expansion, and therefore the gap stays constant. Haynes says to set the gaps cold, to 12 thou / 0.3mm.
Personally I set mine to 0.25mm warm. This ensures the gap is tight (but not too tight) when the engine is hot and gives the valves maximum lift/duration, for maximum power.
Valve sizes, 1275cc (12g940) heads
The standard 1275 mini and metro head, and also the MG metro turbo head have 1.312" (33.0mm) inlet 1.156" (29.4mm) exhaust valves. The turbo head has sodium cooled exhaust valves.
The MG Metro (non-turbo) head has larger 1.410" (35.6mm) inlet valves. This head can also be differentiated from the Turbo head by not having any thermostat bypass hole (the Turbo head has a screw in the hole).
Valve springs, ID'ing uprated ones
Uprated springs are not colour coded, they have 5
windings. Mini Spares do race springs that are colour coded, they are red
though. Std spring 6 windings, full race 5.
Valve Spring Replacement (Without Removing Head!)
Feed some jump-rope sized rope in thru the spark plug
hole and fill the chamber with it. Then turn the motor over BY HAND until that
piston is coming up and compresses the rope against the valve heads nice and
This allows you to release the keepers and change the spring, evah so clevah. Alternately, get the English fella mowog uses to do-up the rear motor mount on a 1100/1300 or the under-clutch motor mount in a Mini to stick his damn finger in the plug hole and hold the valve shut.
NOTE : Apparently, there is a tool available which pressurises the cylinder through the spark plug hole and change the springs.
Valves, Piston contact at high revs
When the piston hits a valve the valve closes, period. It
doesn't care what the cam is doing at that moment. The
cam/valve assembly are a loosely coupled system. Only the cam and crankshaft are directly coupled via the timing
chain. Once you reach valve float the spring can no longer push the rocker down fast enough to follow the cam. It's not really the valves that float, it's the entire valve train.
So, the valve is down but the cam is at it's closed position so the piston is up. Bang! The piston hits the valve forcing it closed just as the cam comes around to lift. Something has to give. If you're lucky, instead of holing a piston or breaking a rod you'll bend a push rod or break a lifter.
The valve system is operated by two things. Most
obviously the cam pushes the pushrod which rocks the rocker which pushes the
valve and spring down.
Less obviously, when the cam stops pushing, the valve spring takes over. It is the valve spring's job to push the valve, rocker and pushrod back, and keep the follower on the cam, and also stop the valve from bouncing on it's seat.
Now, the valve train (all the above bits) has significant inertia. At about 6000-7000rpm, the standard springs are not strong enough to push the valve train back as quickly as the cam is backing off. Then you start to get valve float and valve bounce.
It is this that limits the speed of nearly all standard mini engines. Valve bounce, or points bounce (exactly the same thing, where the points return spring is not strong enough) happens before you are anywhere near the failure speeds of the flywheel, crank or rods (1100 might throw a rod though).
Now, this is where stronger valve springs come into the picture. A stronger valve spring can push the valve train quicker. Lightened valve train components require less force to move them, so you can get away with weaker springs for the same rev limit.
Most of the time you whack on staunch springs if you want to get more revs, if you want LOTS of revs then you get things like carbon fibre pushrods, alloy rockers, lightened followers.
Stronger springs cause more drag as there is more friction on the cam, so you don't want springs stronger than you need.
The recommended distance from the top of the valve guide to the valve spring seat differs in a number of publications:
So it doesn't seem too critical...
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