Commando 850 (1974) Valve seat pressures

Hello all.

I've been a long time reader and this is my first post. Please be gentle.

My recently purchased 1974 Commando dropped a valve seat. Aftermarket seats had been installed by the previous owner. It is now at a reputable machine shop being repaired. The machinist recommended that I bring the valve springs in to be checked out prior to reinstalling. I have not yet decided whether to re-use the original springs, or purchase new ones. In either case, I still owe my machinist the correct specs so the springs can be tested.

The manual was not very illuminating, and a perusal of this forum turned up this:

...which, while helpful, leaves some room for interpretation, as there seems to be a bit of disagreement between the responders.

Would I be correct to infer the the correct seat pressure would be 65-70lbs at an installed height of 1.3" for both the intake and exhaust? And, that the seat pressure should rise to 160-170lbs at 0.450" of lift?

Thanks in advance.

Hi. When you say dropped a valve seat do you mean a valve seat came loose in the head?

If it did I would change the valve springs and the valve because it may have been beaten about and stressed. Valve springs come as a set so I would change all of them.

I posted in the other thread you referenced. My experience with spring setting was around racing norton 500 twin engines.

What are you aiming for with your bike? If you are building a street bike with standard cams I would just buy standard springs from Andover or similar. I just did exactly that with my 1974 850.

In that case you will get around 65 - 70 lb base circle seating pressure.

What you do next really depends on your bikes setup. If everything is stock standard you could probably bolt it up and ride.

But if you have a big cam and high comp pistons and also if you have installed different valve seats you should do a lot of checking.

Get the engineering shop to measure the spring pressure on their spring tester. Plot the lbs versus distance compressed. Record the coil bind distance and make a graph of distance versus pounds.

What I then do is assemble the head on the engine and use a dial gauge to measure spring compression versus cam position determined on a degree wheel. Most importantly the compression distance from base circle to lobe peak. Also measure the remaining distance to valve spring coil bind at the lobe peak by levering down the spring until it's coil bound using a big screwdriver or similar.

Now you can take those measurement and subtract back from coil bind to get the installed and lobe pressures from the graph you made with the spring tester.

It's important you have a minimum of 60 thou off coil bind at peak lobe lift. Some people recommend 100 thou. You should also clay your piston for valve clearance at overlap and again aim for a 50 to 80 thou minimum clearance.

You can pack spacers under the spring seat if you need more pressure but unless it's a race bike I'm guessing your going to see around 150 -160 lobe pressure which is ok for a street bike.

To be honest the only reason I'd do all the above on a street bike is if the valve seats have been changed. I did not do it on my street bike.

@johnm

Thanks for your reply. This is for a stock-standard bike, stock cams, pistons, etc. It's a sunny sunday machine, not a hot rod.

And yes, one of the exhaust seats dropped out of the head. Luckily I caught it before major damage could be done.

The head is now with a local machine shop that specializes in older engines. They have asked me to provide the valve spring specs so they can be tested, but I have been struggling to get check-height. I suppose the way you phrase it is probably best: the seat pressure should be 65-70lbs, full stop.

As far as replacement valve springs go: I have seen far more (car) springs break when new. It seems that once a spring is "seasoned" and has some miles on it, then it can be expected to keep on going. For that reason, I am inclined to re-use the known/good springs, if they check out, rather than replace them. Having said that, I am new to Norton ownership, and am happy to be counselled otherwise.

Thanks,
-Mike

You ARE rebuilding the head, right? New guides?
STOP.
Make sure he isn’t planning to press them in COLD.
Or is it too late for that?
Fair question, normally, (the spring pressure) but it implies unfamiliarity with Notrun engine, and where to get good procedure.

I have never had a broken valve spring on a Norton twin. I am careful to check for coil bind however. I have measured spring pressure changes with use and found the first 5 to 8 % goes early in say 2 hours of race use then the spring pressure stabilised for a seasons racing.

I second the concern of the other post that valve seat and guide replacement on an aircooled head is for specialists workshops