How to make high lift 212cc springs work on a 420cc ?

i would like to make the springs from the 212cc work on the 420cc because they allow over .400 lift. i ran .360 lift on the 212 and i noticed they only sell valve springs that go to .375 for the 420cc and that just wont work for me.
I guess i could stretch out the 420cc valve springs so the coils are spread further and then insert the 212 springs?
thats what im thinking about doing.


I think that's open pressure @ .375" lift not max lift like your thinking


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Dawg 89
At .460 lift you need a spring that will do .500 before coil bind .
A custom spring or one from another application is going too be required .
Jr dragster or motorcycle come too mind still thats a lot of lift .
You could possibly try try Comp 965-1 valve springs. They are made for Jr. Dragster motors and are good .550 lift.
I just bought the spring kit. It is on back order until July. Crazy. Thats 2 months !
Today is 4-23-2021
If i remember to post back the results , and whatever modifications needed if any, i will .
So i installed one of the valve springs. Ended up not using the retainers or keepers that came with it.
The keepers are not compatible with the valves on a Duromax 420 and the included retainers are a bit too small to cover the whole valve spring for some reason. Would think it would cover the whole spring since it was included.So i put the original retainers back on.
Ended up making a makeshift add-on to my Pro-Form spring compressor to install.

Did notice a non-installed height difference. But still has more than plenty of room to operate. still turns over very easily. With the added spring pressure and rpm i should be able to see how far i can take the stock cam.
Will need a valve with a longer valve stem at the least to get max Lift.
Though about grinding the spring pockets deeper but i dont think that should be done because the walls are too thin. the grinding would go through the exhaust and intake port walls.

Grinding away some material for the inner valve spring is required. :
There is a thin metal and rubber valve seal to remove first. Cant reuse the metal part.
Cover everything or clean the cylinder head after grinding away some of the circumference of the circle for the inner valve spring. No need to grind it away completely.. just half of the diameter. The whole circle, but half the diameter if that makes sense.

Looks like the valve spring Lift limitations are more than i thought possible and with about .050 - .060" coil space for safety. :


Now that should hold a decent sized camshaft. Might try less than .510 Lift to see where that gets me first. the duration and LSA will be unknown.
Will report back within 2 weeks about how the extra RPM feels sine i finally have brakes.
No big cam to possibly do wheelies yet. Already partially sheared the valve stem on a tubed tire from spinning on the dirt and gaining instant traction on asphalt i assume. Might do bead locks.