Needing some input. My '70 Cougar 351C-4V engine is being rebuilt. I had selected some stainless steel valves with single grooves. My engine builder has recommended multi-groove valves like the original Ford valves. The difference as explained to me is that the multi-groove valves rotate and the single groove ones do not. I can see some advantage in having the valves rotating. However with the spring always exerting force on the valve lock and no apparent force twisting the valve I don’t understand why it would rotate. I’m having trouble finding stainless steel multi-groove valve since virtually all that are available are single groove. Is it worth continuing the search and potentially delaying my rebuild?
No, multi groove are weaker to.
Correct, multi keeper groove valves allow more rotation then single groove valves. But, that doesn’t mean single groove valves don’t rotate. They just don’t rotate as much and at a different location. The rotation or also can be referred to as carbon scrapping does just that. It keeps the valve seat clean of carbon and deposit ensuring a continuous good seal at the valve seat.
Mulit groove valves spread to load out over a larger surface area on the valve stem which allow it to not lock onto the keeper/keys. The more groves the less it locks and allows more rotation. Some modern engine have triple groove valve stems. With newer harder materials having lots of rotation isn’t a big concern as it use to be. Spring forces of the valve seat are much more accurate then they use to be and don’t overload the valve train more then needed to keep components in control.
Single groove valves will lock the valve stem, keys and spring retainer together. It still rotates just not at the stem/key location. Rotation will occurs at the valve spring interface either between the spring\retainer or spring and cyl head. Majority of the time it will be at the retainer\spring interface. Back when there was leaded fuel and softer seat & valve materials you didn’t want rotation as much due to wear at the seat location. That’s why you can’t find multi grove valves stems for your older application. They’re using the same old design prints just with better material. Now you here people say when they are driving the older car in a spirited manner “I’m cleaning the carbon out”. Actually truth to that. As higher rpm will promote the valves to rotate which helps clean the carbon off the seat. The valve train goes through different natural frequencies at different rpm that promotes rotation. Clockwise, counterclockwise some rpm lots of rotation and other rpm no rotation just depends on the natural frequency of the system.
Multi groove vs single groove breaking at the tip has nothing to do with how many grooves. If your breaking the valve at the keeper groove location it is a combination of stem dia and side loading or heat treat process where the case harden process has gone to deep making it brittle. You should never have a concern of breaking a 351c engine valve at the keeper groove being single or multi groove. You have some very very serious side loading issues if you do.
Conclusion now you know more about valve rotation then you ever cared too. I’ve done many studies of various engines over my career. You’ll be just fine using a single groove valve in your 351c. Put harden seats in it and take it for a spirited drive once in awhile to keep that carbon scrapped off 
Thanks, Brian.
It’s not often I get thorough responses that satisfy both the avid car enthusiast and mechanical engineer in me.