Bicycle Mechanics - Pedal Bearing keeps tightening up

I have Riva Eagle 2 SPD pedals on my MTB, and when I got them, the bearings were extremely stiff. I took them apart and loosened them up, and then rode on them. The problem is, the left-hand pedal bearing keeps tightening itself up. It becomes very stiff after 10 minutes or so of riding. I think the problem is that I can't get the lock-nut inside the pedal tight enough without tightening the cone as well. On most of the pedals I've worked on, there's a keyway on the axle and a keyed washer between the cone and locknut. These pedals don't seem to have that, which is probably why I can't tighten the locknut without moving the cone. It's only the left-hand pedal that does it, the right one is fine. Does anyone have any ideas as to how I might stop it happening?
Thanks

you can make a tool by grinding down some sockets to hold and adjust pedals but probably not worth it. just find a washer from another pedal or get another set

Do like you do with skewered hubs. Tighten the locknut with the cone too loose, knowing that the tightening will snug it up. Alternatively, use Loctite. You need to keep the adjustment fastener (the locknut) from moving when you're riding.

Do like you do with skewered hubs. Tighten the locknut with the cone too loose, knowing that the tightening will snug it up.

That's not the problem. If it tightens while riding, the cone is spinning freely on the axle and compressing everything. No amount of looseness on initial adjustment will solve that. Loctite it.

Loctite the cone or locknut or both?

Loctite the cone or locknut or both?

To be honest i'd like to see the pedal myself, to determine if the OP is missing any pieces. For example the MKS pedals don't have keyways on the axle either. The washer between the cone/locknut allows you to fully tighten the locknut without losing the adjustment. (My memory blows so I can't quite remember if that's *exactly* how it worked).

To be honest i'd like to see the pedal myself, to determine if the OP is missing any pieces. For example the MKS pedals don't have keyways on the axle either. The washer between the cone/locknut allows you to fully tighten the locknut without losing the adjustment. (My memory blows so I can't quite remember if that's *exactly* how it worked).

There's no washer at all in mine. The locknut and cone run directly against each other, neither of them seems to have anything special about the mating surface either. If it's missing anything, they didn't put it in at the factory, as I'm sure I didn't lose anything. I'll probably put loctite on it tomorrow, unless a better idea comes to mind.

As an aside, I'd be quite interested to know how the MKS pedals work.

THen add a washer. The washer will let you spin the locknut and generate opposing forces on the threads without spinning the cone. I've also used needle-nose pliers with 45-degree bent tips for reaching in and holding the cone while I tighten the locknut. If you use a 1/4" socket and ratchet for the locknut, this gives you plenty of room stick in the needle-nose pliers.