Which washer is more important?

[KnifeySpoony]

Question for the learned folks here. I am doing the uprated rear brake upgrade and part of it requires new dedion alloy endplate "ears". I am going from 2deg to 2.5deg ears -because of this, the plate is thicker on the bottom (and is thicker on top too for some reason). Because of the thickness, the two most-rearward bolts that attach the ear to the dedion tube are a bit short. They are advised to have a flat washer and a split lock washer. See pic below for a picture of a stock setup. If I use both of the washers, I'm only getting like 4 threads of engagement - if I omit one of them, I get about 6 threads of engagement, which seems to be enough, but I'm happy to be corrected. Which washer is more important? My thought was omit the flat washer then it will be locked on but with a smaller area to distribute force. If I omit the lock washer, then I could use loctite on the bolt but not sure if that is really equivalent. I can get a longer bolt but they come in 5mm intervals and the hole in the dedion is a blind hole and not deep enough so I'd have to cut some of the bolt off. 

 

Thanks all

 

 

 

[theDreamer]

You need them both. The flat washer is what the lock washer is meant to bite into. I’d buy longer bolts.

[KnifeySpoony]

Loctite not enough to make up for lack of the lock washer?

[sltous]

Two thoughts. 

 

First, 3 threads is technically sufficient but ideal is 1.5 x diameter of the screw. 

 

Second, consider either Belleville lock washers or better yet Nordlock/Wedgelock washers.  Nordlock are excellent locking washers and are not intended to use with an additional flat washer.  https://www.nord-lock.com/learnings/bolting-tips/2010/can-i-use-other-washers-with-nord-lock/ both may be likely to marr the surface of the dedion ears but they should not meaningfully damage it.  https://www.nord-lock.com/learnings/bolting-tips/2016/what-effect-will-nord-lock-washers-have-on-my-surfaces/

[KnifeySpoony]

Yeah I've read 1-1.5x diameter of screw when going into steel. With both washers I'm just under 1x so I definitely need to make a change of some kind.

[sltous]

I pulled up the build manual to remind myself of how big that set of bolts are.

 

For an M10, the stack up comparison would be:

• 2.0mm for a typical steel spring lock washer, 2.0 mm for a typical steel flat washer
• A typical nordlock washer style set from McMaster would be 2.3mm to 2.8mm https://www.mcmaster.com/91074A180/ and for stainless steel a pair would take up just 1.8mm to 2.3mm https://www.mcmaster.com/91812A255/

This seems to indicate that switching to a nordlock style of washer would give back a washer thickness and should be sufficient.  I probably wouldn't bother doubling the cost to switch to stainless steel to gain 0.5mm of engagement but there are still gains to be made there.

[slowdude]

Did this when I redid my rear end a few years ago. 

 

Use the lock washer and the washer.