Prop nut torque value

[sjaltobelli]

I am wondering what the torque value is for my prop nut. I have a 1990 90 hp engine with a lower unit from a 1987 125hp engine. According to a few dealers and Merc, the prop shaft on the lower unit that I have does not use a bent tab washer to hold the prop on. The size of the nut is 5/8-18. There is a spacer washer, then the prop, then a splined washer (instead of the bent tab washer), then a flat washer, then the nylock nut. I find it hard to believe that only the nylock nut prevents the prop from backing out. Regardless, is there a torque value for this nut or should I just hit it with my impact gun?

 

[john from md]

Re: Prop nut torque value

The nylon insert on the nut will keep it from backing off. We have been using them on aircraft for years and they hold well.

As they get used, the nut will loosen up. You can buy a new one or lay it flat on a solid surface and give the top a couple of good whacks with a hammer. This usually tightens them up to be serviceable.

Regards,

John

 

[Frank Acampora]

Re: Prop nut torque value

DO NOT USE AN IMPACT GUN!

The shaft splines keep the prop from rotating on the prop shaft and trying to back-off the nut.

The nut and washer simply keep the prop seated against the thrust washer behind the prop. It does not need to be "Killer" tight. If you snug the nut then give it a little extra, there is very little chance of losing the prop.

My 1988 factory manual does not give a torque value for that nut, however, I have heard values as high as 55 foot pounds given. In my opinion, this is way too much. Last time this question came up, I was so curious that I went down to the garage and checked the torque on all my engines. Average torque on prop nuts that I tightened by hand was 8 foot pounds. As stated above, being a nylock nut, it will hold itself on the threads quite nicely.