New lawnmower blade boss broken TWICE

[Mike1973]

Hello again,
So I decided to just get them to send out another blade boss as I can't be bothered with the hassle of draining the mower of oil and petrol and returning it just for them to send it back with a new boss fitted, it's due to be delivered either tomorrow or Monday.
Is it easy to fit these things?
Also Burnerman you mentioned fitting two allen screws in place of the lugs, how do I go about that?

Regards
Mike

 

[Harry Bloomfield]

Also Burnerman you mentioned fitting two allen screws in place of the lugs, how do I go about that?

Drilled out and tapped - as he mentioned in post #15 above.

 

[Mike1973]

I know he mentioned it in post #15 above, I have never drilled out and tapped a hole ready for a bolt so was just asking how to do it, drilling part I guess is straight forward but what do I need to tap it?

 

[Harry Bloomfield]

A tap of the correct thread and a tap wrench, though you might be able to manage with an adjustable spanner to turn the tap.

I don't know if it would work instead, not having seen the item, but could you simply use plain rod of some type, drill the boss for a tight fit of the rod. Maybe nylon rod would work and still allow for them snapping with overload?

 

[Mike1973]

I do have some threaded bar so maybe that's they way forward, thanks.
I have ordered a cheap tap and die set from screwfix so hopefully I can get it done, I might as well just do it on the existing broken boss.

 

[lostinthelight]

Looking at the blade boss the dowels look quite close to the edge for tapping, not much thickness either

 

[Harry Bloomfield]

Looking at the blade boss the dowels look quite close to the edge for tapping, not much thickness either

I agree, doomed to failure.

 

[Burnerman]

For these, I either drill 5mm for a 6mm thread or 6.5mm for an 8mm one..... not exact tapping sizes I know but near enough.
I’ve never had a boss failure afterwards - there is enough material.
John

 

[lostinthelight]

For these, I either drill 5mm for a 6mm thread or 6.5mm for an 8mm one..... not exact tapping sizes I know but near enough.
I’ve never had a boss failure afterwards - there is enough material.
John

Fair enough if you've done it on one John it looked very tight in the pics.

 

[Burnerman]

The cast knobs are around 10mm diameter so there is enough space, but I admit it’s a little tight.
I only do this when the user is forever breaking them off.....a couple a week on average!
The repair is strong - maybe too strong as the next failure splits the boss lengthwise.
John

 

[Mikemcg]

This just happened to me with the same model , 5 years old , I do recall hitting a tree root recently and I think the blade has gotten looser since , the part is a tenner and there are loads of videos explaining how to do it . Better this boss that a broken engine !

 

[BlueLoo]

You are not tightening the bolt enough.

 

[Mikemcg]

Once the spigots on the boss have snapped off then I don’t think any amount of tightening the bolt will really help

 

[Mikemcg]

I’m more than capable of creating a couple of fresh spigots to hold the blade in place but it’s a safety issue as far as I’m concerned

 

[Burnerman]

I think initially the shearing of the lugs was probably due to the centre bolt not being tight enough, but that’s history now.
Machines that I repair have had a very hard life and occasionally come in in every week for some tlc which is crazy.
Some machines have no lugs and rely on a domed sprung washer for security.
Chinese machines use the alloy boss, Honda use a cast steel one and the smaller Briggs powered machines occasionally bend a crankshaft!
John