I have got a Mountfield SP 550 petrol lawn mower. About 18 months ago I hit a large rock with it and it damaged the Blade Boss. I replaced this, only for the boss to break again on the 1st pull of the cord that started the engine. The 2nd replacement lasted about 20 hours of cutting time until my son decided it would be fun to go over an empty cement bag and the Boss broke once again. I have replaced the Boss twice since then and they both broke on the 1st pull of the engine start cord. There is no sign that the blade is hitting anything, Nor any sign of distortion on the blade or shaft. Any suggestions?.

Juls, April 2009

The issue here is the spigot that the blade fits over is too long, when the bolt is tightened the washer sits up against the spigot and doesn't trap the blade. Simple fix is to file the spigot length down until it sits just below the blade

Mike, July 2023

I've had two of these bosses fracture from the same point in the keyway. Both times the shear pins had sheared and the centre bolt sheared as well due I think to the bolt effectively being tightened by the slipping blade. I've checked the motor shaft with a dial gauge and no runout found so being an engineer I've machined up a steel boss with steel shear pins. The cast aluminium boss supplied by Mounfield is of questionable quality and I'm not surprised that they fracture.

Mike, June 2022

I had the same problem. I hit a rock which bent the crank shaft. I went on utube to straighten the shaft, The boss still keeps breaking It seem the slightest vibration causes it to fracture. My last attempt will be to listen the belt

Keith, February 2022

I've had this problem too, having had a good look at the boss its simply too weak, i drilled two 6mm holes through the pins on a brand new boss, (that normally shear off), and put two stainless steel bolts through the centre of them to strengthen them up and then a nylock nut on the end of the bolts.
That is not all though, because the lawnmower uses drive from the motor to push it along, i also let off the drive belt tension so the belt is allowed to slip on take up and release of drive slightly, acting a bit like a clutch, otherwise the stop start thrutching causes the boss to break as there is no give in the drive train, its`s direct through the belt. so let the tension off a bit so its rather like a clutch is designed to slip on a car on take of drive. It has not broken since! tea daa, Rick Jaguar Technician

Rick Brad, June 2020

I think the blade is not tight enough make sure you use the correct troque setting

steve, November 2019

I have just had exactly the same issue. As I sharpened the blade whilst it was off and fitted a new drive belt I am wondering if the blade balance is enough to shatter the boss. I am going to try a new blade with a 3rd new boss. Take care if you have this problem as I nearly lost a finger. The blade continued to spin even though motor stopped 10 secs earlier. Ouch!!

Andy Kilding, August 2013

try getting a gardener to cut grass problem solved.as you and your son seen to try to do more than just cut grass with your mower.lol

laughing at idiots, June 2012

Hi John. Thank you for your reply. Both aluminium pips are going into the blade holes correctly. The centre bolt washer has not been replaced, as you say it is curved to act as a spring. The bolt was not torqued when replaced, it was done up tightly, but not done up tight enough to damage the Boss. The break on the Boss starts on the groove where the lug slots in.

Juls, April 2009

A curious one this.....I'm just wondering if the blade is fitting squarely into the boss, and that the aluminium pips are actually going into the holes in the blade. Are you over tightening, or under tightening the blade bolts, do you think? Another thing - if the blade has a central bolt and washer, the washer should be replaced as it acts like a spring, forcing pressure onto the end of the crankshaft.

John, April 2009