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Welcome
Because of the large number of fully imported USA spec mowers sold down here we should be able to find you a replacement blade to fit the original blade boss.
Allwe need to know is the diameter & shape of the hole in your original blade and if it is stepped or flat.
If you really want to go the blade disc route then every brand of mowers sold in Australia has an 18" cut model except Honda who do 19" as their smallest cut
Every brand also has a Briggs & Stratton engine either as an alternative or only power source.
SO the first problem is the diameter of your crankshaft 1" , 7/8" , 20mm or 25mm as your engine could have any one of the 4 .
Next, what blade disc did you buy & who did you buy it from ?
We might be able to find some stepped blades that will lower the cutting height a bit.
Next what is fouling on the mower chassis, the blades or the blade bolts ?
Most blade dishes except Masport are cupped and they all fit cup side down so the blades are lower than the mounting bolt for the disc to protect the bolt from being rounded in use.
On top of that some blades are flat while others are stepped and all the steps lower the blade height .
These are the
Blade holders used on most local mowers;
The full disc ones all use the 3 bolt mounting except the Victa one
If you have a 7/8" shaft I would recommend the following
disc which sits on this
boss and is held on using this
washer & this
nut with this
blade set
If money is tight then buy any dead 18" Victa with a B & S engine off ebay and pinch the blades & mounting hardwear off it.
Most regional tips sell the mowers people toss out for a very small price if you are not in one of the major cities.