Match the stone to the material:
Most flywheels are cast iron (soft) but there is an increasing number of cast steel (medium) many trucks, buss’s and agricultural: billet steel (hard) many new European models including VW & late model V8 GM vehicles
How to test a flywheel for Hardness:
For good results, softer flywheels need to be ground with harder stones. Harder flywheels need softer stones. To determine flywheel hardness; spark test a flywheel before grinding. Using whatever grinding wheel is on the machine briefly touch it to the flywheel and watch the sparks fly.
Cast Iron: Soft, sparks make a deep red-orange spark that travels only a short distance before burning out.
Cast Steel: Medium, sparks are a light orange to golden, travel further than cast.
Billet Steel: Very hard, spark is bright yellow to white and travels a long distance
If the stone wears rapidly, it’s likely too soft, try one harder. If it glazes or loads up it’s likely too hard. Try one softer
Coolant: It is absolutely critical that a good quality coolant is used. SAE 1023 is an excellent product. Note: no more than 3% should be used. More is not better as it will load the wheel i.e. becomes soapy and clogged.