I decided to buy the 600 Size tool for my HK550 in the hopes that it would fit. No modding was required to the tool itself, but I did have to file down the root of my Heads button to bring it closer down to hold the tool firmly in place. Other than that the tool is a perfect fit on a 550 head.Would be a great tool if it fit, i have a Align Trex 600 ESP & the legs need to be about 5mm longer as they dont reach the see saw. No big deal as i can just glue a small piece of perspex to each leg to exstend them. Would have given 5 stars alround if it had of fitted without alterations. It may be fine with other 600 size heads.Must have for all 600 Heli fliers with FBWhen You setup pitch and tracking this is very helpfull as the flybar is fixed. If you go for a HK-600, just place one of this in your order, you will not have regreds.Works as advertised, can't beat the price of this simple little tool. Makes the job a bit easier.