Hello -

I'm looking to upgrade the engine on my 2018 Fat Bob some time in the near future and wanted to start a thread to gather some info and user experience on the available aftermarket M8 Big Bore kits that are starting to surface. I know info is probably still very limited right now, but if you've gone big bore on your M8 with an aftermarket kit I'd love to read your input on the whole experience. In the meantime, I suppose starting with a list of what I've found to be available would be best -

Fuel Moto
117" Big Bore Kit (Fitment: 2017-2018 M8 107" Touring Models) - Dyno Chart
120" Big Bore Kit (Fitment: 2017-2018 M8 107" Touring Models) - Dyno Chart
124" Big Bore Kit (Fitment: 2017-2018 M8 114"/117" CVO Touring Models) - Dyno Chart

Revolution Performance
124" Big Bore Kit (Fitment: 2017-2018 M8 107" Touring & Softail Models) - Stock: 77 HP / 98 TQ; RP 124" Kit: 92 HP / 126 TQ; Modifications: Revolution Performance Milwaukee 8 124" Bolt-On Big Bore Kit ONLY

T-man Performance
120" Big Bore Kit (Fitment: 2017-2018 M8)
124" Big Bore Kit (Fitment: 2017-2018 M8 CVO Models) - 120 HP/130 TQ with stock heads and TMP cam shaft

 

T-Man Performance. TR is awesome! I'm doing a full on 124" Pro Series Kit on my Fat Bob. They just received my heads & jugs. I did a T-Man 107" Stage 3 kit years ago on my '09 Street Glide...never had an engine issue and bike still runs strong!

 

I am doing the Patrick Racing 120 kit on my SGS they are right here in Prescott and the best in the business, but I am going to do the S and S oil pump first just for some added insurance.

 

Patrick racing , that crew make some serious engines man.

 

FM 124 kit I believe is not available for your bike nor my 2018 Fat Boy M8 114. Something about exhaust issues.

 

Also FM 117" kit not available at this time they told me, no idea why

 

Revolution Kit, doesn't seem worth it. Look at Fuelmoto and $tonecolds, cam comparison(s)….. you can get close to that kits numbers with a cam change. Good look, CSM-H

 

But those numbers are with stock cams. If you would add an good aftermarket cam it would surely be in the range of the other kits.

 

 

Lots of big bore kits out there. Currently most of them use the stock HD cylinders that are resized. The Stock 107 cylinder can be safely cut to 4.125" and the CVO 114 and 117 cylinders can be cut to 4.187". Any larger than that and your asking for trouble. Rev performance has there new cylinder that is 4.250" but requires the cases to be re-machined for them to fit and there are some that are putting sleeves in the HD cylinders and IMHO your playing with fire doing that. The 4.250" also has issues sealing the head gaskets so you will need to weld the heads and resurface them. The real issue is the sumping problem that occurs much more often once the engine size increases.