Active Member
Oct 3, 2018
Well i only did a piston skirt trim once only because it was not clearing the intake at TDC it does help it breath more on the mid to high rpm but sucks a little power out of the low rpm range it is the easiest mod only way to find out is to take your intake pipe off and check


Well-Known Member
Jul 2, 2018
Be careful skirting pistons just from the cylinder and a loose piston like that video. TDC will depend on base gasket thickness and cylinder casting and can change from motor to motor. By eyeballing and marking TDC as just making the piston crown even with the deck you can easily remove too much of the skirt and have too much duration when assembled, leaving your motor gutless down low.

Proper method is to assemble the top end to the bottom end using the same gasket you will be running, rotate the motor until the piston stops moving up but before it starts moving down (there is some dwell at TDC) and then marking the exposed piston skirt through the intake with a marker. Then you know exactly how much needs to be trimmed. Some motors dont need ANY, and have no overlap into the port while others may need a lot of material removed.


Well-Known Member
May 17, 2010
educate yourself by reading this several times...

buy several cylinders and twice as many pistons (and rings), then get on ebay and order some of these...


dont waste your time with a dremel. they are useless.

you port the CYLINDER, not the head.

when it comes to changing port heights, trim the piston FIRST, test it out, then port the cylinder to the new timing, with an untrimmed piston.

be careful with the intake port skirt trimming. youre better off making it wider in the port to keep the same timing, but giving more area to get more air in there.

transfer matching, angles of transfers, flow of gases...

porting an engine is down to imagination in visualising the gas flow, being sensible, only making one change at a time, plenty of testing, and experience.

run straight intake and exhaust manifolds to analyse any changes in performance.

then tune the manifold to suit the intake timing, and, if youre that keen, design and fabricate a tuned exhaust to take advantage of the transfer/exhaust timing.

throw all the bling at it you want. none of it fixes the issues with port timing though. that you have to do yourself, and has much better results than silly anodised alloy bits.

yes, expect to destroy five or more cylinders in the learning process.

its an idea to get a cast iron sleeved cylinder so theres no problems with chrome plating no longer running into the ports and protecting their edges... and its hard to destroy a cast iron sleeve when something lets go.