How to flycut pistons on GM's LS & LT engines with GPI's Flycut Tool

Want to run the most aggressive cam possible and need a little more piston to valve clearance? Check out our quick tutorial on how to flycut pistons on GM's LS & LT engines with GPI's Flycut Tool.
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[@benjaminrutledge7675]

One point not mentioned is This tool not only gives valve clearance, it also corrects the angle of the valve reliefs in the pistons to match your heads. Manufacturing being what it is, sometimes theyll sell pistons with valve relief angles that arent exactly what your heads are but will work, this way they can kill many birds with one stone by designing making and selling one piston design to suit many combos.. Example, mahle 21 degree valve notches in pistons for 23 degree heads are for any 23 degree head, angle milled or not. If the angles dont match and you over rev and the pistons and valves kiss, it can bend the valve and cause a catastrophe. If they match, for the most part, if they hit, the worst that CAN happen is the pushrods take it and bend. This is race engine stuff with race engine clearances and solid lifters though. If youre haveto watch this, youre likely not concerned about that on your street LS engine.

[@Motor-City-Mike]

Yup, Still Works. This is so old-school that most guys watching this's FATHER'S weren't even born yet, good to see this passed down!

[@seanmesaehable]

This is amazing I've never heard of it done this way.

[@patrickbarnes2686]

Nice. add to cleanup edge of cut. Radius that sharp edge you just made.

[@suicidal_racing]

one thing i may add is to radius the edges of the new notchs once done to help add in flame travel an hot spots. i got lq9 pistons with .150 fly cuts with ls3 heads.,my ls1 is also .1

[@The1st60ft]

Awesome video guys! Proud Customer here 🙌

[@GwatneyPerformance]

Thanks!

[@DynoPete]

The piston should be positioned in the block where the piston to valve clearance is the closest and the cutter would be a the proper angle. The valve is not closed to the piston at top dead center.

[@irnwkrkev]

I thought this as well. Thought a degree wheel was needed

[@Texas_Road_Warrior]

That’s part of the reason the cutting tool is .100 bigger than the valve size

[@mk1cortinatony395]

great point.

[@patrickwendling6759]

True that.. big time..

[@ezmny1387]

Dont need to, theres no head gasket in there, the valve relief will have plenty of room once a head gasket goes in

[@carllattimore8227]

I see big cams in my future builds for sure.

[@SmallBoreGarage]

Can you guys do one of these for checking pushrod length?

[@KnightfallLT]

boosted98acura I’d like it also.

[@GwatneyPerformance]

That's a great idea, I'll definitely get with the team and see what we can do!

[@shaneechternach1459]

What happens is you don’t fly cut on the SS4 VVT?

[@davidciesielski8251]

I had no idea. Thanks

[@joelard2716]

Would this work on cast iron? Needing to deepen valve notches in a 348/409 block.

[@baby-sharkgto4902]

Nice work

[@benjaminrutledge7675]

Obviously youre not going to just hog them out like these guys did, youre going to measure how much clearance you need once youre cam is in there, but I just done my gen1 sbc, i needed .030 extra clearance for the exhaust valve, the volume of the chord it cut out ended up being .3cc measured. that is a third of a gram and it only affected my compression ratio by .038. It went from 11.494 to 11.456 measured. So doing this is absolutely nothing to worry about in the comp ratio or balance department.

[@ironmike742]

I don't think they just hog them out. I'm pretty sure they checked it with the cam and then did the fly cutting.

[@ChargedTTq]

Honestly, I feel like one of these gnarly NA LT engines would be absolutely sick in a C8... How does the valvetrain hold up with this much lift? Is it pretty much a drag combo only? Will it hold up to daily driving? What about extended times at high rpm? I get that it'd probably need some better lifters. Does it need shaft rockers? Cut the valve seats for bigger/taller springs?

[@yeanlingsoda359vortexcars8]

I built a engine for my 81 camaro and it sounded a little knockey or something but I drove it for 3 day and boom take it apart couple months later intake valve broke off and all the valves left a little half moon in the pistons I felt soo dumb

[@coupe50h]

I just put a comp cam in an l83 5.3 with afm 218/220 .525ish lift, and it is running intake valve into piston? Is it the cam? Or should I double check my marks?

[@rustysausage69]

What are the odds on not messing up the valve guides when using a good cylinder head for this process? I don't have an extra LS3 head laying around with the same 12 degree valve angle as my good units.....

[@benjaminrutledge7675]

Dont do as fast as this guy is and use an assembly lube or oil on the guide rod of the tool and it wont burn them up.

[@rustysausage69]

@@benjaminrutledge7675 not gonna blame the tool or the company, its my fault, but guy said in the video "dont overtighten the collet. it will strip" well I took that and ran with it, the collet moved while it was cutting and i put probably a 1/4 deep valve relief in the piston. junked that bottom end lol lesson learned.

[@Ws6Ms]

cool video thanks guys

[@jgarza3997]

Im runnning a 235/252 stage 4 cam with CNC ported 243s 57cc milled .060 on a stock 5.3 block you think i need to notch pistons ?

[@seanmesaehable]

Is the tool one size for intake and exhaust?

[@suicidal_racing]

liddy you can use the same shaft an then order the 2 cutter sizes you need for your int an exh..isky makes a cutter also,the liddy one is easier of the 2..

[@killyonbibbs5601]

Got me thinking about the ss4 flycut instead of NFC

[@GwatneyPerformance]

We've sort of phased the SS4 flycut version out, but you should certainly consider an even more aggressive cam if you're comfortable with flycutting!

[@DannyR2681]

I’m new to this but does fly cutting lower compression? I have a 5.3 with flat top pistons 243 heads milled .030 with stock standard .051 gaskets. I want to run a Texas speed ms3 cam. On e85 fuel will I benefit from going to an even bigger can and fly cutting more or no point after that? Thanks in advance

[@roccobaggiossi3545]

Allows for a thinner head gasket which raises compression

[@benjaminrutledge7675]

Thinner head gasket could complicate piston to head clearance though, might want to check that

[@ironmike742]

Danny... to answer your question. Yes fly cutting a bigger valve relief lowers the compression. To know how much compression you end up with you would need to take a CC measurement of the valve relief and put that measurement into the formula for figuring out compression.

[@ironmike742]

@roccobaggiossi3545 if you cut large valve reliefs into a piston a thinner head gasket is not going to make up for the volume loss by itself. The heads would need to be milled also if you want to have a higher compression ratio.

[@ChristopherJones16]

is that flycut tool able to be used to cut reliefs in smaller hobbyist engines like 1 cylinder 212 cc Predator Motors from Harbor Freight? Or do the motors have different sized valves?

[@ironmike742]

You would need a smaller tool to do small engines

[@brandonmoore269]

Is this tool ok to use on a gen 4 ls3

[@mikefox546]

Can you use this tool to fly cut an ls2 with ls3 heads?

[@benjaminrutledge7675]

Dont measure it with calipers like he is, use a depth gauge with a pin attatchment inthe deepest part of the relief... and for the love of god if you DO try doing it like this guy, take the tape off the piston before measuring it, the tape can be .015 thick, also dont take his word for the acceptable depth to cut, consult the piston manufacturer, they know waaaaay more than this guy about how much and where its safe to remove material from their pistons. Id also recommend putting grease on the take to catch the chips if you do this, assembled shortblock or mocked up, its better to catch the chips than let them end up who knows where. I feel sorry for anyone who has an engine this guy has anything to do with..

[@ericperry4934]

How about .240 . On stock lq9 piston

[@03c5z]

What kind of power difference could you gain by going from a cam that doesn't require fly cutting vs one that does? We just got one of your SS4 non fly cut cams.

[@GwatneyPerformance]

You could make a considerable amount more power, especially if paired with an appropriate manifold like the Holley Hi Ram or CID.

[@03c5z]

@@GwatneyPerformance good to know. Thanks for the help. Looking to do some more business with you guys.

[@YoungerPrice]

Pure genius.

[@GwatneyPerformance]

We try lol, but thanks!

[@jeremyeubanks2387]

Can you perform this function on stock lq9 pistons

[@GwatneyPerformance]

Yes you can!

[@ericperry4934]

Gapped 1and 2 piston rings

[@jaywalkin8205]

Can I get this for a ls1

[@gadgetdeez7069]

I like how you add 0.090" on the valve axis, but insist checking depth in the piston axis. Lol

[@benjaminrutledge7675]

He does it on to of the tape with his calipers too, skewing the results even more

[@Westbankfireman]

@@benjaminrutledge7675 the tape thickness skewing the depth of the initial measurement stood out to me as well.

[@Artoconnell]

Kinda rough..detonation?

[@07STINKYBOY]

Ceramic ball on a dremel will clean that up in 2 minutes

[@reyls3728]

Does the crank have too be balanced after flycutting?

[@shanesink8073]

It was stated in the video.. no worries about balance

[@jasperpipes4222]

A pro would Rey, but this isn't that type of video...

[@mikeeagle2653]

Look how much “chatter” that cutter leaves Bunch of hot spots

[@ryangulley1082]

Oh hell I've been doing this for 20 years myself with a dremel.you can sand off hot spots.this is no 100 k race engine

[@Ep1cPvP]

What brand of impact wrench is that?

[@benjaminrutledge7675]

Is a regular electric drill. Dont use an impact. The reason it seems that it is, is because hes going a million rpm with the tool and each time a cutter on the tool hits the piston it bounces, causing chatter. Thats the awful wavy appearance of the relief on the piston when they show it close up. Also, dont use this tool at a million rpm in the drill.

[@Ep1cPvP]

@@benjaminrutledge7675 nah buddy the green one he uses to bolt the heads on is a small 1/4impact wrench, that's what I'm talking about not the drill he uses to make the flycut

[@tyleroneill6319]

Snap on unit, not sure on part number

[@dr.michaelbennett8597]

Yikes! Run, run very fast!

[@mikeeagle2653]

Fly cutting the pistons does not does not reduce combustion size or raise combustion. Removing material would lower the combustion ratio Wow. What wrong information

[@jasperpipes4222]

And why would you cut reliefs at TDC when PTV clearance occurs BTDC / ATDC ! Another dirty workshop pretending they know enough to "share".

[@MrJayo73]

@@jasperpipes4222 how do you check ptv clearance? At tdc?! It's the same concept.

[@jasperpipes4222]

@@MrJayo73 I check my clearance like any knowledgeable engine builder. Your cam selection will dictate where in BTDC to ATDC sweep that the PTV clearance is closest. If you just check at absolute TDC, you are not looking at the full picture and still run the risk of a PTV contact event.

[@MrJayo73]

Well just like the old saying goes there's more than one way to skin a cat right. I've never had an issue doing it that way and I'm sure there's a whole lot of other people out there that done at the same way as I have never had an issue so I'm for you to say that they're pretending to be knowledgeable is probably a little bit of of a misconception

[@benjaminrutledge7675]

You dont check piston to valve clearance at tdc either, the valves are nowhere near the pistons at tdc. An engine is dynamic. Watch this animation. m.kzbin.info/www/bejne/g5-tiKClaqidd9U Yes its not an LSX engine but it illustrates this perfectly. Imagine the valves at your angle not the angle in the vid. At seconds 2 and 58 youll see where the valves are closest to the piston in their respective events. THIS is the point where you want to check piston/valve clearance, not tdc.

[@patrickwendling6759]

Please, Thoroughly dismantled that short block, Thoroughly clean it. Do not assemble as is..

[@GwatneyPerformance]

If full disassembly was desired, we would have just flycut them in the bridgeport lol. This is common practice to do this in chassis..... Hundred of them done over the years!

[@jgarza3997]

Im runnning a 235/252 stage 4 cam with CNC ported 243s 57cc milled .060 on a stock 5.3 block you think i need to notch pistons ?