How to flycut pistons in an LS1 for free! (almost)
While degreeing the cam in my new LS1 build, we found that my piston to valve clearance was only .055 on the intake side, so they needed to be flycut. Unfortunately, the cheapest tools I could find we're over $100 and that was just for a single cutter and mandrel. Add one for the exhaust and you're looking at $200. The only other option is to rent a tool to do the job and that too costs about $200 to use. That wasn't in my budget, so I started brainstorming and came up with an idea that works perfectly and costs barely anything. It was free for me because I already had the materials needed, but if you have to pay over $5 to do this, you're doing something wrong.
Items needed:
1) Adhesive backed sandpaper, I used 40 grit
2) Stripped cylinder head - I used an old one I had lying around because my ported heads have stock size valves. If you have larger valves or do not have an extra head, USE ONE OF THE HEADS YOU ARE PUTTING ON YOUR CAR! This method of flycutting will absolutely NOT damage your cylinder head in any way unless you really screw something up.
3) Either an air drill or a cordless drill with a lot of power. We used a Milwaukee 28V cordless.
4) Masking Tape - Tape across the entire deck on both sides to keep grit from contaminating your engine. If you do this correctly, there will be no path for anything to get inside.
5) Some sort of tool that can be inserted between the chuck and the top of the valve seal to give repeatable depth results. We used a small flat wrench that measured .080 thick and it worked great. You could even make one out of a thin piece of bar stock.
6) Shop Vac - to suck up all the aluminum and grit that's produced during cutting.
7) Some sort of cutting tool to make small sanding discs slightly larger than your valve. We used a 2" I.D. exhaust pipe with the outer edge chamfered to make it sharp for cutting. If you have larger valves, you could chamfer the inside edge until you get a suitable diameter, or you can use any sort of round tube that you can just barely fit around your valve. Having a slightly larger diameter disc causes the paper to cut slightly larger at first, then closer to the actual valve size as pressure is applied. This bevels the top edge of the cut to eliminate hot spots in your combustion chamber.
Now that you have everything needed, let's get started. First you will have to cut out your sanding discs. We used a press with the aforementioned piece of exhaust tubing which gave us accurate, repeatable results. If you don't have a press, lay your sandpaper on a block of wood grit side down and use a hammer to strike the tubing to "cookie cutter" out a disc. If you have a pipe end that you could slide over the end of the exhaust tubing, that would be VERY helpful here. Cut out 8 discs for the intake side, then switch to your exhaust cutting pipe and cut out 8 more ( this is optional, as I only needed clearancing on the intake side ) . You should be left with something that looks like this:
Now clean off the deck of your block with whatever you choose, but make a final pass with lacquer thinner or electronics cleaner so your masking tape will stick. Tape off the ENTIRE deck so there are no paths for grit to get into. However, you will need to punch a small hole in the center of each cylinder to allow air to escape or be drawn in. If you tape it off well enough, a piston can produce so much suction that it will rip your fresh masking job. Not good. Here's the final product (note the small hole in the center):
When that's done, you're ready to start flycutting. You should have already determined what your piston to valve clearance is, and in turn you should know approximately how deep you need to cut to get proper clearance. We went for about .120 clearance ( from .055 measured ), so we needed .065. We found a small wrench that measured .080 thick and used that, figuring the paper itself was about .010 and we wanted to err on the larger side as to not make multiple cuts on the same valve. Place an adhesive disc on the face of your valve as show below:
Once that is done, turn your engine over until it is at TDC (doesn't matter which pistons you start with, two will come to TDC at the same time) and then back it off slightly (LS1 pistons come out of the bore, so if you go to true TDC, your head will not seat properly). Once this is done, determine what chamber your valve needs to be in to make the cut on the first bank of cylinders and insert the valve into the valve guide (a drop or two of oil should be placed on the stem). Pic below:
After inserting your valve, place the head on the block and lightly bolt it down. We used 3 bolts in a triangle shape, as it made for quick installation and removal. Two in the dowel holes and one in the center between cylinders. You don't need a lot of pressure to make these cuts, so there is no need to clamp the head down hard. Once the head is bolted down, turn the engine over so the piston butts up against the head. DON'T FORCE IT! All you need is for it to touch the piston. Then take your finger and push the valve stem down until it touches the piston:
Once this is done, place your depth tool (wrench or whatever you've located) on the valve stem:
Then take your drill and put it over the valve stem while tightening the chuck - the object is for the fingers of the chuck to be resting snug against your depth tool. This will NOT damage your valve if done correctly, my valve looked perfect when done:
Remove the tool and visually verify that the proper clearance exists between the valve seal and the drill chuck. You can do this without a seal installed, but your chuck may not seat far enough down on the valve:
Pull the valve up slightly off the piston and start the drill. Push downward using steady pressure and watch the gap between the drill chuck and the valve seal decrease until the chuck contacts the seal. At this point, your cut has been made.
Remove your drill and your cylinder head, then use a measuring tool to verify your cut is as deep as needed. Use the vacuum to suck up all the debris from the cut you made and in the combustion chamber/ports, then attach a new sanding disc to the valve and insert it in the corresponding chamber for whatever piston is at TDC on the other bank of cylinders. Once you've made those two cuts, tape over the cuts with a piece of tape and rotate your engine so the next pair of pistons hits TDC and repeat the process. When you're done, again go over the engine with the vacuum and then remove the tape. You should be left with nearly perfect flycuts and a lot more money left in your wallet! ( As a side note, if you DO use a head that you are planning to bolt on your car, clean it thoroughly with soap and water and an air gun after doing this process or you will get a lot of debris in your engine. )
__________________ '98 NBM Formula M6/ Hooker Super Comps w/TSP True Duals/ lid/custom 85/85 FAST/ CNC Ported 241's/ Streetsweeper 228/232 on a 111 LSA/ Six Speeds Inc. Stage 4 T-56/ Hurst Billet Plus with chopped stick/ Monster Level 3 clutch/ S60 w/ 4.30s and a spool www.michiganspeed.com
Be sure to let us know how that holds up, Gutsy move, I am all for saving money, especially not fond of spending cash on something that I know I am only gonna use once.
__________________ 98 T/A stock short block, PRC 5.3 heads, custom cam, ported fast 92, QP ford 9" tuned by Texas Speed, 463 rwhp/428 tq on motor
Never save anything for a special day, THIS day the Lord has given IS special.
Be sure to let us know how that holds up, Gutsy move, I am all for saving money, especially not fond of spending cash on something that I know I am only gonna use once.
What do you mean? Do you think it wouldn't hold up? The flycuts are nice and smooth to the touch and if I really wanted to, I could polish them so they were perfect, but I don't think it's needed.
Thanks for the compliment on the paint. I was going for an old school Pontiac vibe with it.
__________________ '98 NBM Formula M6/ Hooker Super Comps w/TSP True Duals/ lid/custom 85/85 FAST/ CNC Ported 241's/ Streetsweeper 228/232 on a 111 LSA/ Six Speeds Inc. Stage 4 T-56/ Hurst Billet Plus with chopped stick/ Monster Level 3 clutch/ S60 w/ 4.30s and a spool www.michiganspeed.com
Yes, I used 2.00 valves to flycut with, but the sanding discs I made were about 1/16" larger than the valve, so they actually cut bigger reliefs in the pistons as they were being used. The paper wrapped around the edge and only toward the very bottom of the cut did it finally deteriorate to the point where the valve was only cutting the true diameter of the valve. Why would that be an issue? Just curious, as I have never did this before. We checked before and after and P to V went from .055 to .115, so I know the valve is not hanging up anywhere in the valve relief.
As for piston thickness, LS1 pistons have more than enough material to take .050-.060 off the top with no ill effects. Many people have done this previously using different methods with good results, so I figured I could do it this way and be perfectly safe. The reason I went with these is because I got a full set of brand new rods and pistons from Katech for free, basically. This is a budget build and a new set of pistons and rods was not in my budget, unfortunately. Otherwise I would have gotten pistons with valve reliefs in them.
__________________ '98 NBM Formula M6/ Hooker Super Comps w/TSP True Duals/ lid/custom 85/85 FAST/ CNC Ported 241's/ Streetsweeper 228/232 on a 111 LSA/ Six Speeds Inc. Stage 4 T-56/ Hurst Billet Plus with chopped stick/ Monster Level 3 clutch/ S60 w/ 4.30s and a spool www.michiganspeed.com
What do you mean? Do you think it wouldn't hold up? The flycuts are nice and smooth to the touch and if I really wanted to, I could polish them so they were perfect, but I don't think it's needed.
Thanks for the compliment on the paint. I was going for an old school Pontiac vibe with it.
What I meant......... was Gutsy move, Its a compliment bro, to take the chance and idea of what you had in your mind and use it on your short block, And yes it does look good Let us know how it works out for you when its all back together, You put up a great post with pix that can help guys do what Most of us like to do, modify and save money.
__________________ 98 T/A stock short block, PRC 5.3 heads, custom cam, ported fast 92, QP ford 9" tuned by Texas Speed, 463 rwhp/428 tq on motor
Never save anything for a special day, THIS day the Lord has given IS special.
It's NSW full size, and I don't send out PMs. There are a lot of pics of my girl over at SSU. Let's keep this on topic As for how much sandpaper I went through, not even one sheet. We have adhesive backed sanding sheets for floor sanders here at my shop and I just used one of those to punch the discs out of. Used about half the sheet. Each piston uses just one disc per cut. I suppose if you REALLY wanted to, you could go back in and hit it with a finer grit to make it even more smooth, but it's not really necessary. Hopefully the engine is back together and in the car within the next couple of weeks so I'll be able to post up how it comes together.
__________________ '98 NBM Formula M6/ Hooker Super Comps w/TSP True Duals/ lid/custom 85/85 FAST/ CNC Ported 241's/ Streetsweeper 228/232 on a 111 LSA/ Six Speeds Inc. Stage 4 T-56/ Hurst Billet Plus with chopped stick/ Monster Level 3 clutch/ S60 w/ 4.30s and a spool www.michiganspeed.com
Yes, I used 2.00 valves to flycut with, but the sanding discs I made were about 1/16" larger than the valve, so they actually cut bigger reliefs in the pistons as they were being used. The paper wrapped around the edge and only toward the very bottom of the cut did it finally deteriorate to the point where the valve was only cutting the true diameter of the valve. Why would that be an issue? Just curious, as I have never did this before. We checked before and after and P to V went from .055 to .115, so I know the valve is not hanging up anywhere in the valve relief.
You should measure the eyebrow, the outside arc has to be at least .04 inch bigger. When hot everything expends and the piston actualy rocks. If less than that, your valve could catch the edge and bend or worse.
Just make sure.
Also you might want to deburr any sharp edges to avoid hotspots.
Easy way to test is to clay the relief flat and then free fall the valve. Measure the distance between valve contact at clay and edge of flycut.
If you have .04 or bigger you are good. I would repeat that on all pistons.
Excellent advice, I will make sure to do this when I get back down to work on my engine next week.
__________________ '98 NBM Formula M6/ Hooker Super Comps w/TSP True Duals/ lid/custom 85/85 FAST/ CNC Ported 241's/ Streetsweeper 228/232 on a 111 LSA/ Six Speeds Inc. Stage 4 T-56/ Hurst Billet Plus with chopped stick/ Monster Level 3 clutch/ S60 w/ 4.30s and a spool www.michiganspeed.com
very nice! this def. good starting point, i cut my releives and then come back with a shapr razor and deburr the shap edges, very carefuly, ive even used a stock machined valve i made but it worn down pretty quick.
im going to look into this mod for another build, but i think im gonna buy a single 2.02-2.05 valve and use that instead of the stocker.
How to flycut pistons in an LS1 for free! (almost) 
Never save anything for a special day, THIS day the Lord has given IS special. 
As for how much sandpaper I went through, not even one sheet. We have adhesive backed sanding sheets for floor sanders here at my shop and I just used one of those to punch the discs out of. Used about half the sheet. Each piston uses just one disc per cut. I suppose if you REALLY wanted to, you could go back in and hit it with a finer grit to make it even more smooth, but it's not really necessary. Hopefully the engine is back together and in the car within the next couple of weeks so I'll be able to post up how it comes together.
