danglerb

Looked at wikipedia, searched the web, found some info on ebay, but plenty more questions. Here is what I have found so far, please correct and add to as you can.

When the desired valve travel is great enough to contact the top of the piston, a piston with valve relief's can be used, or in some cases the existing piston can have material removed to create valve reliefs.

Fly cutting comes from the type of tool used on a milling machine, a Fly cutter. Its a shaft that fits into the rotating chuck of the milling machine, with a fixture on the end to hold a cutting tool perpendicular to the shaft with the radius of the cutting tip from the center of the shaft adjustable. Its a dandy tool for cutting precision sections of circles of various radii.

How does the fly cutting process work in practice?

Do you give the machine shop a blue print or something like it, or just give them the heads, valves, and pistons and tell them what the lift is?

jon928se

Unless you want to pay them lots of $$$$ to figure it all out your are definitely into working out the angle the valves make with the centre of the piston relative to the centreline of the pistons movement (This may be zero degrees for a 2v motor), the diameter of the circular cut, the distance from the crown of the piston to the intersection of the centreline of the valve with the centreline of the piston and the depth of cut relative to the to the intersection point above. For a 2v engine where the move parrallel to the piston this is simplified to a distance off the centre of the piston to the centre of the circular cut and the depth of cut measured from the crown of the piston.

danglerb

Thanks Jon, exactly the sort of answer I was hoping for.

Would fly cutting pistons be something a machine shop would have some kind of "jig" setup to do, or treat it as general machining and just clamp the piston somehow, measure and cut?

John Veninger

Why not talk to Gregg Brown, your future engine builder?

danglerb

I ask Greg plenty of questions, this is more of a curious aside than a question though. Besides with all the conversation about fly cutting I suspect I am not the only one that is curious about the process.

123quattro

I would guess the piston would be chucked with a soft metal between it and the jaws. It wouldn't be worth making a jig unless someone was doing a big run of pistons. After you get the piston set up on the correct angle on the mill, all you would need to do is raise the bed however much you wanted to cut into piston crown.

largecar379

I assume (very bad thing to do, but I do it well) you're building a hybrid (ex: 5.0 short block with 16V heads/top end, probably Euro parts)?

If so:

The easiest thing to do is take one of your 16V pistons and the 32V pistons to your machine shop and have them duplicate the valve notched i the 32V pistons.

Any competent machine shop will be able to handle this. You must do this because the 16v pockets are different that the 32V pockets. Jim @ 928 Int'l cautions me that you should not use later than '88 32v pistons, as they will cause a compression ratio of less than 8:1 or so, making the engine barely run-able.

Short of that, give Wiseco a call and order up a set of new ones to your specs.

regards---largecar379

mark kibort

i have a stensil of the cuts, angle can be figured out by the size of the cuts and the deepest point. i can get an angle if anyone wants it. I modeled them after a stock euro S piston from '83 . However, in 80-83, the valve cuts were real deep. (ie .350 " or so) on the 85 US 4 valve pistons, they are not thick enough to go this deep, nor do you need to. the 84 euro pistons have only 2ccs of valve cuts. remember the only difference was the duration of the different euro S cams, not the lift.

anyway, we went slightly deeper with the 85 4 valve piston 2 valve cuts just in case.

i have the drawings pictures and measurements. my machine shop did a jig based on a euro 2 valve piston scaled to the larger 100mm 85US piston. If i do it again, ill mark them up using my stensil and cut them myself,or use a machine shop to make cuts. ill make a safety area about 1-2mm wider all the way around just to be sure.

let me know if you want a copy of the actual stensils, providing when we clay the pistons, that nothing is hitting at .5" lift

Mk

Namvet

The piston being flycut needs to be checked in thickness at the crown and also the top deck, before any flycutting begins. Cast Pistons are never a Go, without careful inspection.

Blank forgings are much better as ring groove location and style are machined as needed. FORGED R BEST!

Flycutting the proper clearance without regard to the thermal load on the Pistons, thickness of deck surface, etc, is a disaster that will happen. If pin end of rod bore is not Bronzed and floated to tight clearance, the piston may be blamed when it was just along for the ride.

Mods are Time and Money.

Be careful!

mark kibort

we are only going down .175" as a valve cut. this is fine for all porsche pistons. now, try and do a euro 80 valve relief in a 85 US 4 valve piston and you could have problems! the 85 pistons are not forged and work well with these mild mods to the piston faces.

mk

mark kibort

Notice how we now have an "S4" like orientation with regards to piston offset??? now the pistons have to face the front of the engine via the arrow, rather than toward the exhaust side, which means the offset is proper to one side and compelety wrong on the other cylinder bank.
One expert in the field says this better rod -to- piston angle is worth 10hp on race motors. so, maybe its worth 5hp on ours???? i dont know, we will see in a month or so when i get this thing running!

MK

Namvet

Thanks for the great pics! Yer lookin good!
All best wishes from here! GO for it!

NV