Valve face recutting VS grinding/lapping

[gearhead13]

I was planning to just lap the valves on my engine, as the seats arent that bad. Then the guy at the machine shop was touting how much more power with a new recut. He was probably just trying to bring in more business. But, are recut seats that much better than a grind/lap job?

 

[hampshirehog]

If your seats are ok just lap them. They only need to make a seal and if they do that you can't make them any better.

 

[asetech]

If you lap them make sure you clean the lapping compound out really good or it will pit the valve face.

 

[rapidray]

If you want more POWER then you could have them BACK cut. I do this to stock valves all the time. Chances are, for what YOU are doing, lapping will be fine. Ray.

 

[gearhead13]

Ya, I am doing a back cut for sure.

 

[Guest]

A high performance seat flows more than a stock type. It will make more power. Apparently this is what the guy is talking about.

 

[headsbikesmopars]

I am with you Ray. On my flow bench, my GS heads LOVE the Back cut. Back cutting the valves @ 37 degrees will pick up the flow just about 20% under .300 lift. Hemi chambers like 35 to 37 degrees . The GS's respond better to 37 degrees. Wedge chamber heads (V8), (chevy, Mopar, ect) like 30 degrees. Anything other than that will HURT flow. This simple mod uncovers more seat area at lower lifts and starts the 'column' of air in the intake port sooner in the lift cycle. This is TORQUE big time. Simple and cheap too. $ 20. bucks at most shops. Hope this helps.

Terry

 

[Nessism]

Simple and cheap too. $ 20. bucks at most shops. Hope this helps.

Terry

Hey Terry, good info. Thanks. Can't say I agree that $20 is "cheap" if we are talking about each valve (which I assume we are).

 

[gearhead13]

HMMMM, I was just planning on putting my valves in a drill press and running a file on them at the appropriate angle :-k

 

[82Shafty]

HMMMM, I was just planning on putting my valves in a drill press and running a file on them at the appropriate angle :-k

File... Angle grinder son, it's what all the latest tuners use LOL.

Oh ya, don't forget the protractor to help set the angle . ^o


On a more serious note, for anyone who cuts the face and seat, be sure to double check installed height.

I don't have a FSM, does Suzi recommend a 1 degree interference angle?

Make sure you check all guides/stems before you do anything to the valves/seats.

 

[headsbikesmopars]

Nessism, no that is per set. Not expensive at all and will pick up flow more than anything you could do for the money. If you were to do anything for your heads it would be back cutting of the valves first, and then put a nice gentle radius on the short side radius, of the port floor where the factory cutter leaves that sharp edge. Air does NOT like corners anywhere in the port. Smooth and radius this area and clean up any casting flaws with a cartridge roll and you will have a nice piece. Flow will be in the 90's and that is enough to make in the 160 hp range. You don't need cutters for this, use a strip of abrasive emery cloth 1" wide torn into strips of 1/4" and 24" long. I use 80 grit to remove this 'edge' and follow that with 180 grit. Place head on work bench with chambers up with valves removed, fish the strips of emery cloth thru the port and with a back and forth motion pulling up, you can quickly remove and round (radius) this edge. All you need to do is take this edge off and you are done. It won't take long and you don't need to remove anymore than this. You will be amazed at the results. Anyone can pm me if they want anymore detail on this cheap and simple mod. You can do it. Good luck.

Terry

 

[Flaggo60]

If you want more POWER then you could have them BACK cut. I do this to stock valves all the time. Chances are, for what YOU are doing, lapping will be fine. Ray.

What is "back cutting"?

 

[not a biker]

To wide or to narrow a valve seat will shorten seat life and air flow.also lapping may cause a step in the valve seat on the valve. Laping is fine for inspecting seat contact.

 

[Nessism]

What is "back cutting"?

Google "valve back cutting"

 

[drhach]

Isn't that also referred to as a 3-angle valve job? In other words, two separate angles are ground on the valve face and then a third, different angle is ground in to the valve seat.

 

[Nessism]

Isn't that also referred to as a 3-angle valve job? In other words, two separate angles are ground on the valve face and then a third, different angle is ground in to the valve seat.

A three angle valve job refers to angles on the valve seat. Suzuki uses two angles on the valve seats 15 degrees and 45. Valves are typically cut at only one angle - 45 degrees. Back cutting the valves results in two angles on the valve; the standard 45 degree cut (part that contacts the valve seat) and a second cut to aid gas flow at low valve lift.

Examples of a three angle valve job on a Honda auto engine...

 

[headsbikesmopars]

Flago60/notabiker, Back cutting does nothing to the valve seat itself. It is a cut that is above the seat (45) on the valve itself. It removes the 'corner' at the very top of the 45 cut on the valve face. Has noting to do whatsoever with the valve angle on the head or valve. The 45 is not effected at all. If you lap your vales in you will see that the face of the valve is longer than the 45 seat itself, (on the valve). There is margin left. Back cutting up to the edge of the 45 with a smaller angle takes off that part of the valve, knocks the 'corner' off so to speak. Opening up more clearance (valve area) and exposes more of the port for air flow. The valve job isn't effected at all. This is part of the valve that doesn't touch anything. This dosn't affect the longevity or reliability at all. Nothing but a positve gain here. A win-win deal. Check out the fine photo's Nessism posted. Just trying to help out a fellow GSer. Easy and cheap, My flow bench likes it too.