30 degree back cut valve

[Noddle]

Hi,

 

I was reading a thread here, (it in the shed now),

 

but BRAAP mentioned "stock valves that have been given a 30 degree back cut."

 

since the search engine here freaks out with search terms less that 3 characters,

 

I was wondering if anyone had some picture they could share showing this, and explain how it's done.

 

Is this a easy DIY thing to do on standard Nissan valves ?

 

Nigel

[BRAAP]

Nigel,

This thread covers it a little as well as some of the other facets in custom cylinder head building;

http://forums.hybridz.org/index.php/topic/42991-custom-cylinder-head-building-what’s-involved…/

 

 

Most Valves have the faces ground on a 45 degree angle. The backcut merely helps to blend the transition from the back of the valve to the seat face. Like porting the back of the valve. The purpose is to help improve air flow across the back of the valve head itself allowing the air flow to make the transition to seat easier with less loss, especially at high velocities as the valve leaves the seat or is coming back in contact with the seat, if flow is still high in that region.

 

These two pics should help illustrate that more clearly;

Courtesy of WS6transam.org

 

 

 

Any automotive machine shop that grinds valve can do this operation. Some of the later Nissan intake valves come already with the back cut.

When a machine shop is doing this, they need to know how far to go so they don't cut into where the actual valve sat is at. They need to leave some margin. If the the shop has the head as well and the seats are faced, (ground/cut), they can simply lap them to see where the face is at and back cut to just shy of that.

 

Hope that helps,

Paul

[Noddle]

 

Any automotive machine shop that grinds valve can do this operation. Some of the later Nissan intake valves come already with the back cut.

When a machine shop is doing this, they need to know how far to go so they don't cut into where the actual valve sat is at. They need to leave some margin. If the the shop has the head as well and the seats are faced, (ground/cut), they can simply lap them to see where the face is at and back cut to just shy of that.

 

Hope that helps,

Paul

 

Thank you for this,

 

I have access to a old valve grinder at work, that I have used before, I'll look at doing this when I do my MN47 head,

 

Nigel

[KillerBjt]

Braap, would that be considered part of a 5 angle valve job, or would that be done in addition?

[ozconnection]

 

since the search engine here freaks out with search terms less that 3 characters,

 

 

I hate that deficiency...cripples an otherwise good site...needs to be fixed ASAP

 

The valve stuff outlined here is very interesting though

[Tony D]

Braap, would that be considered part of a 5 angle valve job, or would that be done in addition?

5 angle refers to the seats and making an ersatz 'radius' to flow.

 

Working the valve is a different procedure, this is not part of a '5 angle' valve job.

 

15, 30, 46, 60, 75 were the cuts I 'think' are commonly used on a 5 angle. The 1 degree difference between the 45 face cut of the valve face and seat is what gives your contact point to seal the valve. You then make two cuts below and above that to 'smooth radius' the seat. There are mechanical radius cutters that will do similar things now without multiple angles.

[Noddle]

15, 30, 46, 60, 75 were the cuts I 'think' are commonly used on a 5 angle. The 1 degree difference between the 45 face cut of the valve face and seat is what gives your contact point to seal the valve

 

I would have thought, on the exhaust valve anyway, that you would want to have 45 degree cut on the seat and the valve, to have a larger contact area to help remove the heat from the valve ?

 

see picture

 

Nigel

Edited September 2, 2010 by Noddle

[BRAAP]

1 degree separation between the seat and valve face is quite common, helps with the initial sealing due to differing angles upon contact, but it is shallow enough that the material of valve will conform of the seat as it runs, sort of like lapping, but different.

http://www.tpub.com/content/construction/14264/css/14264_95.htm

 

 

A couple other key points to valve seats that we don’t see discussed much. Hopefully Tony will chime in to fill in the holes I left, etc.

 

The valve to seat interface is where the valve dissipates most of the heat it acquires from the combustion process, transferring that heat back into the head via the seat, casting into the cooling system. The stem to guide interface is the other area the transfers heat from the valve back into the head, but not nearly as much as the seat interface. This is especially important for the exhaust valve as it has extremely hot gasses blowing across it that are hot enough to melt the metal which it is made from, (by the way, ALL factory L-series exhaust valves are stainless steel!), so if that valve does not get the chance to get back in full contact with the seat, it does not get the chance to cool down, which leads to a burnt valve. A burnt valve typically looks like a chunk has been broken off of the valve, (many mistakenly think and tell other that), but if you look closely, using some magnification, you'll see what looks like itty bitty torch streaks across the affected area, just like a cutting torch cut it. That is essentially what happens, the exhaust gasses "cut" that material from the valve. Valve lash being too tight not allowing the valve to seat, a piece of carbon/crud between the valve and seat, extremely late ignition timing, (combustion still taking place across the valve into the exhaust port, torching it), etc are common causes of a burnt valve.

 

Valve seat width is also critical. A valve seat that is excessively wide has a lot more surface area when seated. For a given valve spring, the pressure exerted on the seat interface will be less per square inch with a wider seat than with a narrower seat. Where this comes into play is the wider seat will allow carbon to more easily build up or get stuck between the valve face and seat, which doesn't allow the valve to seat directly in contact with meat valve seat in order to transfer it heat into the head. Also, and excessively wide seat means a smaller I.D. of the throat which in high-performance applications is not desirable. A seat that is too narrow in width and there is not enough surface area to transfer the heat for the valve back into the head. This hotter valve also expands more so if the valve to guide clearance is not adequate, the valve could "stick" in the guide.

Accepted valve seat widths as used on most 2 valve engines is .040"-.080" such as the Datsun L-series, SBC, SBF, SBM, etc, For the most part 040" is common for the intake, .060" is common for the exhaust. Depending on spring rates, materials used for the seat and valve, application it is used in, those seat widths could be different. For most stock to even moderately hot street builds with stock seats and stock valves, between .040"-.080" for either the intake or exhaust is considered safe.

 

 

Tony covered the 5 angle seat. Ideally we want a radius to form the transition from port wall to the seat continuing onto the chamber walls. These added angles help to get us close to that ideal of fully radiused, close enough that going to fully radiused vs 5 angle is only worth a gain for engine built to levels we would never run on the street, such as Bonneville car or drag car to push some record run, etc.