Valves: OS Vs. Stock

Left side 1mm Oversize Stainless in a Spain cast Marnel head

Right side  n/a Stock Replacements in a Mitsu. "T" casting head

 
The OS valves have less contact in the seat area compared to the wide area available in the stock replacements, these replacements were using the outer most contact area, they should have been more in the center and probably were before these seats were reconditioned

OS on the left Vs stock on the right shows the contact area after lapping.

 

 

 

 

 

The OS valves fit more flush into the chamber

 

 

 

 

 

These two pictures have the valves swapped, the OS valves are in the stock seats on the right, stock valves in the OS seats on the left

 

 

 

Seat diameter into the pocket is the same on both the intake ...

 

 

 

 

... and on the exhaust

 

 

 

 

 

 

 

Left, the OS valves seat contact area center diameter is greater than the overall outer diameter of a stock replacement, GREATER than the 1mm they are named to be.

 

The OS exhaust valve contact seat center is larger than the stocker seat contact center but not as much of an increase as the intake valves are oversized (outer edge of stocker to one side center contact area)

 

intakes left, exhaust right, the OS valves are on the left in each picture

 

OS seat contact on the left and stock seat contact on the right, the right side has a second angle below the contact seat -this was ground away on both the intake and the exhaust seats when the OS valves were fitted, the Marnel head would benefit from removing that lip above the seat that isn't really there in the stock head to make the top seat angle have greater area, just some mild porting will fix that or that could have been done when the seats were ground.

Exhaust seat contact. OS left, stocker right

 

 

 

 

OS seats left, stock seats right

There are still 3 angles after the seats were cut for the OS valves.  The top angle although very thin on the intake side is there and I feel that could be blended into the head better, the seat angle and the throat angle.  The throat angle on the stock head has two angles, the top angle on the OS ground seats is the same as the stock seats top angle.

 

 

 

If the stock replacements are swapped into the head with the seats ground for the OS valves

 

 

 

 

The amount the seat is recessed into the head in regards to the chamber roof is greater in the Marnel casting. 

 

 

 

The greater recessed seat in this Marnel head could be removed with some porting.

 

 

 

 

 

Even without porting, the 1mm OS valves make for a smoother chamber roof compared to these stock replacements.  Smoother edges help control detonation, i.e. sharp edges get HOTTER

 

 

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