CJS Customer Projects

996 Head Repair

Repair and Re-Work of a 37/31  Valve Conversion

A customer from abroad approached me needing some detonation damage repaired on his ported elsewhere 996 big valve heads. At the same time as the repair I was asked to give them a look over/flow test and see if any improvements could be made.

Looking at the heads all that had been done was a fitting of the larger valves and a mirror polishing of the ports and combustion chamber. After my first initial flow and velocity test it was revealed improvements could be made.

The first modification was correcting the valve seat angles that were cut into the combustion chamber way too deeply.   Too deep and the shrouding kills the low and mid lift flow respectively. I applied a shallower top cut then set about re- blending the combustion chamber around the seats to remove the intrusive material. A flow test revealed a significant gain had followed.  The race valves previously fitted had taken quite a hammering. .There was extensive pitting on the valves that could not be restored so some of my own designed 37/31 valves (as pictured below) were in order. The valves use a different clearance, are wasted, very light and have a different shape aiding airflow.

I changed the port size and shape to optimise it to work with  748r cams.  For longevity I set about installing some of my own designed colsibro race valve guides. The colsibro race guides are a little shorter than stock, very wear resistant and are actually a little undersized. This means the guides can be fitted reamed and micro honed to the correct size of the valve making them a nice snug fit unlike the rather loose Ducati OE items. A loose or worn out guide amongst passing oil into the combustion chamber can destroy a new valve job in a short period of time due to a symptom called valve walking.

The next batch of modifications were a re-contouring of the inlet port short side radius pictured below. The port radius shape and width is critical for high valve lift flow and making sustained high rpm power.  This was re-shaped and re-checked on the flow bench several times.

Inlet port short turn pictured left.

The temporarily roughed in, inlet and exhaust seat cuts were finally precision cut using the PEG FM1 to some different seat profiles below. As the seats had already been cut very deeply before, it was hit and miss on whether they could be saved. Deep seats end up with shimming problems. Luckily these were saved by using very large 3.9mm closing shims supplied by Mike at EMS.


Finally after welding up the detonation damage, skimming the head and repeating the operation three times we finally had a satisfying repair and much improved cylinder head.

The final flow graph. Blue is a stock 996 , Black is this big valve head before  modifications and red is the same head but repaired and improved by me.


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