On this page we will show and enlighten you some about what is done when we rebuild/replace your valves and guides. Many shops will tout the much vaunted Serdi machine. What needs to be understood is that a Serdi is a generic device meant to increase productivity, not what we desire when rebuilding heads to porsches-or better specs. The Serdi is convenient and fast but has limits. It should be noted that concentricity should be held to .002" per Porsches specs, which is, of course, correct.I have seen many seats around .005" to as much as .010 non-concentric which will not last long as they would were they held to a tighter tolerance. . Though they work reasonably well they are not as accurate as locating each guide by hand-takes a bit more time but we get a more accurate seat. As with any tool, the results still depend on the skill of the operator We will cut your seats initially on a Bridgeport milling machine-and index every guide relative to the seat.This alone makes for a better seat but once done we then test for concentricity on each and every seat and hand cut the seat to keep it within factory specs. It is attention to such detail that Exotech prides itself on.
After the head gets torn down and a first cleaning the guides are core-drilled, driven out and replaced. They are then reamed to size. The reason we core drill is to prevent damage to the guide bore so we generally don't need to use oversize guides.
Once the new guides are in place the seat is cut. This type of cutter does five angles in a single operation. It is critical that the alignment between the mill and seat is as close to perfect as can be so that only the minimum material is removed. Over the life of the engine parts tend to warp making it very easy to "overcut" the new seat. Keeping the cut to a minimum and maintaining correct seat width will help airflow which as we all know is what makes power.
Now the seats are checked for runout and touched up by hand if needed, an important step sometimes left out by high volume shops.
After measuring the valve stem for wear the valve face is ground and if required a backcut may be made. This particular valve is a custom Ti piece-a bit tricky to grind since if one is not careful it can actually cause the coolant to catch fire-annoying.
Once all the machine work is done the head gets a final cleaning, the valve is test fitted and the spring height is set. It is then assembled with new seals. By the way it should be noted that Teflon seals are NOT the hot setup, ultimately they will in fact wear your valve stems prematurely. Another thing often found is incorrect valve springs. Sometimes referred to as "safety" springs they usually have far too much seat pressure, resulting in premature valve and cam drive wear. We have seen springs used giving150# of seat pressure, twice whats needed for all but the most exotic lobe profiles which does nothing but what is referred to as "tuliping" the vales and pounding the seats, both recipes for a short lifespan. The correct spring for high performance use is one of a fairly high rate-around 400# per inch that is also the correct free length.