BUY YOUR PARTS  from the OEM or the STC Holder. By having even one part made by someone that doesn't keep track of, or improve on the product, you risk larger problems down the road.

In the case of an operator that had a exhaust made from the sample of what came off the airplane, the exhaust shop did not use the special fixtures or processes to manufacture it.  As a result he could risk several things including a possible engine fire from the hot exhaust being too close to the push-rod tubes (and melting through the soft aluminum tubes) or too close to the cylinder cooling fins causing hot spots and possibly blowing a cylinder head off. Not to mention if he ever needed a mating part from the factory, it probably won't fit.