Once the collectors are built we flare the 1 5/8" OD tubing to meet up with the 1 7/8" ID oval ports on the cylinder head.From here each tube is slipped into the merge collector and TIG welded. Every flange on these exhaust manifolds is TIG welded from the inside and out! All of our stainless is backpurged when welding to prevent crystalization of the stainless steel which could lead to fatigue and possible failure. After the welding process is finished we hand finish the exhuast manifolds on the inside to maximize flow and performance. Every set of headers we build has a special Speed Force Racing logo welded to the collector so you can be sure that you are getting a high qaulity product and not some cheap knock-off. We can also offer these headers in a variety of runner diameters for people who are looking to make extreme amounts off horsepower.You can call or email us for any questions you might have.