Next, the valve seats are...
Next, the valve seats are cut with a pilot that fits tightly into the valve guide and cutters. Howard used a 45-degree seat with a 75-degree throat cut and a 30-degree top cut.
Now you're done cutting. Next, get your sanding mandrel, chuck up some sanding rolls, and polish the ports and combustion chamber, taking care not to scratch your valve seating surface. Blend all of your cutting marks, smooth the surface, and polish out all of the sand-casting pits in the ports. It doesn't help much to make things mirror smooth so don't bother going finer than 220 grit. For our head we didn't even bother to do much more than smooth our own cutting areas, but you might want to polish your whole port to feel better about your work. In the combustion chamber you'll do the same thing where you unshrouded your valves. It's also a good idea to break the sharp edges of the combustion chamber. This will make detonation less likely. Don't go crazy; lightly break the sharp edges with your sanding roll.
Congratulations, you've successfully ported your own head, saving thousands of dollars. If you want to get more power, by all means spend the money and have your head professionally ported but, like we said earlier, you just got halfway there without spending money.
20. The 45-degree seat is...
20. The 45-degree seat is cut first. The minimum amount of material is cut to straighten out the seat and remove any traces of pitting.
The next critical part of headwork is the valve job. The valve job ensures that the valves' seal well, which is important to maintain compression. Fortunately for us, this being a low-buck build, the stock Nissan valve job is a performance-type three-angle valve job from the factory. (This is true for most Japanese engines.) A valve job requires special expensive equipment and skill to do, so this isn't something you'd want to do at home. Howard Watanabe of Technosquare performed our valve job.
First, Howard cut a new 45-degree valve seating face on the valve seat. Then, he used a 75-degree throat cut and a 30-degree chamber cut to narrow the seating area down to .04 inch on the intake and around 0.05 inch on the exhaust. He then refaced the 45-degree seat on the valves themselves and did a 30-degree back cut on the valves to the 45-degree seating surface. After the valve job, it's important to measure the valve stem tip height and correct the spring installed height with shims if necessary. (It's usually unnecessary in an SR motor). It's also necessary to check the side-to-side valve stem height on the intake and exhaust valves and make them equal. Nissan sells select fit shims that sit on top of the valve tips in the spring retainer to do this or the valve stem tips can be ground in a valve-grinding machine to do the same thing. This procedure is somewhat tedious and perhaps impossible for a lame machinist used to domestic engines. It's helpful to refer to the Nissan factory service manual or stick with a reputable racing head porter that has Nissan engine experience.
The freshly cut seat is clean...
The freshly cut seat is clean but wide. The 75-degree throat cut is next which unshrouds the valve seat to the port, helping ease flow. Next, the seat is narrowed further by a 30-degree top cut to help flow into the combustion chamber.
SR20 engines, like most finger follower valvetrains, are also somewhat hard on its valve guides. Our head was in surprisingly good shape but others often need new guides. Using new stock Nissan guides is a pain. The Nissan replacement parts are oversized on the OD and undersized on the ID. This means that the guide holes in the head have to be reamed oversize and the guide's inner hole then has to be reamed to the valve stem diameter-a lot of work. If we needed to tighten up our valve stem to guide clearance we would've used silicone bronze inserts, a common machine shop fix for loose guides.
With cheap low-mileage bluebird SR20s so common in import junkyards like Sokken, it may be easier and cheaper to simply find an engine with less miles with a better condition head to start the build on. Most Domestic SR20s found in Sentra SE-Rs typically have really high miles and usually need to have the valve job and guides renewed. Our engine started out this way and even though it had many hard racing miles, the use of premium synthetic oil with frequent changes perhaps spared it from excessive wear. We got lucky and our head's excellent condition enabled us to stick to a really low budget.
Here's the completed valve...
Here's the completed valve job. The three angles can be seen here.
Next, a cartridge roll is...
Next, a cartridge roll is used to blend the 30-degree cut smoothly into the combustion chamber and the 75-degree cut into the port wall. The roll is also used to lightly polish and deburr the combustion chamber to eliminate sharp edges that can cause detonation. For a big-buck porting job, the head porter might spend a lot of time polishing the ports and combustion chamber but since we're poor, we only hit on the important rough areas.
Scotch-Brite red is used for...
Scotch-Brite red is used for additional polishing and cleanup.