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JFairman JFairman is offline
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Join Date: Jul 2006
Location: S. Florida
Posts: 7,289
Quote:
Originally Posted by 911nut View Post
Wouldn't it have been easier to have a machinist mill that port down to the bend?
Are you using a bore gauge to keep the ports round as you go?
Are you going to get the heads flowed when your done? Would be nice to know that the mass flow into each cylinder is somewhat equal.
Yes, it would have been easier to take them 25 miles up the road to xtreme cylinder heads and have Bill do them but thats expensive and I have more time than money and I'm really good at doing this by hand. I have very good eye-hand mechanical skills and I've done it many times on other cars and motorcycles with great success.

I don't have a bore gauge, I use a digital micrometer to measure and then I mount my 40mm aluminum injector blocks on the heads and use a sharpie pen to draw a back circle and a machinest scribe to mark the port matching 40mm circle I'll hog them out to.
The rest I do very carefully by eye and feeling the inside of the ports with my fingers. Kind of like doing body work on cars and block sanding the fiberglass and body filler to get it shaped perfect and I've done tons of that too.
If you know what you're doing, you have a good eye, and know where not to take too much material off > like the short side radius then you can do a good job by hand.
I will be doing the intake manifold ports the same way. I wish I had another CIS intake manifold so I could do that now and have it ready to install along with all the other parts but I only have one intake manifold and I'm still driving the car right now.

I'm not going to take any material out of the bowls.. they're big enough already but I will sand away and blend/smooth the crude casting marks along the sides and radius and smooth the origonal semi crude rough edged machining done by Porsche all the way around about 8-13mm before the valve seats.

These are 1976 930 heads and I was hesitant to do this because the value of '76 930's seems to be going up so the origonal parts from them would be too.. but I have these already and the the exhaust valve guides in my '87 930 are worn even though they only have around 40,000 miles on them since the previous owner had the top end rebuilt so it makes $ sense to me to do this. These heads have new valve guides installed and fitted correctly and a fresh valve job done to them already with new exhaust valves.

The '76 930 is 95mm bore and my '87 is 97mm bore. To use these '76 heads I had to have a 1mm wide bevel cut into the edge of the 95mm combustion chambers or else the inner edge of the sealing surface would become a sharp edged 1mm squish band. Sharp edges in a combustion chamber run hotter and that could cause detonation and the pistons might hit it.
I took these heads up to Xtreme cylinder heads and Bill cut the 1mm wide bevel into them a few months ago.

On another note, I have an old '75 BMW R90S motorcycle I bored out to 980 CC and installed new R100/7 pistons in way back in 1988. Those hemi combustion chambers are very similar to 911 heads.
Instead of paying someone else I made the same type of bevel in the edge of the squish band by hand with a round sided metal file, then hand sanded it smooth, polished the combustion chambers and it worked out perfect.
I didn't want to do that on six 930 turbo heads so I had Bill at Xtreme do that on a machine.

When I finish these ports I'll take pics of them. I won't be done and satisfied with them until they look as nice as any other hand finished ports I've seen pictures of.

Last edited by JFairman; 05-20-2013 at 07:02 AM..
Old 05-20-2013, 06:58 AM
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