73.5 CIS Rebuild 2.4 stock or go for higher displacement?
 

I rebuilt my 73.5 CIS 2.4 Targa engine a few years ago I am finally over the blue smoke enough to pull it out and do it again. It’s a shame because otherwise I’m pretty proud of my first time rebuilding a 911. No oil leaks and it runs strong despite the moderate level of blue smoke.

The initial smoke after my build I thought might clear up once the rings settled but has never stopped and I am starting to think I might have broken a piston ring during assembly. A normal Compression test reveals good cylinder to cylinder variance but I will do a leak-down tomorrow to try and isolate the problem.

A big problem is that I cannot seem to locate the correct piston rings online - everywhere I search turns up no longer in production for my CIS. As a result I am considering whether it is worth it or not to upgrade the displacement but am not sure which setup would be best. I have the 7R mag case and 915 trans so I want to do what would be best for moderate spirited driving, nothing on a track. Any advice with upgrading to a 2.7 piston setup or just locating 2.4 cis piston rings would be appreciated.

You need to buy a new set of pistons and cylinders , with new rings only it will still smoke . Just buy the stock size , the cylinder heads and ports are not for a 2.7 .

Case work if you go to 2.7 for clearance.
I was able to get a set of oem rings a few years back for the CIS, but now it seems tough.
I did notice that Pelican has a set of Deves rings that are correct dimensions, 84mm, 1.50 x 1.75 x 4.00mm - Deves Piston Rings, https://www.pelicanparts.com/catalog/SuperCat/5878/POR_5878_ENGPIS_pg1_piston-rings.htm
but I don't know if they work with original pistons.

Ok, finally completed leak-down tests all around … 100psi

#3. (26%). #6 (20%)
#2. (25%). #5 (25%)
#1. (23% ). #4 ( 20%)

So, yeah based on these consistent numbers it looks like my 2.4T is just plain tired. All leakage sounded like it was inside the crankcase. I suspect the oil smoke mostly is the valve guides but these pistons and cylinders are likely just at or near EOL. It looks like I will be making a decision to either size up my piston’cylinder or trade for a rebuilt engine altogether.

Any leads on a nice 3.0 I guess are welcome.

Check out Mark Epsteins Euro Carrera 3.0. I built one up with 9.5:1 JE, mod S cams, and PMOs. It's quite a unique engine from Porsche, aluminum 930 case but with 6 bolt 2.7 crank. The engine I built is better than my old favorite 2.7RS with Webers.

If you have the funds, build a 2.7RS plus engine. You can see machining costs on the internet at Ollies. You'll have to get the cylinder spigots enlarged, the heads machined for 92 mm pistons, and the ports opened up. Also, a set of Webers or PMO carbs and S plus cams. You might call Henry at Supertech, his company has built many of these engines. This is a very nice engine.
Regard, Dave

I recently had my top end rebuild on my 2.4 (1973.5T) after excessive and continuous smoking. They found a broken ring in cylinder #5. To add a bit more compression, longevity and performance I opted for the "Nikkies" (pistons and cylinders). They are not much more then the factory replacement. Runs like a charm.......