1973.5 911T 2.4 CIS New Pistons

[Chickenstrips]

Very soon I will be starting a rebuild on my 73.5 911T with 123,000 miles, which is a German smoke bomb on hard accel/decel. I didn't want to hijack Nathan's thread looking for old 2.4 CIS pistons, but I have a very related question that probably belongs in the technical section here. For no particularly well informed reason, I was hesistant to do all that work and spend all that money and not have new pistons. But I think it's the case that we 73.5ers are in the particularly unlucky 6-month window spot of needing domed CIS pistons, but in the 2.4 size, before they went to 2.7. Are these all the options and what are the pros and cons:
1. Reuse or find old one's that are not too worn?
2. Wossners?
3 JE?
4. Porsche new ones?

I think I could in theory bore it up to 2.7, but I got this car from original owner, who never changed a thing. So not too keen on blowing originality. Any input appreciated!

[Gumby]

Have you determined that the pistons are out of spec?

[Chickenstrips]

I have not yet taken it apart. For no good reason I was thinking that at 123,000 miles they would be too worn. Would be awesome if not,

[edmayo]

If the piston top ring lands are not too worn you might be able to overbore worn 2.0 liter cylinders to be correct for your 2.4 pistons. I've not done this, so it would depend on if 80 mm cylinders can be overbored that much. Just a thought.

[Tom F.]

When I needed to replace pistons in my 2.4 CIS in the 90s, I used 2.7S pistons and cylinders. I had the cylinder openings machined to accept the larger bore cylinders and converted to 24 Dilavar studs. No other modifications were needed. The car worked great. The small inlet ports and intake runners restrict flow, so you won't see the power of a factory 2.7.

[HarryD]

I faced this dilemma a while back. My thoughts:

1-At your mileage, it is very possible you merely have worn valve guides. A compression and leak down test will help determine of your pistons are leaking by. If the pistons are ok, I would opt for a valve guide replacement and move on.

2-If you pistons are worn, you have to make some decisions. Basic options:
a-Have machinist inspect the existing pistons and see if they are serviceable with a re-ring. do a top end and don't split the case. Keeps you car numbers matching with original engine configuration.
b-Locate a set of good 1973.5 pistons and cylinders and pay whatever the owner wants for them. You will not need to split the case to do this. Car stays number matching.
c-Get a set of 2.4 non-CIS pistons and convert to carbs or some sort of EFI system. Will not need to split the case but meaningful bucks for a decent conversion. Upside is you can get more aggressive Cams and more power! Car is still numbers matching.
d-Get a set of 2.7 Pistons and cylinders, split the case to enlarge the spigots, do the other machine work necessary to a mag case ($$), install the fuel delivery system of your choice (carbs, MFI, EFI, CIS). Your car will still be numbers matching.
e-Take the 2.4 engine out and replace with a nice 2.7/3.0/3.2 motor. Mods will be minimal and you transmission may need a bit of work. Your car will not longer be numbers matching but will make more power.
f-Install a 3.6 motor. Major mods.

When i bought my car in 2002, it had worn valve guides with 116,000 miles. New guides (option 1) and it was good to go!

In 2015, with 160,000 miles I had a huge backfire (should have tuned the car before!!), cracked the air box (yes I had a pop off) and dropped a screw into a cylinder that damaged the piston top and head. After top end disassembly and inspection by the machinist, we were able to polish out the damage and reuse my existing pistons. We did option 2a.

Do some investigation and decide how far you really want to go.

Best of luck. Keep us posted.

FWIW, I think the 2.4 CIS long nose is a sweet ride as it came from the factory.

[HarryD]

When I needed to replace pistons in my 2.4 CIS in the 90s, I used 2.7S pistons and cylinders. I had the cylinder openings machined to accept the larger bore cylinders and converted to 24 Dilavar studs. No other modifications were needed. The car worked great. The small inlet ports and intake runners restrict flow, so you won't see the power of a factory 2.7.

Tom,

Question. You did not need to split the case for machining the spigots and as a consequence, need to address the warpage of a mag case?

[HarryD]

If the piston top ring lands are not too worn you might be able to overbore worn 2.0 liter cylinders to be correct for your 2.4 pistons. I've not done this, so it would depend on if 80 mm cylinders can be overbored that much. Just a thought.

Ed, At the OP's mileage, I am thinking it may be just worn valve guides. What testing/inspection do you recommend to determine the extend of the necessary repair?

[Tom F.]

Tom,

Question. You did not need to split the case for machining the spigots and as a consequence, need to address the warpage of a mag case?

The case needed work. Aftermarket AC plus no front oil cooler had done their work on the #1 main bearing, which was out of round. I had the 7R case honed back to standard size (by Competition Engineering as I recall). I don't recall that it needed any other work. It did not need case savers. I agree that the original pistons and cylinders are the way to go, if they measure up okay. The piston ring lands were worn in the engine in question, there was no saving them. I had a set of very nice used nikasil 2.7S pistons and cylinders on hand for free, which influenced my decision. It was a swell upgrade and made for a great low end torquey motor for autocrossing.

Another 2.4CIS engine I had went through a top end rebuild without needing new pistons. The original pistons had very little wear at 80,000 miles. That car did not have aftermarket AC.

[Chickenstrips]

Thanks for the highly informative replies. After reading the last few I realized I probably should have pointed out that I did a leakdown test and was at about 10% leakage on all except cyl 3 which was 40% leakage. I could hear that one leaking through the oil filler, that was also the cyl where I found a mouse nest packed around the cylinder after racking up 500 miles like a dope. I thought I was being so meticulous on restart, but missed that until pulled the alternator. Pulls great, runs great, just a smoke bomb and oil leaks. Will pull the engine soon - first time! This car is so original - bought it from original owner, no mods, perfect records, original sale slip with engine numbers that I feel the need to stay numbers matching 2.4, but will see how things look and report back. Possible I cave, Maybe I will only need one piston? My next one will be a hotrod for sure.