I've got a 2.4T (73 1/2) that I have torn down. This is what the pistons look like that are in the engine...
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What's the difference between these and these that are typically on eBay...
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Also, These are both 84mm. Since they list these pistons as for a 2.2 / 2.4 is it just a different stroke between the 2 engines? If that's the case are the cylinders interchangeable?

That's a CIS piston that you have, you need that weird lump to swirl the mixture with CIS but not with carbs. The other pistons should be a bit higher compression. You can run wilder cams with the normal looking pistons than you can with the CIS pistons.

2.4 CIS pistons are not very common. However, they are not very desirable either unless you find someone who is trying to stay stock.

I'm trying to stay stock (CIS) but I want to replace the old iron jugs with something that doesn't hold heat as much.

You can go up to a 2.7L and have CIS, but you will at least need the p/c and I am not sure about the rods and crank. I have a 2.4L and a 3.0L CIS set up spares. Mmm, good beer, making typing difficult

2.0 & 2.2 cranks and rods have the same dimensions. 2.4 & 2.7 ditto. 2.2 & 2.4 pistons are the same diameter, but the pin location is different because the stroke is different. 2.7 pistons are larger in diameter than 2.4, but the pin height is the same. You want to be careful what compression ratio you end up with for a street engine. You can flycut the heads for larger pistons, but you have to pay attention to the edge of the combustion chamber.

The Cap'n

If you stay with CIS, then you need a CIS style piston
(as well as mild CIS cams, and mild compression)

you can bore the case spiggots, buy pistons, and buy cylinders to go to 2.7
(it would be a shame to bore those spiggots on the 6month only block you have, better to buy a complete 2.7 core and start from there... sell me your block!)

or you can buy 2.4 CIS style pistons

or you can change the injection and use other styles of pistons and cams.

time to read
I'd recommend the pelican rebuild book, and the bruce anderson 911 performance handbook.

true, but the factory T. E. S piston dictates wich cams you can use.

The rods make up the difference. but the piston dome is greater or lesser changing the rated CR.

Hey everybody thanks for the great info. Maybe I didn't make my question clear though; I am happy with the 2.4T CIS as it stands except that I'd like to ditch the old iron cylinders and use something like the birels to get better heat dissapation. Other than the 84mm ID, are the jugs the same dimensions for both the 2.2 and 2.4 engines?

Yes , but there is 0, 1, 2 factory tolerance groups.

I have 6 new 2.4 cis pistons .
I used the iron cylinders, as they were new.

2,2 911E and S used the Biral cylinders. Largely unobtanium since they've been out of production for more than 30 years and racer-types needing stock cylinders buy them up when they can find them.

So this is where you really need to ask yourself what it's worth to you...

If you -really- want cooler cylinders, you can get a set of Nickies, and Charles will precision-hone them to whatever piston clearance is required.

Long way to go for not much, IMO.

2,2 911E and S used forged pistons, which have expansion characteristics that match the Biral cylinders. Similarly the cast T pistons and the iron cylinders.

I don't think a 2,4T, especially a CIS one, makes enough heat for it to be an issue (another reason the 73,5 T engines are prized by racers - they have seen very little stress in life...)

Sixnot four - Excuse my ignorance but what is a "tolerance group"?

Rich - So are you saying that it is not a good idea to put the cast T pistons into Birel cylinders?

There's a nominal cylinder diameter, and then there's reality. The reality is that some will be a teeny bit bigger or smaller than nominal due to production tolerances. So the plan is measure everything, then match the big cylinders with the big pistons.

Group 0 - 84,00 - 84,01 mm
Group 1 - 84,01 - 84,02
Group 2 - 84,02 - 84,03
Group W - mother-rapers, father-stabbers, father-rapers...

There are matching piston diameters, which vary depending on whether they're Schmidt or Mahle pistons, and the differences are all in the 2nd decimal place (mm). (Only T's used Schmidt pistons - E and S used Mahle only with the Biral cylinders.)


I'm certainly not going to recommend it, because if you ran a Group-0 cylinder with a Group-2 piston, and didn't warm things up carefully, and siezed a piston, you'd be pissed at me.

The reality is I suspect lots of guys can't and don't measure this stuff to 0,01mm and just throw stuff together and it mostly runs fine. Face it - if your pistons have time on them, they're not their original nominal size any more anyway...

But this is really cart-before-the-horse stuff. Go see if you can even -find- a set of Biral cylinders before heading down that path. If you can, they can be honed for whatever clearance your cast pistons will require. But you have to expect a certain amount of oil consumption and piston slap if you wind up with bigger clearances than a matched set would allow.

I'm not sure it can't (or can) be done...

theoretically it should work to combine T pistons with birals, but as mentioned above the factory used metals with similar expansion rates and now your mixing things...

plus when people buy used, they may have to have pistons made to match the bore of used cylinders..... your going at it the opposite way, so you would have to find cylinders to match the wear of your pistons.. that would be difficult to say the least.

if your happy with the CIS, the Cam choice, and your stock-ish compression then you should be absolutely fine with your steel cylinders you already have.

cylinders (nickies at least and maybe all?) come in height groups as well as tollerance (bore) groups. So if you decide to find one cylinder to match a set, make sure you watch for that also.

Often people buy complete sets of Pistons and Cylinders together at the same time to avoid mis match.

or they buy new

or they buy cylinders matched and then get JE pistons made to match the cylinders they have

I doubt JE makes a CIS style piston, but I've never looked.

Would adding Biral cylinders require head studs and/or case work (inserts)?

I thought one upside of the iron cylinders is their expansion rate matches the head studs.

one school of thought says you oughtta do case savers and TimeSerts on any mag case that hasn't had it done already...


a Biral is a steel sleeve with a finned aluminum muff. I don't think the axial expansion is so much different as the diameter...

Have you check ring side clearance ?
Rich whats the spec ?

you're dreaming if you think I'm gonna retype this table ;-)
included the cylinder-height spec too...Click to view attachment

The rebuilt 2.7 I bought has the factory 911 CIS installed on it and the paperwork (from Competition Engineering) indicated JE 9.5:1 pistons. So I'm not sure what the hell is actually in there. Going to have to take it apart one day...

Otherwise good looking pistons may be toast depending on the ring land to ring gap. Wear limit is .004 "......that means that you're gonna break something sooner rather than later. A feeler gauge will tell the story. A good number is .002-.0025. The top ring is the one to check.

Mahle made Nikisal 84 mm replacement clys within the last 10 years. An ad on the Pelican 911 used parts forum should find you some cyls, but as others have said , the iron jobbies are fine if they are in spec......most aren't.

A quick & dirty check is a feeler gauge between the bottom of the piston skirt (the widest point) and the cyl bore.....moving up & down the length of the bore. .004 is very iffy, .002 or less is good. You'll need room to hone for ring seating so keep that in mind. Engine rebuilding is an inexact science. If you want new spec perfection buy a new engine....or in this case, new P/Cs. Either way, bring money.

Anyone foolish enuff to measure after market pistons against an in spec cylinder is gonna find that Porsche's tolerances are out the window. BTDT. A leap of faith in your piston maker becomes the tolerance......if the cold slap goes away when the thing wams up you're good to go.

It sounds like you are happy with the stock '73.5 911T CIS engine. I'm extreemly happy with mine!

If the pistons, cylinders, etc. are all within spec: clean them up and put them back in with fresh rings. Since you have it apart: get the heads rebuilt, get fresh seals, then expect to get another 200K trouble-free miles out of the engine.

The CIS pistons and Cylinders will last forever. Mine are original and have lasted 290K miles so far. Also the iron cylinders have the same heat expansion rate as the iron head studs so stock '73.5 911T 2.4L CIS engines never pull head studs.

Heat has never been a problem with my stock '73.5 911T CIS engine. I don't even have an external oil cooler.

Why did you tear the engine down?