enlarging valve pockets in early 911S pistons

Terb Grubmeel · 10-03-2005, 07:05 AM

I have a 911T engine (1969/70) case with a counterbalanced crank and a set of heads with 38mm and 42mm valves. Since the pistons and cylinders out of this engine are at the wear limit, I am trying to match this engine with another 2 liter set of early 911 S hign-compression pistons with the biral cylinders.

However, the pistons have valve pockets that are too small for the 911T valves and need to be enlarged. What is the porcess milling out the valve pockets in the pistons, and what are the approximate costs? Or am I better off to purchase a new set of 2.4 liter set of pistons and cylinders, cost wise?

thanks for the input!

TB

Eagledriver · 10-03-2005, 07:53 PM

This thread would get better results on the engine rebuilding forum. I recommend you give Ted Robinson a call. He runs German Precision machine shop and can do this work. I had him deepen my pockets on my 2.7 pistons. His number is 408-747-0728. He advertises in Panorama and Excellence and is located in Sunnyvale CA.

-Andy Simpkinson

jluetjen · jluetjen's Garage

In what way are the valve pockets not big enough? Are the valve diameters begger then the pockets? Or are you saying that the pockets need to be deeper? The reason that I ask is that even though the earliest 2.0 911S's (42I/38E) had smaller valves then the later 2.0S's (45I/39E), they were still the same size as the 2.0 T's and E's (42I/38E).

So I'm trying to figure out where the issue is.

Terb Grubmeel · 10-04-2005, 05:47 PM

Being a Newbee, my replu hopefully will come out correctly... Yes my questio would be better posted on the "rebuilding forum"!

Andy, thanks for the reply, I will Ted Rbinson a call at German Machine shop! that certainly will give ne answer.

Thanks for the 'Heads-up' information.

T.G.

John, the valve pockets on the early 911 S pockets are smaller in diameter, and are deeper than the 911 T valve pockets. The diameter of the valves of the early 911 S, are as follows: I 39mm E 35mm. Which begs the next question, that I will post next, and that is "can I turn the piston 180 degrees? That would then at 39mm be sufficient for the 911 T exhaust valve to "reach" into the valve pocket of the 911 S pistons. However, are the wrist pins offset?

Thanks for your help!

Air-cooled

TG

davidppp · 10-05-2005, 12:09 AM

Hello TG.

The pistons you have here are not S pistons at all..

They are normal 2L pistons..and should be cast not forged if you look inside them..

All S pistons will accomodate 42mm inlets...and the later S ones, which were one year only, 45mm..

Not only would you have to machine for a larger diameter, but also deeper, if you want to use s cams..

Kind regards
David

ErVikingo · 10-05-2005, 04:57 AM

FYI, I'll most certainly have a set of Cosworth high compression 2 liter S pistons for sale very soon. I ran them for 12 hours at the most (including dyno time)...

More stuff will also be available. PM me if interested.

Terb Grubmeel · 10-06-2005, 11:32 AM

David,

Thanks for your note re. the "911S" pistons. I purchased the pistons and Biral cylinders from a person I know, who told me that they came from an early 911S. I then cross-referenced them with Bruce Anderson's Performance Handbook, p. 108, where I found the 901/01 engines had I 39, E 35 mm valve sizes. So it appears that they are not from an "S" but an early 911...

I don't know how to tell if the pistons are forged or cast, are there indentifying marks inside the piston?

Thanks again very much for trying to help!

Air Cooled

T. G.
1974 Carrera
1958 CAB
911 Engine

Juan,

Thanks for your offer of the Cosworth (great make!) high compression 2 Liter pistons, but I am now re-evaluating the entire project... I will keep your info incase I change may mind!

Thanks again for contributing to this forum!

T.G.

1974 Carrera
1968 Porsche 356A Cabriolet/hardtop
911 Engine

davidppp · 10-06-2005, 12:04 PM

Hello again.

The forged pistons are sheped so as to allow the die to enter and withdraw..they have "draw" or slight taper built into them..

Some will have machined undercuts for lightening...

The cast pistons have usually cast in ID marks and numbers inside them..

I have one set of brand new pistons for the 2L cars..they are not forged S but will fit all 2L cars including the S and work well..they are made by Kolbenshmidt and are OE items..

Kind regards
David...

Terb Grubmeel · 10-07-2005, 11:37 AM

Hi David,

Thanks for enlightening me on the forged/cast pisons issue! The pistons I have thus appear forged, and are actually in good shape, checking piston ring tolerances. I appreciate your offer of the 2L pistons, and, yes, Kobelschmidt has been making pistons for Porsche for some time as OE.

I have to pass on your offer, but thanks also for contributing more info on this thread!

Air-cooled Regards,

T.G.

1974 Carrera
1958 Porsche 356A CAB/HARDTOP
911 engine ( 2Liter), without a car...