Increasing the Spigot Bore on a 7R Case

[tom1394racing]

I have one of the unusual 7R cases with the 92 mm spigot bore. As I understand it, this case can be used as is to build a high performance motor at 2.4 liters and below because these engines all use cylinders with spigot diameters of 92 mm. As best I know, the only difference between the 92 mm 7R cases and the larger 97 mm 7R cases is the spigot bore.

My question is: Can the spigot bores on this case be enlarged to 97 mm to allow the use of larger cylinders say for a 2.7 RS, 2.8 RSR or 2.8 short stroke?

[Steve@Rennsport]

Tom:

This is very easy to do on a milling machine (by competant personnel) as we do these all the time. Any machine shop doing 911 work can do this for you and Case Savers are strongly recommended at the same time.

Boring them to 97mm permits the use of 90mm & 92mm P/C's for 2.7 & 2.8 litres.

A 2.8 short stroke needs to be done with the early 3.0 Turbo aluminum case for the 95mm x 66mm combination.

[len911]

Just out of curiosity... How do you center the milling machine to make sure the new hole is concentric with the old one? Thanks! Lou

[kenikh]

You chuck up a tool in the head of the milling machine that finds the center ( can't remember the proper name; it's been 8 years since I've used one). I know the the newer ones use a laser and are combination edge/center finders. All machine shops have this kind of equipment.

[euro911sc]

There are several versions of center finding tools one might use. Also, since centering is a very common task, almost all the digital readout (DROs) systems offer this function. Once centered you can check it easily to make sure with a differnt style of dial indicator.

Its really very easy. Biggest issue is backlash on an older machine if you dont have a DRO. Most shops doing very tight work will be all digital. Though by hand is also just fine with a well kept machine.

-Michael

[davidppp]

Hello there.

Given the rarity and good price you can get for the unmolested case, it will be much cheaper to start with a proper 2.7 case surely?

they are easy to find..

kind regards
David

[Steve@Rennsport]

This is a matter of opinion, but the vast majority of 7R 2.7 cases have been subjected to some big heat stresses.

In most cases, some expensive line-boring (or more) is needed to straighten the main bearing bores that have distorted from heat. If that doesn't fix it, one has to remachine the case parting lines on each side and re-bore to standard.

2.4 7R's have not see the temperatures that distort these cases so usually all they need is a line-hone to straighten things out nicely.

Bottom line: Its less $$$ to use the 92mm spigot 7R versions.

[tom1394racing]

Once again, I am in awe of the knowledge and experience of the people who frequent this forum.

Steve Weiner...Thanks for sharing.

[Steve@Rennsport]

LOL,..Always a pleasure, Tom,....

[Henry Schmidt]

Two thoughts.
First
Supertec now offers 86 mm Nikasil cylinders for this case and all early cases.
With these cylinders you get the cooling benefits of Nikasil and retain the small spigots of the 2.4 case. It makes a 2450 out of an engine that starts as a 2341.
One of the problems with the 7R case is the the spigots when machined to 97 get too close to the head stud holes making this a week part of the case.

Second
If you do choose to bore the spigots, I would recommend boring the spigots to 95 and trimming the barrels. This keeps as much material around the stud holes as possible. This will not work with 92 or 93 mm cylinders.
Truly a great modification we've used in many racing engine. Especially good with 89 mm cylinders.
Good luck

[304065]

Tom,

One other point I would add is that Charles Navarro, from LN engineering, makes an 89mm thinwall "Nickie" cylinder that's had the skirt turned down to fit the small case spigot, cryo-treated with no windage notch. 87.5 Nickies that fit the small case are common.

I have been scratching my head wondering, "how can I make the power I want without cutting my early aluminum case?" I built the attached chart to help with that, to give me a sense of the power bump between cylinder sizes, so I could assign a dollar value to a marginal increase in horsepower. Hope it's helpful.

Disclaimer: The attached chart has been prepared using linear interpolation between published horsepower ratings of factory 911 engines. It is not intended to be absolutely accurate.

[Henry Schmidt]

Nikies are nice but compare them at our price of $1,360 a set and for a cost effective option, our QSC 86 mm Nikasil cylinders are hard to beat.

Also available with custom built JE pistons (any valve pocket, any compression ratio) for $ 2,340.

[blue72s]

Originally posted by Henry Schmidt
One of the problems with the 7R case is the the spigots when machined to 97 get too close to the head stud holes

even without case-savers?

[Henry Schmidt]

Yes, even without case savers.

[blue72s]

Then why do you recommend 2.6SS?

[Henry Schmidt]

Performance is sometimes more important than reliabilty.

More, more, more and the 2.6 Short stroke is more.

[blue72s]

Originally posted by Henry Schmidt
Yes, even without case savers.

But I think a 95mm case with case-savers is the same thing as a 97mm case without case-savers. Isn't the wall thickness of a case-saver about 1mm think?

[kenikh]

Damned good value; the market has been jonesing for a small spigot bore Nikasil cylinder forever. These will sell in droves; no more scrounging for good used Birals!

[michel richard]

My thought exactly. The way to make a small spigot 7R into a big spigot 7R is to sell the small spigot one for big $ and buy a big spigot for fewer $.

but then it's just my own .02

Originally posted by davidppp
Hello there.

Given the rarity and good price you can get for the unmolested case, it will be much cheaper to start with a proper 2.7 case surely?

they are easy to find..

kind regards
David

[tom1394racing]

What is the going price for a small bore (92mm) unmolested 7R case? I saw one on Ebay go for $1000 a few months back. I have one offered for sale in the classifieds. However after all of the discussion in this thread, I am thinking of keeping it for a future project.