2,0 -> 2,7 in 901/03
 

Hi there,

It is quite common to bore the Cylinder spigots from 92 to 97mm to insert 90mm barrels into a 7R mag engine case.

Actually i have got here a 901/03 (some 1 or 2R) and the idea came up to upgrade this one to 2.7L.

To me the contact surface where the cylinder would sit on would appear a bit tiny plus the pressure on the copper ring would probably be exceeding.

Question: Did anyone do such conversion on the earlier mag cases and if, what are longterm experiences?
Cylinder/Case oil leaks?
Cylinders "sinking"?

Thanks,

Robert

Hi Robert,

The early generations of magnesium cases were much more flexible compared to the later versions and as such, were prone to cracking.

Given the necessity of the extensive (major $$$$) machine work that ALL Mg cases need to work, those early ones are not a good choice when building a 2.7 due to their fragility.

I suppose if one used "T" cams to keep max revs below 6500, it may last acceptably long, however I could not offer such options to my clients, simply to protect their financial interests.

JMHO, but the 4R/5R & 7R cases are far better choices for a 2.7.

Thanks Steve,
i know about the overall fragility of the early cases. So not a good choice, i know. Especially the spigot area and its potential issues actually are my concern. Can you report issues with it when boring to 97mm as longterm experience?
Since i am my own machine shop i am not concerned about the labour.
I just never did such conversion on the early cases but on the later ones for the same reasons you state so i have a lack of experience here.

When we enlarged some VW cases for bigger cylinders...we bored them with a step in the cylinder hole and inserted a steel ring (with as large a diameter as we could get in) and let the barrel sit on this.
It distributed the load over a larger area...and made the whole deal worth while.
Other than that...I would go with Steve...and get a later case...or grab an older Aluminum case (stronger) to use.
Bob

Thanks Bob, you made it that way for the same feeling and reason i never did the conversion i talked of.
Regardless of that: Anyone here who did it and has proven it can work?

Cracking is the problem here. Spigot cracks: main web cracks behind #3 to the flywheel seal area are the two biggest problems. This is why I strongly recommend retaining the 92mm spigot size since the very LAST thing one wants to do is remove any material from these flexible cases. I've seen a lot of this when these engines are operated over 6500RPM.

Thanks Steve, good explanation, the case remains as is.

Back in the day when there were limited suppliers of P&Cs we would convert 2.0-2.2 to 2.5 by installing the 90mm cylinders.
The concerns for cylinder support lead us to a compromise.
Instead of boring the case to 97 mm, we would bore it to 94mm and trim the cylinders to fit. This compromise added to our peace of mind and produced a noticeably more reliable engine.
With the advent of reasonable after market cylinders, our preference for these early cases is to leave the case spigots stock and run a 70.4 (2.4-2.7) crank with 86mm cylinders.
This produces what we call the 2450. With 9.5:1 compression, 36 mm ports and Mod "S" cams these little Weber carbed engines make 180 RWHP @ 6400 rpm. Well within the reasonable performance limits of the case.

Good point Henry, but that gave you a cylinder skirt of not more than 2mm. Has that been reliable with Nikasil Cylinders?

Although not a perfect solution, this modification was reliable.
Keep in mind that the early 911 engines had a very nice rod length to stroke ratio (1.97:1) so there was very little side loading on these cylinders.

Hmm..
i am still a bit ambivalent as i actually ran out of cases.
I still have got a 4R, which already is (slightly) reinforced in the area Steve has mentioned.
But even if, it still apears to me difficult to let the barrels sit on a "flange" of remaining 3mm with spigots bored to 97mm (not really much better when boring to - say - 95mm).
Opinions?