mag case question
 

hi Guys

I have a 2.7 magnesium 7r case

i just tightened my studs over the weekend (some leaking at cylinder 6) and found they all nipped up to the required torque, except one on the exhaust side at #6 which turned freely :mad:

So i have to make a decision as to whether I strip this one down, or drive it until i can sense more of a problem. Right now it drives well.

My question is relating to machining of the case. i am aware that the magnesium relaxes when the barrels and heads are taken off, so decking the case halves is needed. Im also aware that when the case is split, the bearing surfaces are deformed somewhat.

So...I dont want to use oversized bearings, so I presume that line boring back to standard size will require a certain amount to be taken off the case deck. How do you compensate for this ? I presume it would just increase your compression ratio? Do you use a thicker gasket between the case and the barrel?
Does anyone have the right dimensions for the bearings that I can tell my machinist?


Thanks in advance

Daniel
Australia

You reall have to treat every case on an as needed basis. The mainline-if it's out- can often be just honed back to spec-they tend to go undersize actualy. As far as decking spigots theres no problem getting thicker base gaskets to compensate.

I had my machine work done at a reputable US left coast machine shop. When they returned the case, they included thicker cylinder base gaskets sized to compensate for both the spigot decking and align bore/case half surfacing. The gaskets come in different sizes, .25, .50. 1.0. There's some disagreement about stacking them but Wayne says it's OK in his book. You can have the machine shop make the right size but I think that would get a bit $pendy. I'm looking at a double whammy. I have a thicker gasket for the case work and I have to add in what they took off my heads.

I believe you'll find the mains measurements you're looking for in Wayne's book. I noted Chris Seven quoted some numbers in this thread presumably from Wayne's book and the PAG manual.

Good luck!

It may be risky to run it with a pulled stud. Magnesium is a bit unstable even when everything is right. A pulled stud will put unbalanced forces on the case. It might be fine (certainly many engines are driven in that condition), but proceed at your own risk...

Hello

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Thanks guys
If anyone knows of a rebuildable core aluminium case, let me know!! I do love the 2.7s and their ability to rev freely! Everyone bags the 2.7, but having owned sc's and Carreras, the lightness of the mid years are great.
I have some decisions to make!

Daniel
were there any updates to your thread?

Another thing pointed out to me, is that if the case halves are machined, then also the intermediate shaft will need boring and also the distributor will need to be relocated and re-aligned...

I'm curious, when Ollies machine a case half, do they allow for the distributor somehow?