Magnesium 911, cylinder matting surfaces

[Fede44]

Hello,
this is my first post to this majestic forum. My name is Federico, i am from Argentina, and PP usual buyer.

I am rebuilding a magnesium block from a 911 S 2.4 which had the broken studs problem.

In the mean time, i found that the matting surfaces between block and cylinders have some Little damage. Some parts are missing ! no more than 0.1mm. depth, i think. but missing anyway.

IŽd really appreciate your experienced help on this engines, since i don't think it is worth to machine the block for this Little, but i don't want to have oil licks nor inappropriate cylinder guidance or deformations when hot.

Thanks a lot.

PS: also, note the scratches in the connecting rod...i donŽt know what happened there (probably made by the man, not the common use or break of engine).

Fede.

[Walt Fricke]

Federico

Have I circled what you call scratches on the connecting rod?



If they (especially the upper one) are as deep as they look, I'd replace that connecting rod. Even if they have not directly weakened the rod, they can serve as "stress risers," which are points of concentrated stress where a fracture of the metal can start. Sort of like how a small chip in the glass windshield can suddenly spread across the whole windshield. Or like tearing cloth - once you can get a small tear started on an edge, you can then rip the whole piece in half.

The case cylinder sealing spigot problem is not something I have seen. It almost looks like someone used a sealant, and part of it flaked off. I suppose you have tried to see if you can scrape a whole layer off without trying too hard (so you don't do more damage. You might need to have a machine shop take off 0.1mm from all three of the spigot tops on that side of the case.

But maybe one of the professional engine builders knows what this is.

[Fede44]

Walt,

thank you for your answer. I took the desition to disassemble the hole engine. Specially because cleaning the block after making the 24 holes will be real tough, or nearly impossible. Also, iŽll make the holes with the milling machine referencing from the half carter.

With the con rod, iŽll take it to magna flux, verify the squad, and clean the imperfections. About the mating surface, iŽll just fix it with a little sealant.

IŽll keep you informed with pictures and the procedure.
Federico.

[ajwans]

I'm with Walt, throw that conrod into the steel recycling bin. Unless your magnaflux also has a flux capacitor and can go 88mph.

andy

[ajwans]

Machine the cylinder mating surface flat and compensate with a thicker cylinder base gasket. Or machine the same amount off your chainboxes to maintain geometry and get a compression bump.

If you substitute 901-104-191-02 for the regular base gasket and remove 0.25mm you should be back to standard.

andy

[Walt Fricke]

Ajwans - I think he should give the connecting rod the float test: throw it into a lake. If it floats, retrieve it and reuse it.

A river of swimming pool would do. I'd be careful about sea water, though. More buoyant. And I don't know just how the anti-cyclonic forces in the Antipodes affect this otherwise time honored test.

[AlfonsoR]

I am no expert, but if money is an issue, you can have a reputable machine shop make a detailed inspection of the rods including dimensional inspection (both ends), magnaflux and shotpeen to get them to a serviceable state. Of course the best would be to replace.

If you go through all that trouble, you will have spent some good money for used rods that are marginal. You may want to consider buying good set of used rods that may be ready to use. Make a separate post in this forum and in the parts classifieds to see what's out there.

I agree on the case machining. I would think it's a must do. Machining the case is one part of making sure that when assembled, you want the cylinder heights almost exactly the same from cylinder to cylinder because you will assemble the cylinder heads and then the cam housing on top of these surfaces. You ideally want them to be flat across all three, but the tolerance (cam housing to cylinder head) is something on the order of up to 0.001", but don't quote me on that, look the specification up in your manual.

If you do not have a good reference, do a search. Wayne's book is pretty good. Not sure what other manuals are good for the early engines, but there is a series of videos on you tube that is pretty informative...Nick Fulljames Pt 1 911 crankcase stripdown - YouTube

[docrodg]

I agree with the others. Replace the conrod, there could be cracks from the damage you don't see in magna-flux.
Machine the case, after how many thousands or more heat cycles the spigots are probably not flat in the same plane.