How Deep to Go?

[cpcooper]

I dropped the motor in my '78 SC to take care of some oil leaks and a few other small issues and, of course, my luck has somewhat run out and I came across two broken head studs (on one cylinder) plus evidence of exhaust getting by the head gasket ring. My mentor in things Porsche is out of town, but I'm pretty sure he is going to say if two studs broke, the rest are just waiting to and replacing a single piston/cylinder is not as good as doing them all, and while we're in there... Bear in mind this is a 4th or 5th car that goes maybe 500 miles a year, ran strong with no smoke or weird noised, save the ratta tat at full throttle I attributed to an exhaust leak, and I guess was, just from the top of the cylinder, not the exhaust system. So, do I stop at repairing the two broken studs and replacing/salvaging the one cylinder, (head looks OK) or is that really shoddy and I should really replace all the studs, pistons/cylinders, valve job, and on?

Some additional factors:

I believe it is a ROW car, engine type is 6382473

I think someone has been in there before as the two broken studs are nonferrous, but I can stick my magnet to the other two.

Cylinder is Mahle 95ZN1W2. What can I tell from that?

Any and all input appreciated, including "Go buy the parts to fix your car you big cheapskate" Or, the admittedly less likely "Gee, I have everything you need cluttering my shop and was hoping to get rid of it"

Thanks,

Craig

[fred cook]

Since you have the engine out and (at least) partially torn down, you probably should go ahead and replace the bottom row of head studs (Dilivar) and get a valve job done on the cylinder heads. The least expensive option for the head studs is to use steel studs like the top row. And, of course, fix any leaks that you can take care of without splitting the block. The easiest way to remove the cylinder head studs that I have found is to use a propane torch to heat the block inside the spigot (hole for cylinder) next to the stud. When you get it hot enough to expand the block and melt the thread locker, the stud will turn out pretty easily. If you break any exhaust studs, use the same process to remove them. Actually, I would replace all of the exhaust studs while they are easy to access.

[cpcooper]

Thanks Fred: So the bottom studs are Dilivar and the tops steel? That would explain magnet sticking to the tops. Exhaust studs are all new... see last week's hard luck story.

Craig

[Trackrash]

Originally Posted by cpcooper

Thanks Fred: So the bottom studs are Dilivar and the tops steel? That would explain magnet sticking to the tops. Exhaust studs are all new... see last week's hard luck story.

Craig

Correct. The top head studs are steel. Same as was used since day 1 on 911s. They work fine and are basically bullet proof. Only the bottoms are dilivar.

If you don't replace all the bottom studs now the rest will sooner or later break, so you may as well replace all the lowers now with steel. AHIK.

Now is also a good time to inspect the cylinders for wear. If pristine, leave well enough alone. A valve job is probably a good idea as well.

What you posted for the cylinder number is the casting number. On the bottom side of the cylinder will be a number in a triangle and another number stamped near it. Those will be the size and height of the cylinder. I doubt the size matters at this point, but you will want to match a replacement with the same height.

[cpcooper]

What's the thinking on replacing a single cylinder? My Porsche mentor is somewhat aghast at the thought, but everything mikes out within spec, both the cylinder and piston. Sadly, there is damage to the top of the cylinder where a small part of the seal blew through. It's a size 1 and the height is 6.

Craig

[911pcars]

You have some leeway with the stack height (cylinders, cylinder head, cam carrier). Component repairs and rebuilding techniques often include slight machining to arrive within factory tolerances and the owner's/builder's objectives. All attached components from the crankcase outward should have the same stack height. Identify and measure the existing cylinders, then source a replacement cylinder that matches the installed height of the other cylinders.

The installed cylinder height can be machined to match the others. This assumes the cylinder-to-crankcase spigot surfaces are in good shape. Cylinder head rebuilding is typically performed at this point (valve/seat refacing, valve and valve guide inspection/replacement, cylinder head mounting surfaces, etc.), and especially if there was a combustion leak at the cylinder-to-cyl. head interface. Your builder should also inspect other engine bits (camshafts, rockers, etc., then recommend either re-using, refurbishing or rebuilt parts as needed.

You may want to stop at just prepping the cylinders for new rings. Delving further, e.g. con rod bearings, usually requires rebuilding the con rods. Your choice. General engine condition and mileage play a big role. However, 911s have 8 main bearings and unless metal bits are in the lube system or the crank rides on a Mg case, they are typically long-life parts. Once the bottom end comes apart, the machining budget can expand to the point where sourcing a good used, later engine might be more cost-effective. It helps not to compare rebuilding costs with domestic makes, VW or other common import brands.

Your builder may want to replicate what the factory did, but it's not necessarily mandatory. Most pro's want to achieve maximum confidence with a low percentage of come-backs. The only variable I see would be the somewhat imprecise prediction of piston ring break-in with good used cylinders, a common variable with engines of this era. But I'm not the pro in charge. Talk to your builder.

Outside of that, if done right, your build should be lots better than any currently engine in service with more than 20K on the odo.

MHO,
Sherwood

[yelcab1]

I would replace the one cylinder and replace all Dilavar studs with steel studs. This is a repair, and not a rebuild.

[gtc]

I'd find a good used cylinder of the same size group, have all six cylinders machined to the same height, and use a thicker cylinder base gasket to make up for the material removed.

Originally Posted by cpcooper

What's the thinking on replacing a single cylinder? My Porsche mentor is somewhat aghast at the thought, but everything mikes out within spec, both the cylinder and piston. Sadly, there is damage to the top of the cylinder where a small part of the seal blew through. It's a size 1 and the height is 6.

Craig

[3rd_gear_Ted]

Originally Posted by yelcab1

I would replace the one cylinder and replace all Dilavar studs with steel studs. This is a repair, and not a rebuild.

Correct,
Clean it to inspect it. Inspect it as needed for repair. Repair it as needed to improve reliability