Case machining confusion

[nickonickolaw]

My main question here is how does the amount of times a case have been rebuilt/cut/align bored limit my option of the size of the piston/bore?

The reason why im asking is because i'm getting a 1915 built by a mechanic. I don't understand what hes trying to tell me but he said the case has been cut 3 times and something along the lines of its gonna be too thin for a 94mm piston and something about the stud..... and that the stud wont hold the cylinder heads or can fall off. so he suggested that i go for a 90.5mm piston.

so i'm genuinely confused. How does a case that has been machined 3 times limit the size of the bore? from my understanding, you can always remove material for bigger jugs and pistons and that the holes for the pistons can only be cut once....

[minirailz]

Does he mean line-bored, for the crankshaft?
Or does he mean machined for larger cylinders?

The line-boring can be done multiple times, but it's not advised to have more than +02.00mm larger bearings.

I'm unsure about the cylinder bores. I guess it doesn't matter how often it is machined, but you do indeed have to have enough meat/metal for the studs.

Regards,
Dennis

[modok]

Best to have 12mm head stud inserts to go 94 bore.
Maybe it has larger ones.

[Rome]

My guess is that your mechanic is describing two situations on your case, which may have melted together into only one when you were listening.
1. Align-bore the case for the crankshaft bearings. I've seen oversize bearings up to the 3rd cut size (1st cut would be 0.020" larger, 2nd cut 0.040", 3rd cut 0.060"). If your case is already at the 3rd cut and now needs yet another cut, I think you're out of luck with the case. I'm not aware of a 4th cut bearing size being readily available.

2. Bore the case openings (often called spigots) for larger cylinders. If you truly intend to run the 94 mm cylinders for a 1915 cc, the openings are bored so large that the edges can weaken the case if the case has 10 mm cylinder head studs. What happens is that 10 mm head studs "should" have case savers installed, which are threaded steel or brass inserts that fit into the case and are flush with the case surface. The inserts have inside threads into which you fasten the 10 mm head stud ends, and also external threads which hold the inserts to the 16 case holes. But the inserts have material thickness. If you drill the case for case savers for the 10 mm studs, when you bore the cylinder holes for the 94's, the machining will likely break through the case material and expose the external threads of the inserts. (sharp64 gallery)

If you limit the larger cylinder size to 90.5 mm (1776 cc), the case opening does not need to be as large, leaving a small amount more case material on the hole edge to the external threads of the insert.

You have yet another option; have the machinist bore the case for 90.5 mm cylinder size, then buy 92 mm "thick wall" cylinders (1835 cc) on which the bottom ends fit into the 90.5 case size, but the head end needs machining to the same size as for a 94 mm cylinder. The fins and cylinder top are thicker material for strength. Make sure your machinist knows about those thick wall 92's, because for decades the 92's were only available in the thin wall style. The thick walls were only on the market in the past ~ 15 - 20 years.

[Cusser]