Soft Seize?

splatt said:

Not likely they are putting 100's in a finished bore is it, either going to be 200 or 300 to clean up seize marks, one thing I thought was strange was that it seized on only one side of the piston pin, it should have been pushing on both sides

Click to expand...

No, I wouldn't use 100s except to remove material quickly. Like if I wanted to remove .006 or more.

Of course I seldom use the hone to remove material quickly. I always bore them first and then use 300s or finer to remove the last couple thou.

I have seen wear marks like that before on those pistons. I looks like the cam grind could be a bit off. Jim

I just put my 72 Combat back on the road after a complete engine rebuild (using advice from this forum I might add). Re-ground rod journals (1st oversize), new rod bearing shells, re-used the old Superblends (actually SKF brand with tapered rollers) as they were fine, new valves, guides, intake seals and seat grind. Also replaced connecting rods with used ones that had much better fit on the gudgeon pin end, and balanced the heck out of everything. Piston clearances ended up at .035" on left side, .040" on right side with cast pistons (not sure of manufacturer, but they look like JCC) and US made rings.

Reason for post: I installed the rings DRY which I learned on this forum, which went against my "traditional" thinking. Actually I had installed them with oil at first, but after researching on the forum I removed them and got rid of the oil!

I am happy to report that the rings were broken in after less than five minutes with straight 30W oil. The .035" side blew a small amount of smoke for about 2 minutes, the .040" side blew a bit more, but cleared up completely by the 5 minute mark. Then it was re-check the timing and do a test ride which ended up being about 40 miles. I had some mild concern about the .035' side being too close to the lower limit on clearance, but after driving it "like I stole it", on a very hot and humid day, there were no issues and the power level is awesome.

So I am happy to report a successful rebuild and wanted to say thank you to all of you who took the time to share your experiences on the forum. Now it's on to my 74 850, which I hope is a top end job only, and then the 73 750. The 73 has a cracked cylinder flange but is otherwise ok (except for the oil weepage you would expect from the cracked flange). But if either of them turns out to be a complete rebuild I now have a collection of jigs and other tools I had to fabricate for the 72 and I expect the process to take about 1/4 of the time the Combat took. Knowledge is power!

Congrates breaking ring install traditions but are ya sure its really a Combat ya stole : )
Felt like finger nails on chalk to me 1st time I slide pistons in with dry rings but always sealed smokeless within a min or so - half dozen times. TotalSeal rings took a clue from the Chevy 1950s Bon Ami treatment to sell a similar power to put in which seems like finger nails on sand paper aggressive seating.

Ok it's definitely a Combat. Got the right "C" marking stamped in the head, the bottom fin is flush with the head gasket surface where it was machined down, and it has the SS camshaft. And it's a b**ch to kick over, and if it snorts in the process it could take your ankle off! So no question about its lineage.

The photo in my signature is the 73, so maybe that's why it doesn't look like a Combat (because it isn't). The 72 has the proper black painted cylinder barrels and all the other Combat stuff mentioned above.

Just rattling cages on Combat funny farm but am keeping track of close clearances gotten away with a long time.

One does feel a bit like a forensic examiner when digging into an old British engine, especially a Norton with all the labor and financial issues that interrupted their parts supply and forced them to use whatever was on hand (mind you, being cheap may have had something to do with it too). My 72 was actually made in 71, at least the frame and s/n are from 1971, but it left the factory in 72 and some parts like the wiring harness appear to have been left over from 1970 or 71.

That's half the fun, isn't it?

hehehe half the fun but cause of most the bickering like your confusing date of build vs date stamped. I got forced to learn real mechanics both because of Norton fault plus hard prior use so not much fun to me. I look up to those that like fixing these clunkers.