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What does "WTF" and "butthurt" mean in plain English?
WTF = “what in tarnation”

butthurt is that feeling you get when someone pulls your head out of your a$$ so fast it pops....
 
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my inclination, being on the other side of the pond, would be to dial up the shop foreman at KB-silvolite and request a couple of appropriate supereutechtic slugs with sufficient meat where needed to finish. pistons that will reliably run 0.0005" skirt clearance are very gentle on the bores and will run much thinner rings without flutter or other sealing issues

it may not be 'ol skool' traditional, but, nothing wrong with taking full advantage of modern tech and processes especially when nobody will ever know, except to wonder why yours makes so much more power, and sounds uncommonly not like a loose wheat threshing machine

horses for courses etc
Nothing wrong with using modern materials, I do so all the time, super to hyper or more tradition forged types pistons. Usually availability or cost drives the decision on client bikes or a decision not to wait for a manufacturer to make a new batch. Had a problem with Xr1000 pistons a few years ago, nobody could supply higher comp version, I so used modified forged Venolia items intended for a Ducati, they worked a treat.
 

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Discussion Starter #43
Ok I stayed up late and removed most of the material from the bottom of the tank. It was bondo covered with risen. Tonight I'll clean it up and get pics posted. I also picked up a Triton yesterday, slimline frame, 750 Bonnie engine, Ceriani forks with a Norton ls2, Trimuph swingarm and hub. There is a road holder front end with it. Checked the engine numbers, it's from the year Triumph went on strike. IMG_3694.PNG
IMG_3695.PNG IMG_3696.PNG
 

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Nothing wrong with using modern materials, I do so all the time, super to hyper or more tradition forged types pistons. Usually availability or cost drives the decision on client bikes or a decision not to wait for a manufacturer to make a new batch. Had a problem with Xr1000 pistons a few years ago, nobody could supply higher comp version, I so used modified forged Venolia items intended for a Ducati, they worked a treat.
Would you run them at .0005 ? My way of thinking it would pretty much have to be almost a perfect engine. Seems to me that nothing is perfect in British motorcycle engines of that vintage. The OP's engine with the cast iron lump for cylinders is different than what I'm working on, and slightly newer, but I couldn't get away with a piston that snug. By the time you deal with tolerances in line boring, "straight" rods, small end bush parallel to the big end, crankcase cylinder mating surface parallel to the crank, cylinder liners perpendicular in the muffs, muffs perpendicular to the cases, centering the crank ..... the list goes on... and then add heat and things trying to fly apart. Regardless of a low expansion piston only needing .0005 to survive, it doesn't allow room for the havoc that goes on inside some engines....imho. and I used a torque plate.
 

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Would you run them at .0005 ? My way of thinking it would pretty much have to be almost a perfect engine. Seems to me that nothing is perfect in British motorcycle engines of that vintage. The OP's engine with the cast iron lump for cylinders is different than what I'm working on, and slightly newer, but I couldn't get away with a piston that snug. By the time you deal with tolerances in line boring, "straight" rods, small end bush parallel to the big end, crankcase cylinder mating surface parallel to the crank, cylinder liners perpendicular in the muffs, muffs perpendicular to the cases, centering the crank ..... the list goes on... and then add heat and things trying to fly apart. Regardless of a low expansion piston only needing .0005 to survive, it doesn't allow room for the havoc that goes on inside some engines....imho. and I used a torque plate.
I think you ( ) mean 0.005 not 0.0005"

On Brit parallel twins (vertically split cases) I don't run pistons tight, that said it depends on the piston as soft slugs tend to work better even in race conditions as they are more ductile, no point going high silicon as the pistons are too hard, even some forged pistons can give trouble. As you say there's too much movement - flex in a standard crank, small ends are reamed so that the pin drops through the bush under its own weight. I always check the top deck too on race engines, its never perfect but you can correct and get quite close. "WE" don't have the benefit of the factory sellecting a set of perfect cases from 4000 machined castings in the factory stores. On road engines I just assemble the bottom but leave the crankcases semi tight (gasket jointing is a non hardening type like Wellseal), no pistons on the rods, then fit the block and tighten in down. This allows the cases to straighten up against the block (for what its worth the factory have matched the cases previously), tight the case bolts. remove the block and start to assemble the top end. Not perfect I know but it seems to work.

Jap engine are different of course. I guess we need to remember that sometimes highly accurate modern replacement parts are just too good for pre CNC era engines. A well put together original Petty, Lacey, Lancefield Manx engine might make 50 or 51 hp for say 250 miles but a modern replica Manx engine (full CNC one piece crank, high silicon piston, Nikasil liner etc) with the same bore and stroke will do 55 or 56 all season long. Same applies to replica Commando, Vincent, Nourish engines they are just made so much better than the originals.

Chatting with gentleman who raced for Francis Beart - "Once the engine was warmed up we were allowed a few easy laps, then three flying, that's it. If you did 4 flying you would be looking for another job."
 

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I think you ( ) mean 0.005 not 0.0005"
No, I really mean 5 ten thousandths of an inch.

If I was as concerned about blueprinting green and unstable castings or thought my machine work might yield an engine out of square, true, in alingment etc, I might open that clearance up to 0.001".

Cyorg................

Tell me of this "line boring" done on a brit twin. I can only imagine getting anal and bashing in some cam bushings resulting in the need for line boring, honing and lapping, but, your post seems to indicate this is something special done to cylinders which is beyond me. Had you said a triple, I could see actual line boring being valid under unusual circumstances although involving the crank main bearings.

KB eutectic's are amazing, you might consider trying some with some thinner and lower tension gapless rings.

I've run them at the 0.0005 in much older engines and having much larger bores than the flat topped and relatively not large domi.
 

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No, I really mean 5 ten thousandths of an inch.

If I was as concerned about blueprinting green and unstable castings or thought my machine work might yield an engine out of square, true, in alingment etc, I might open that clearance up to 0.001".
Good for you. My old Bridgeport would be in knots trying to achieve accuracy of 5 ten thou of an inch.

My experience is mostly road and road race 60's though the 90's, I have never seen anything running that close either 4 or 2 stroke, air or liquid cooled.
 

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ya might consider getting a good honing rig, like a sunnen grasshopper, since generally, you use the boring bar to get purty close then finish with a hone

I'm glad I don't have to bore cylinders on a R5 mill
 

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The line boring was just part of a general ramble about things that eat up clearance. I’m not suggesting that it applies to the op’s engine or most British engines of that vintage.. An example of where it could apply, would be a Vincent twin and yes it would involve main bearings. One (and not the only one) of the reasons the outer main bearing races walk around and start running against crank pins is that although the cases are matched (and stamped with mating numbers) they are not always right on the money. Relatively easy to correct with a mill of suitable size (which I don't have)…. and while its in there true the mating surface for the cylinders so that they are perpendicular to the centerline of the crank.... because that eats up piston clearance as well. I suppose one could argue that clearance in the roller bearing big end would help compensate for any effect that out of whack mains would have on piston clearance…provided the rods are properly centered in the bores.
I don’t doubt that they are great pistons and they have their place. For me personally, even getting a cylinder true within .0005 would take some luck and that’s with using a boring bar to true them up and finishing with a rigid hone. I don’t pretend to be an expert at it and even if I was, I would have to sub it out, because my mill is a step down from Jalsteve’s.
This doesn’t apply to the OP’s engine, but with alloy cylinder muffs and pressed in cast liners, you probably have more than .0005 in variation due to expansion of the alloy. The liner needs to be a snug fit for heat dissipation, so when the alloy expands more than the cast iron, you get some distortion where the liner sticks out below the muff. With the Vincent engine there is also the stress member factor and the fact that on some engines the lower part of the liner is a “snug fit” in the cases and some are not…
Jalsteve covered this, but just to add my 2 cents…. You mention unstable castings and that certainly must be factored in. Some of the post war stuff has the consistency of dried snot , so all the CNC work isn’t a guarantee things will stay where you put them. Anyway, I could prattle on and none of it really matters anyway. Given all of the things that eat up piston clearance in these old things, I just can’t see running them that tight. There are folks that have been using low expansion pistons that come with a recommendation of .0025 to .003 and they have run into trouble. Granted, its not the pistons and its due to all the other shit going on, but you can end up with a shitty result.
Again. I have no doubt they are amazing and obviously have their place, but don’t see the advantage outweighing the risk in the lumps I’m dealing with.
Anyway, just my opinion... don't claim to be an expert and there are occasions when I think I know what I'm doing and the motorcycle gods hand me my ass.
 

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Discussion Starter #52 (Edited)
That tank is a hacked around early Manx tank. These tanks with the narrow V (albert modified) at the front of the tank was fitted to pre 56 Manx Nortons without oil in the left down for primary chain lubrication. The later frames needed a wider V to avoid the oil filling point.

Its probably worth reinstating the rear section as was, if you can find someone over there to do it correctly. Or send it this way and I can have it sorted for you. Or leave well alone and have a new tank made and put the original tank in the heritage box.
Ok now with Bonneville over I had some time to finish pealing back most of the hack job on the tank. I have remove almost 10 lbs of body filler, some kind of white sealant, fiber glass and resin. The bad news, 1, what I think are the original baffles are cut to shit. If they are not available I pretty sure I can reproduce them, but I don't know how they were placed 2. the bottom of the tank was cut open like a sardine can and I'm only missing the section that was on the back. The good news (i'm making some assumptions here as I have never seen one of these tanks in person, just pics, the back of the tank may not be hacked up that bad. It looks like the curve on the bottom is 80% there, there are some welds that still need to be removed ( it was covered up with aluminum and bondo) Please tell me if I'm wrong but the repairs for the tank do not look all that bad now that all the garbage has been removed. IMG_20180823_085747258.jpg IMG_20180823_085708566.jpg IMG_20180823_085756957.jpg IMG_20180823_085951087.jpg
 
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