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Discussion Starter #1
OK, got back from my trip to the machine shop yesterday and the jugs on my 69 tiger are pretty worn out, or so says the guy at the machine shop ( he has never seen triumph jugs before) I know that they are already bored .040 over ( stamped on the top of the piston)

The question is: how much bigger can I go with the jugs before I have to worry about trashing them ( .050, .060 )? what options do I have to get my tiger on the road?

I think i'm gonna invest in a bore guage and check them myself since there isn't anyplace that I have found here in southern oregon that knows what they are doing with old brit bikes.

Oh, ya one other thing, when the guy at the shop checked my cylinders he said that a variance of .0061 from the bottom of the cylinder to the top of the cylinder was too much.
 

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the shop manual gives you the allowable bore taper, and there are .080 over pistons that work just fine with 650 jugs.

have the machinist cut to .055 and measure; if they look like they'll clean up okay at .060 (he'll know) get the pistons, then have him do the final cut according to the actual pistons to be used, plus the appropriate tolerance for that piston's construction.

Cast and forged are two totally different tolerances, and some brands of cheaper pistons like JCC (which are re-branded with several names) actually have less expansion than pricier ones like Hepolite.
 

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I've got JCC pistons in mine and they seem fine - with Hepolite rings. If you can, get ahold of some new .040 rings and put one in the top of the bore - about an inch from the top (make sure it's level) and measure the ring gap. If the gap is .008 you're still good to go.
 

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I've got JCC pistons in mine and they seem fine - with Hepolite rings. If you can, get ahold of some new .040 rings and put one in the top of the bore - about an inch from the top (make sure it's level) and measure the ring gap. If the gap is .008 you're still good to go.
Won't help a whole lot to do that measurement on cylinders that are known to need a re-bore.

That's why I suggested the machinst do the first step of the re-bore, then take the measurements and inspect the progress to see if he believes the next overbore (.060) will do, or if he'll have to go beyond that (.080).

Before doing the final honing, the machinist SHOULD have the pistons and rings in hand to ensure the best results.
 

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Discussion Starter #5
Ok,
Updates from the shop. After spending the last hour checking and rechecking my measurements ( picked up a new bore guage last night) I have found a few things that just dont seem right.
The bores are nice and round, little to no variance there (.0001 to.0003), and the pistons had a few scuffs on the thrust faces but they cleaned off with a rag and there is no measureable differance there. Now this is where it gets wierd.... the difference between TDC asnd BDC is .002 on the left bore and .0009 on the right, but the difference between BDC and the bottom of the sleeve is .003 on both bores! Now for those of us that don't like math ( me included) this doesn't mean much. I decided to try a simple test...... I took the correct piston and put it in the correct bore ( minus the rings) and used a blade guage to see how much gap there was. When I got the piston down to where it was at BDC and everything checked out to what all the math said it should be I checked the bottom of the sleeve, and guess what!!!! the ring section of the piston wouldn't fit easily through the bottom of the bore!!! ( when I removed the jugs it was a fight to get them out, but I figured it was just rust that was the cause)

So to make a long story short, The last machinist to touch these bores did a terrable job and the guy that owned it never had a clue as to what his motor was like inside..... glad I tore it apart :)
To be on the safe side I think I'm gonna get another set of pistons and just have them rebored :) now I just have to find a machine shop that can bore triumph cylinders right...........
 

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cyl bores

Many years ago I used to run an automotive engine rebuild shop. I had the first Sunnen CK10 in the province.

The barrels of a Triumph and most motorcycles are not so easy to do. I had to learn..... The problem with the final hone of the last 2-5 thou is that the barrels have fins from the top till about 3/4ths of the way to the bottom. The fins tend to hold the bore steady. When the hone reaches the lower portion without the fins, it tends to flex with the hone, and the hone therefore looses tension and takes out far less materiel than in the upper portion of the cylinder.

What the machinist must do is manually adjust the hone stroke to spend more time at the bottom of the cyl than at the top. This can be done with the Sunnen CK10 by dwelling the stroke 50% more times at the bottom than with the rest. Additionally the hone will need to be adjusted to less tension for the final strokes. I know this sounds a bit strange but if you have ever witnessed a Sunnen hone in action you will know what I am trying to say. :confused:

It (the machine shop job) can be done well. I used to do a couple of Harley cyls a week and a Triumph/Norton or two a month. I built a custom jig to hold em and got lots of business from the local bike shops. I required lots more time than my billing rate.... Bike engine rebores were not a profit job for my shop.....

The skill of the machinist is paramount. Care and lots and lots of measuring is needed. You will never get a perfect bore... Even factory new cylinders are only within .001-.003 thou from top to bottom. A skilled machinist can get you within .001 thou inch.

I always preferred to hone a loose cyl than a tight one. If you have forged pistons you will need 3-5 thou, if you have cast pistons 1-3 thou. I favoured .003 thou on the castings and .004-.005 thou on the forgings. On racing auto engines you could hear the pistons rattle as I honed 6-8 thou on 4 inch bores. They used to rattle for about 3 4 mins till they got heated up. However bear in mind the engines I used to prep for racing were never intended to last long....

As far as wear in periods... I disagree with many... The lapping in of the rings to the bore only takes a few hundred miles. During that time the engine should never see top gear. Avoid "lugging" the motor. Keep the rpms up above 2500 and below 3500. Change out the mineral oil at 200-500 miles. The tappets and camshaft surfaces, are critical. It takes about 30 mins of running time. I assume all camshafts are "tuftrided" or nitrided surfaces. Coat them liberally with a molybdenum grease when assembling. Break-in the cam surfaces by running the engine at 2500 immediately for a few mins at steady rpm (20-25 miles in third gear at 2500).

Regrinding camshafts and new tappets are recommended for a complete overhaul, unless they measure well. I never "played" around with the cam profiles. I always just reground the cams to stock, or recommended new parts.

I was lucky here in BC to have a friend who worked at an aerospace facility (McDonald Detwiller/MDA) and he coated all my camshafts for me. (he also did NDT testing with industrial xrays.... Nice to have friends in high places).

I never had a problem with the Harleys or Triumphs, but I will admit to never doing the actual assembly. I only did some of the machine work for the local bike rebuilders.

And finally this work I did was in the mid 1970's. Lots of equipment has changed since then (although Sunnen CK10s are still the standard). I used to grind valve seats with abrasive stones!!!. Now the new cutters do a fantastic job. Relatively unskilled labour can do a perfect 5 angle valve grind.

What I could have done with the new cutters on the old Chrysler hemi engines.... sigh:eek:
 

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If you have a shop locally that does work on vintage Brit bikes, talk to them about who they have do their cylinder work. Of just have them send it out for you. Don't trust your vintage jugs to a machine shop accustomed to cutting small block Chevy cylinders ... it is a much different science.
 

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Discussion Starter #9
Thanks for all the info so far:)
I did some remeasuring on my cylinders and found something pretty odd.
I took measurments at 4 places down the bores as well as at the top lip and the bottom of the bores. since most of the wear occurs in the top half inch of the bore i expected the wear there.
on both bores there was a variance of .002 from the bottom of the stroke to the top lip.and from the bottom of the stroke to the bottom of the bore the variance is .003.
The strange part is that the ring gap and piston clearance is well within factory specs. If it wasn't for the variance on the bottom of the bore it wouldn't seem odd at all.
I think mikestp was on to something about the bores being off at the bottom if the machinist doesn't have any expierance.

I wish I could find someone that works on old british iron here in southern oregon, but I haven't found anyone anywhere close yet.
 
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