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Matthew in TO
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Engine top end is apart and pistons look ok. A little wear on front and back of piston skirt about the last inch. They are .040 over. There is a ridge at top of cylinder. I am takeing it to work tonight to maisure bores. How much wear is permitable without reboreing? The bottom of the cylinder looks a little thin to take it any further. I am hoping I can get away with honeing them. help

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I think you can go with rings maybe .030..at most..but sometimes you pull her down that far better to just go .010 over with new pistons rings ...but i think you could just do rings thats what i would do..

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.010 more......

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Hello.
Just a little remark. - After honing before mounting new rings, I would follow (Dyno)Daves procedure for preparing the cyl(recently posted on this board):
1. Wipe cyl clean with dry cloth (real clean)
2. Put cloth in a dash with oil and wipe over cyl
3. Then wipe cyl clean once more - oil just reside in honing "scars".
4. Clean & Dry piston rings

(Maybe best reading Dave's posting - Norton Board I think).
I'm in the middle of "my new piston ring exercise" (Norton Commando 850). Honed, new rings, wiped precisely after Dave's receipt, carefully assembled. Now, after approx 4 hours (between 3000 and 4000 rpms) results are:
- No smoke what-so-ever (just come in after 5 quarters)
- Oil level dead constant (seems almost lifting on the pin)

I'm aware of yet to low mileage, but so far it seems very successful (my engine burned approx one litre per 1000 kilometers! It was way to much and it's funny/interesting to go in depth in this theme)

Guess it's a goal getting the oil consumption down to "an agreed norm" (less than half the mentioned rate). - For your BSA too, learningcurve, I hope :rolleyes:

regards
jangg


'73 Commando Basket - new aluminium cyl
'93 Ducati 900 SS

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Beezerbill, I took what you said to mean that I can buy a ring set for my .040 over pistons that are + .010 more in the rings. Where can I get these?
Or did you meen to go another .010 over and I would be ok with regular rings for my .040 over pistons.
I maisured both bores with dial bore guages, And they ranged from .002-.006 more than the 2.992 they should have been. Maisured at 6 places up the bore, also checking for out of round condition..................Any ideas welcomed.

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You're already on +40 thou. There's a ridge at the top of the bore? You need a rebore to +60thou, new +60 thou pistons and rings. No problem. A little thin? Let your reborer see it and decide.


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Learningcurve, A question about that "ridge"...

Is it a "metallic ridge" or is it the normal "carbon build-up band" around the top of the bore ?

If a metallic ridge, then I agree with the others' advice on a rebore. If it's the normal carbon band & you feel that the bore is O.K., you might be able to just go with a new set of .040+ rings w/o rebore. In which case, you want to leave that carbon band undisturbed but still rough up the rest of the bore with 300 grit.


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Is there space for a new casting?

May I see a picture?

When some people want to change their two-stroke cylinder radically, they get a new casting in, but I'm not sure that there is space enough for that on yours.

http://www.lokke.dk/steen/kreidler/78cc_cylinder/78cc_cylinder.html

You can see some of the process of getting a new casting in here!


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Hone the bore first, then measure.

If the piston skirt clearance is anywhere near .004" then slap some new cast iron rings in there and run it. Piston scuff marks don't mean anything except the piston is scuffed. You're not going racing. You're not even going to put 3000 miles on this bike this next year.

Spending money to bore the cylinder and buy new pistons when there's no need is how this bike got out to +.040" to begin with.

That whole ordeal is going to set you back about $500 when you count in gaskets and all the time lost. Tell you what. You send me $250 and you keep $250. I'll send you a certificate that says, "My bike was just bored." You can frame this certificate and hang it on the wall. Then when friends ask, you can point to the certificate and say "My bike was just bored. And I saved $250!"

What a deal.

:bigt:


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Although I don't quite agree with Mr. Whatley's advice, I do agree a rebore is not always necessary.

What you half to watch for is the wear at the rear of the cylinder wall on the bottom and the front of the wall at the top. The wristpin location in the A65's is quite low and that makes the piston tend to rock back & forth at TDC and BDC and makes the cylinder wear in an oval shape in those locations.


Jeff


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My A65 was also out to .040" and I was worried about the liner. I just had the barrel bored until it cleaned (about .010), then had Venolia make up a set of pistons to fit. Cost about the same as the old crap stuff, and you get nice modern piston, chrome top rings, 3-piece oil rings and a looong time before your next teardown. (There are other piston makers out there who will also make nice pistons to order.)


When people who should have known better cautioned me about the dangers of motorcycle racing, I always told them that a fear of death is nothing more than a fear of life in disguise.
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The cylinders can be resleeved so it is not question that you can't make something work. If you have.006 at the bottom, it won't be long before the pisons are slapping, but I have rode a long way with slapping pistons.

I have not heard of using larger rings than what is supposed to be in a piston. I am assuming (perhaps incorrectly) that's what "beezerbill" was saying. Am interested in more comment on this. When I was in college and broke, I would hand hone the cylinder to break the glaze and just stick new rings in it. Not always the best solution but you do a lot of things differently when you are broke.
Mr Mike

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When I was a little drag racer the ring manufacturers used too and probably still do make piston rings in over sizes such as .025" .035" etc. The reason for this was so we could file the end gap on our rings to just exactly what we needed for clearance... The ring manufacturers usually err on the side of safety in that they give the end user enough end gap that they won't have any problems and that has to suit a wide range of applications and driving conditions...
I would tear the engine down for inspection every week and it'd make me very happy when I could see that the rings ends were very lightly touching each other, it just don't get no better than that!!! It was also very easy for me to control the conditions under which my engine was being run, the temperatures, pressures and such, I would never suggest running the ring gaps that tight in a street driven engine where a person is likely to encounter traffic tie-ups and long periods of slow speeds...

Cheers...


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Using a ring .010" larger diameter than the bore size requires a lot of metal to be removed from the end(s). Circumference = Pi X D, so .010" subtracts .031" from whatever end gap the ring was designed to have.
Assuming you want to do this, the ring manufacturers are almost universally against it because the increase in radial tension ("springiness") is high and results in premature wear and higher surface temperatures.
The .005" interval oversizes are of course intended for this use, and generally higher quality than normal replacements (ductile, steel, stainless rather than cast iron).
The major piston mfgs. (JE, Venolia, Ross etc.) will make any oversize you like, but not all will supply only 2 pistons.

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Again -
Hone the cylinder first. You're never going to assemble it back with the old rings and you'll have to hone before adding the new rings, so the as-honed measure is the TRUE measure. You only need the finest grit "glaze breaker". You do not want to remove metal.

Secondly there are several items to measure, not just ring end gap. First you measure the piston-to-bore clearance. Ignore the scuff marks. What does this measure?

Then if this measures more than .005 or .006" you have to determine is it piston wear or bore wear? Usually it's some of both.

If the bore is good, sometimes you can have your pistons re-sized and the extra slack will disapear without having the expense of new pistons. This is very common for car pistons. This is a practical solution since piston-to-bore only results in "piston slap", which is a tinkling noise at idle.

Then measure the bore for out-of-roundness.

If that's OK, then buy a new set of +.040 rings and check them in the bore (.30" down from the head gasket surface) for correct "end gap". I prefer black cast iron since they retard bore wear, seat almost instantly, and are easy to fit/file. I would not try +.050 rings since that would indicate excessive bore wear that would be discovered in your bore measurements.


There are 2 things young people are very apt to forget about these bikes:
1) These engines were designed well before the science of engines came into being. If you were raised on Honda tolerances and fits, then forget everything you know when you work on Brit bikes. These bikes are perfectly happy and will run for years with fits that would make a Jap bike sound like a punch press.
2) You bought the bike to ride. If you wanted a bike to worry about fits, then go get a Honda and you can work to 5 decimal places. If you want a bike to polish, then go get an H-D.

You can spend a million dollars if you like, but a) it won't go any faster, sound any better or burn any less oil, and b) all the parts will be out of tolerance again within 6 months. And that's how your bike got to be +.040 with only 12-15,000 on the clock.

:bigt:


Don't hide 'em, Ride 'em !!

RF Whatley
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Make life easy, bite the bullet and contact TMS in Nottingham UK. They can supply new +.060" pistons from stock and are good quality plus good service all for 90 pounds [email protected]
Regards, Stoney.

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not knowing your intended use I go with whaley. If it is not exssesivly out of round or .008 piston to wall clearance put in new rings after honing and ride it.
Tony

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I have been away for a week,and have not posted.
Thanks for the replys. I will hone bores this weekend and maisure. I would be happy to get away with doing this and rings being the only expence.
I intend to ride the bike as much as possible next season. Not planning to hotrod it too much, but we all like to grab a handfull of throttle now and then. Just want a good running engine after rebuild.


Moderated by  Allan G, Jon W. Whitley 

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