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October 23, 2018, 05:42:22 AM
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  Need boring or replacing?
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   Author  Topic: Need boring or replacing?  (Read 174 times)
oly_man
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Need boring or replacing?
« on: May 31, 2018, 05:49:00 PM »
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So I finally got deeper into the 71' 640 tnt engine.  I have new STD pistons for it, but I think that the cylinders are no good to just install these pistons.  I only have a set of flat feeler gauges to measure with so I took the new top L rings and put them inside each cylinder.  Started at 10mm from the top, measured the ring-end gap, then 20mm, 30 etc.  They are definitely out of round.  Looks something like this:

mm  L    R
10  23    21
20  24    22
30  28    24
40  30+  28
50  32+  30
66  32+  28

Those are "thou" numbers, and the + just means that its close to that number but smaller than the next size I have to measure with (35 thou).

Am I right thinking that these are too far out of round?  Time for better cylinders?  Or finding good non-tnt cyls and doing the dremel work to make them tnt cyls?

« Last Edit: May 31, 2018, 05:52:07 PM by oly_man » Report to moderator Logged
oly_man
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Re:Need boring or replacing?
« Reply #1 on: May 31, 2018, 06:18:02 PM »
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See if this works...
https://photos.app.goo.gl/jZh3l20QGBt5I50N2
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GaWajn
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Re:Need boring or replacing?
« Reply #2 on: June 1, 2018, 05:01:20 AM »
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Before giving up on this ... try to beg/borrow a bore gage or an inside telescoping gage to try and get an accurate measurement.

If that doesn't happen ... maybe your local engine rebuilder shop can measure them for you and give you an estimate on the honing/boring job. The measuring is a free service at my local builders shop. Maybe it is at yours???
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1970 Ski Doo TNT 292 Bogie (Restored)
1974 Ski Doo Elan (Habs - custom)
oly_man
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Re:Need boring or replacing?
« Reply #3 on: June 1, 2018, 10:34:33 AM »
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Good point.  the jugs are off and cleaned up now, so likely worth bringing them in for a measure.  Should bring the slugs in to measure the slop at the same time.

Should invest in a bore gauge too...
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Green and Gold
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-Doos, Superbirds, & SAC forever

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Re:Need boring or replacing?
« Reply #4 on: June 1, 2018, 12:22:36 PM »
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Your question brought back some flashbacks (not all good) from HS geometry.    There’s a classic puzzle/riddle/thing out there that goes something like “if I  had a rope pulled tight around the earths equator and added a 3 foot section to it, how far about the earth would the rope now be? (Assuming it was supported equally)  Here’s a web page that explains:

http://puzzles.nigelcoldwell.co.uk/fortyone.htm

The math on the page points out that you can’t add a linear amount (your feeler gauges thicknesses) to an arc/circle segment (your rings) and directly use that as the amount your circle diameter (cylinder bore) increased...      So you can’t compare the measurement you took to any Cylinder wear STDs without additional computations.    If I read it right, you’d have to add the feeler gauge thickness to the original circumference of the rings then divide by 2PI.    That would give you a radius, which you’d double to get a diameter/bore.    Whew….. Some other potential measurement error doing it this  way, for example stiff rings bridging oval areas in your bores.

Have you got the tech data/wear limits for that engine?  If not I can dig them up.

What you need are a set of telescoping T gauges and a big enough micrometer and you can go to town.  With the right technique you can get repeatable results with inexpensive Ts, but don't skimp out on the micrometer.  Pretty good quality used analog ones are often for sale on our favorite auction site; stick to a name brand (Mititoyo, Starrett,  Brown and Sharpe, etc).  Try to buy one with the correct standard included so you can calibrate it from time to time.

The dedicated bore gauges you see also require some sort of standard to calibrate them for use.  The  3 point kind use ring standards, which get kinda pricey.    The two point gauges can be calibrated with less costly stuff (mics, calipers, height gauges) but offer the same kind of accuracy you can get with the simple T gauges at a fraction of the cost. 

And yes, take the pistons with you.  A good machine shop will bore to the correct size only after measuring the piston.  You'll have to let them know what kind of clearance (piston to cylinder wall) you want.

IMHO, If the shop balks at that and says they just bore to a book standard, pick up you're stuff and find another shop.  Too much variance in piston size.
Good luck
   

 
« Last Edit: June 1, 2018, 12:25:03 PM by Green and Gold » Report to moderator Logged
oly_man
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Re:Need boring or replacing?
« Reply #5 on: June 1, 2018, 04:35:56 PM »
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Man you make a lot of sense.  I know exactly what you are talking about with the earth and rope theory.  I was going along with that ring end-gap theory but all that really tells you us there is "a" difference.  I am going to try a local shop that a friend of mine uses for his vintage cars.  Interesting to see where that leads.  And I will be happy to keep this link active with what I find.

I tried yet another pic hosting site... see if you can actually see a picture without going to a link of mine... let me know.  Might be a full size pic...

there thats better.  Finally.
« Last Edit: June 2, 2018, 07:49:59 AM by oly_man » Report to moderator Logged
rkalter
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I love TNT's

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Re:Need boring or replacing?
« Reply #6 on: June 2, 2018, 06:53:14 PM »
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Hi Vince. That pic showed up perfectly all on its own.
Ray
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1970 Skidoo TNT 292 Ratsled
1971 Skidoo TNT 340
1971 Skidoo TNT 775
1976 Skidoo RV340
1996 Skidoo Formula III 600
2008 Skidoo Renegade 600 sdi
oly_man
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Re:Need boring or replacing?
« Reply #7 on: June 3, 2018, 07:02:32 AM »
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Thanks Ray.  (Lensdump.com by the way)
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