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Tractor Talk Discussion Forum

pistons and ring question

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dx29

12-05-2006 11:33:33




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Whne you buy a piston overhaul kit that is .045 over - is it just the rings that are .045 over or is the piston bigger also?
I thought wheny ou go .045 or bigger you just use your standard size pistons with larger rings to make up the difference (.045 in my case)




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davpal

12-05-2006 22:18:31




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 Re: pistons and ring question in reply to dx29, 12-05-2006 11:33:33  
I think you might mean something different than you are saying. If you still have standard pistons then you don't have a .045 oversized cylinder because you would be able to see about 1/16 of an inch of daylight between the block and the piston and the tractor would not even run. 45 thousandths is quite a big overbore. I would bet your machinist meant .0045 which is four and a half thousandths. That would be about the right piston to bore clearance. That is why engine builders get sizes like 4.035 and 4.065 and 4.005 so they can file fit the rings to the correct bore size. A lot of race engine builders like the extra .005 thousandths on there to custom finish it. In other words, yes the piston needs to match the bore size too! Good luck.

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dr.sportster

12-05-2006 13:42:45




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 Re: pistons and ring question in reply to dx29, 12-05-2006 11:33:33  
If your at .045 over now and you have .008 taper in the cylinder then you would need ,060"over pistons.Also it seems highly unusual that you could be at that much wear from standard pistons.Are you taking the dimension of stock piston from the manual or miking the ones from the motor[which might be .040"over with .005 clearance and wear]Pistons come in .010 increment oversizes normally.What Im trying to say is there is no way you have standard pistons with .045 wear,or could be miking the piston and finding the cyl bore .045" bigger.When parts books state oversize ring sizes it is for that oversize piston also.You need to give more info for this to make sense.

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noncompos

12-05-2006 12:03:32




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 Re: pistons and ring question in reply to dx29, 12-05-2006 11:33:33  
glennster has a good point: if that"s a modern, or fairly modern engine, 45 over is getting to the point you might not want to stress that engine too much, to avoid cracking, BUT it depends on the engine, how much material you can bore out and still be within design limitations.
As an example, I"ve got a piston catalog from 1924, when US engines were much heavier, that says there"ll be a surcharge for pistons cast 60 to 80 over!!

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glennster

12-05-2006 11:38:43




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 Re: pistons and ring question in reply to dx29, 12-05-2006 11:33:33  
the piston and rings are for a .045 overbore. the block should be bored to match the oversize pistons. thats quite a bit overbore, has the motor been bored before, or is there a cylinder wall problem?



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dx29

12-05-2006 14:00:59




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 Re: pistons and ring question in reply to glennster, 12-05-2006 11:38:43  
I have a JD model 50. The bore was measured and I was told by the machinist that it was bored .045. The pistons are original cast iron. I guess my question is - Is it possible to just get rings that are .045 over and put them on a stock standard size piston or do you have to get a larger piston also?



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dr.sportster

12-05-2006 18:16:07




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 Re: pistons and ring question in reply to dx29, 12-05-2006 14:00:59  
If he is measuring the old worn bore you need to make a very cylindrical hole again to see where you stand.A worn cylinder bore has taper and wider areas.You will need pistons to fit the size of the clean overbored dimension.If there is scoring it might be best to bore out first in case of ordering the new pistons one size too small.If you are at .040" piston now and need .050"piston plus running clearance ok.But if at that dimension the cylinder is still not cleaned up then youll have to go up to the next size piston .060".Plus running clearance.Its hard to mike a cylinder with gouges. You have to estimate the depth of damage or refinish cyl to a round bore then order the size you need.Putting new pistons and rings without resizing the cylinder bore back to round is a waste.Another way is to go two oversizes at once to ensure any damage is gone after boring.Most gouges are not any deeper than that.The trouble with that way is your losing one future top end job.Oversize rings are for the oversize piston.Example;.040" piston takes .040"ring.If your cylinders the machinist measured are refinished, bored and honed,to .045" now and the running clearance is .005" then all you need is .040" pistons and rings but you would still have to mike out the pistons and determine their true size.Some manuals will tell you how to use a shim or feeler strip and pull scale to determine this clearance without mikes.Using the feeler strip of the dimension given for clearance say .002 and attach a scale to a small hole in the strip you put the new piston into the bore and look for a certain amount of pounds pull.If the spec is 7-18 lbs pull and you have 25 lbs on the scale you must hone more as the piston is too tight.If you put the scale on the strip and you get less than 7 lbs stop honing right now as the piston is becoming too loose in the bore.Dont force the piston into the bore hard against the strip or you will scratch the side of the piston.Its possible to not use the scale and just drag the feeler strip by hand,maybe not as accurate but I doubt it will cause engine seizure.I had the strips before I had the scale.The feeler strip method is fairly accuate as there is nothing lost in the math of mikes and snap gauges.

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Gerald J.

12-05-2006 16:07:45




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 Re: pistons and ring question in reply to dx29, 12-05-2006 14:00:59  
Yes, you can put over size rings on a standard piston. But you will be annoyed by a loud bang as the pistons flop from side to side in the cyinder right about the time the plugs fire.

Gerald J.



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dx29

12-06-2006 03:36:06




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 Re: pistons and ring question in reply to Gerald J., 12-05-2006 16:07:45  
Thanks,
Ya'll cleared it up for me.



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