Re: pistons and ring question

Posted by dr.sportster on December 05, 2006 at 18:16:07 from (67.72.98.89):

In Reply to: Re: pistons and ring question posted by dx29 on December 05, 2006 at 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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