correct piston installation

The usual rule of thumb is to install the new pistons dry in the bore but I am having a problem with vertical scratches forming very quickly front and rear. I am VERY CAREFUL to clean the cylinders after a fresh bore / hone using ATF then soap and water then more ATF. Takes a long time but necessary. I also wipe down the pistons and rings. So why am I getting these scratches?

seattle##gs said:

The usual rule of thumb is to install the new pistons dry in the bore but I am having a problem with vertical scratches forming very quickly front and rear.

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Try fitting new pistons with lubricated rings instead of dry :wink:

Well no offence, the cleaning is absolutely necessary, there´s a lot of sh-t left in the honing patterns. But installing the pistons dry is new to me, doesn´t make sense, they will run up and down in oil when you start the engine, so why install them dry?
Tommy

After cleaning your bores after honing (they need to be rid of all the crap from honing), I like to use min of oil in the bores after fitting the rings as less oil will bed in the rings quicker, but you do need to put oil on the rings to slide them in the cylinders with ring compressors, if you don't put oil on the rings they will scrach the bores, so only enough oil to lube the rings (don't drown them in oil) no need to put oil on the pistons just the rings and they will bed in quicker, to much oil in the bore and pistons and you will or might have the risk of glazing your fresh bores, it has happened to me the first time I rebuilt my motor as I drowned the pistons and bore with to much oil.

This is my opinion only others will have their own opinion, so for me the less oil on the rings better breakin of the rings, to much oil and you risk glazing the bores.

Ashley

From what I gathered the "dry assembly" method started when modern oils came into existence. These oils (SL and SM) with friction reducers prevented the cast iron rings from doing the final bit of metal removal between the freshly honed bore and rings. The result were glazed bores and smokey engines. Practices were changed to include using the earlier type (SG) oils for break in, coarse honing, scrubbing the cylinders with hot soapy water and just a spot of oil on the piston skirts, per John Healy's at TIOCs numerous posts on the subject. That's how I did my 72 750 and I got excellent ring seating. Little bit of smoke on start up for about 30 seconds and then great compression.
Question is what contributed the most. My money is on the switch back to the earlier type oils. Before the newer oils, people lubed the rings and pistons and had minimal trouble seating rings. I'm getting ready to build an 850 and will use the SG type oil but maybe a bit more of it on the ring and pistons. Just seems risky to have two new moving parts running against each other with no lube in place until the crank splashes some up there.

never did dry....don't do synthetic oil, particularly on build up/run in.....don't do ring compressors either....its why you have 'opposable thumbs' :roll:

"its why you have 'opposable thumbs"

I thought it was so they didn't get broken swinging from limb to limb.

I have never had a problem with rings sealing on dozens of rebuilds, 2-4 stroke, air & liquid cooled, big & small. The dry fit thing will never be used in my garage as long as I'm still there. Goes against everything logical. :idea:

Every engine assembly line I've ever worked on (quite a few now) has always lubricated pistons upon assembly.

Most modern engines now come with crazy 'first service' mileages, so they clearly cannot be built with 'bedding in oil'.

So there is something about this dry piston idea that I don't understand.

I have never built an engine with dry pistons either.

Fast Eddie said:

I have never built an engine with dry pistons either.

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Or me.

There's an interesting discussion archived on the NOC site (http://www.nortonownersclub.org/support ... ston-rings) which is rather much in favour of the dry ring method, including support from several people who's views I respect - Kevin Cameron, Vern Fueston, etc.

seattle##gs said:

The usual rule of thumb is to install the new pistons dry in the bore but I am having a problem with vertical scratches forming very quickly front and rear. I am VERY CAREFUL to clean the cylinders after a fresh bore / hone using ATF then soap and water then more ATF. Takes a long time but necessary. I also wipe down the pistons and rings. So why am I getting these scratches?

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not really a rule of thumb so much as a quick and dirty method for breaking in track bike engines where frequent top end work is the norm. having ensured clearances on pistons and ring gap, install with 30W on the surfaces

I have talked with 3 shops who have been building motors for a couple of decades or more and they recommend fitting them dry so I took their advice. Next time I will try a thin smear of oil and see what happens. It makes sense to lubricate it slightly but I have been following instructions from people who know a lot more about it than I do. Time to try it with a little oil