piston to cyl wall clearance

I just tore my motor down and found the pistons had scuffed the cyl walls pretty bad and I am taking it to the machine shop to have it honed if possible. What kind of clearances do you guys recommend. I have a 4.5" bore Merlin Pro cast iron block Arias 11.5:1 pistons, running 35+lbs of boost no water in block. I like my cyl temps in the 1150 range. .0080 was what Arias recommended. How big can a I go and still be safe. I don't want to buy new pistons.
Thanks
Adam

 

I don't run less than .011 clearance. Also, make sure you don't gap your rings too tight, it will have the same result. I run .035 or more on the top ring, and .032 on the second.

 

we run exactly what Ken said in our motor......seems to work really well.
Darren

 

Also, your oil can cause this.

 

Thanks for the replies. The wierd thing is that it is scuffed past the ring land area on the cyl walls almost to the very top of the cyl. I will gap my rings to your recommendations. I am running Hell Fire top ring.

 

Almost sounds like your piston skirts are worn and letting the piston rock in the bore. Does the top of the piston above the top ring show any scuff?

 

Are the piston skirts scuffed also? If they are, stage temps to high will cause this.

 

Yes, the pistons are scuffed above the ring lands. I measured the clearance last night at the skirt and I could barely fit a .008 feeler gauge between piston and cyl wall there was no way a .0011 gauge would fit. I did not notice skirt being scuffed but will check tonight when I get home. If this helps any, it seems like the left bank did not do this near as bad but the right side esp toward the front cyls are scuffed more.

 

Dude,Piston to wall clearance is measured with a dial bore gage and in your case a 4'' to 5'' micrometer your not adj. valves. when you get a mic. measure 180 degrees across from center of wrist pin, measure bore with bore gage subtract mic reading from bore reading and thats your clearance. By the way do the pistons have black scuff marks?

 

I know to use a dial bore gauge but I don't have one or a mic big enough to go around the piston so I was just trying to get some idea of what might be wrong. It's going to the machine shop today or tomorrow no matter what to see what can be done. I think/hope it's just too tight but who knows. Losing a flexplate out the bottom of your car just kills the race car budget.

 

Hey Adam....my pistons measure 4.300" across from the pin and my sleeves are 4.310" - 4.3105" when new....I have also had some scuffing issues....some are due to lean or loose bore (above the ring) or rich (skirt damage) fuel conditions.....so I'm gonna try coated pistons for 2010 with new sleeves....mine were all measuring 4.314"-4.316" and .002" - .003" out of round/taper....basically I ran em too long....and also had issues with blow by/puke tank mess that is probably related....so this is off topic a little....coated pistons with .010"-.011" - new round cylinder bores - new on board computer so I can see just how balanced the cylinders are....I know this is more about my situation than yours....but it's info for you on where I wanna start for 2010...hope it helps....

 

Thanks Scott, hopefully it's not bad. The pistons don't really look that bad but 3 cyls on right bank have grooves in various spots.

 

Are they partial grooves (eyebrow shaped)? Could be borderline detonation, since I know you've run this combo quite a while without teardown, even extremely minor detonation can rock a piston over time enough to cause the ring grooves you're describing. Also, not sure if you're running blower specific pistons, they normally have more skirt surface than a lighter weight piston to give more stability under blower loads, just curious.

Scott, some of your piston issues might be related to Mohawk Boy's right foot......

 

Yes, they are blower specific pistons. Slight detonation could be the problem. All the grooves in the cylinders are perpendicular of the wrist pin and toward the outside of the block particularly on the right bank. I was inspecting the pistons tonight and found a slight even wear mark around the middle portion of the skirt, right where you measure the diameter of the piston for installed clearance. I don't tear my stuff down very often, this is only the 3rd time in 4years I have had the pistons out. I have replaced all the pistons 1 time and a couple holes have had them replaced twice.

 

That would be the thrust side on the right bank, look closely on the left side cylinders, there may be similar grooves on the inside (cam side) of the left bank.

 

If the pistons are scuffed above the ring lands, then the skirts are collapsed. Time for new pistons. Where in the cylinder is it scuffed? If it's towards the exhaust, it could be fuel wash.

 

Adam, one other thing I forgot to mention...my new pistons have .003' - .0035" or so taper - so the skirt is 4.300" at the top and 4.303" at the bottom....and I run em until they are zero taper and then pitch em...tear down every 10 runs or so and measure...

 

Also, I was told by Joe Pisano @ Venolia Pistons that a piston has an A,B,C,or D, cam ground into them?

 

Took pistons and block into machine shop and he said the block is not the problem. Piston skirts are collapsed on at least 4 pistons. He also thought it was wierd for them to collapse like that since I run a full skirt. He recommeded getting a .005 bigger pistons and maybe change manufacture he also thought there could possibly be a problem with the metal. Has anybody had any concerns with Arias. I run there pistons in my NA injected alcy car also.
I just got my crank back last week from having it balance b/c I put in thicker wrist pins. How many more grams on the piston can I go before I have to have it balanced again? He said if I go with a .005 to .010 bigger piston that I would still be fine. I need a sponsor
Thanks for all the replies and help.
Adam

 

If you have the weight of your old piston, you can have bigger pistons made to the same weight by the piston manufacture, just tell them the weight you want, I balanced motors for 20 years, had my own balancer, when some one tells you you can get away with 5 /10 tho. its bullshit, ask them if he would let some machine work go out the door thats .002 smaller than it has to be? your talking about $50,000, motors everything is to be perfect.