File fit or not

[Geoff 2]

Nobody has mentioned the most important thing when filing Moly rings, which most top rings are & sometimes the 2nd ring. The Moly is an insert. You should file/grind inwards so as to not chip or dislodge the Moly.
Numerous tests have shown that slightly larger ring gaps do not cause any noticeable HP loss & practice these days is to make the 2nd ring gap slightly larger than the top ring.
The worst thing you can do is have the gap too small, which can cause bore scoring, glazed bores, etc.

 

[HawkRod]

I have file fit rings with a hand file, and I totally agree to only file inwards. This also avoids sharp edges that can score a bore.

File to fit rings are tedious unless you have one of the below. Hopefully I'll be using that this Saturday to do mine...

 

[BSB67]

Should I use file fit or standard rings?

Standard rings.

 

[DanielSlocum]

for every .001 increase in bore dia. ring gap will open up .0035 . so if you increase the bore dia. .002-.003 to get proper forged piston skirt clearance..... then your ring gaps will open up .007-.0105. most likely your non file-fit ring gaps will then be close to .030 which is way too much. if your bores measure exactly .030,.040,or .060 over std. ........then non file-fit ring gaps will likely be about .020 which is very good. hyperutectic pistons may require wider gaps due to the ring grooves being closer to the piston tops.
good luck.

 

[dvw]

for every .001 increase in bore dia. ring gap will open up .0035 . so if you increase the bore dia. .002-.003 to get proper forged piston skirt clearance..... then your ring gaps will open up .007-.0105. most likely your non file-fit ring gaps will then be close to .030 which is way too much. if your bores measure exactly .030,.040,or .060 over std. ........then non file-fit ring gaps will likely be about .020 which is very good. hyperutectic pistons may require wider gaps due to the ring grooves being closer to the piston tops.
good luck.

You are correct about .001 bore increases the bore .00314. However skirt clearance is built into the piston. Example for a 4.00' bore. A .002" clearance piston would measure 3.998". A .006" clearance piston would measure 3.994". Tight ring gap really isn't a large concern. Sealing the ring to the wall and piston is where the power is. You could run a .030" gap and it really wouldn't phase it. To small a gap will kill it.
Doug

 

[Geoff 2]

And the larger ring gap with hypereutectic pistons is not because the rings are closer to the top. Nearly all pistons are made that way to reduce crevice volume. The HE piston material runs hotter than other alum grades, heats up the rings more, hence the need for bigger ring gaps.

 

[RemCharger]

And the larger ring gap with hypereutectic pistons is not because the rings are closer to the top. Nearly all pistons are made that way to reduce crevice volume. The HE piston material runs hotter than other alum grades, heats up the rings more, hence the need for bigger ring gaps.

Yes exactly on point! Crevasse volume was never a concern from a engineering view, but the cheap expense of a durable and stable material for a given amount of heat and hp were.
You're on track!!

 

[Roger63]

Slightly different direction, I'm putting in the crank and I checked clearance on each main journal as follows. #1.004 #2 .003 #3.004 #4 .003 #5 .003. Am I good? It's a forged standard crank.

 

[DanielSlocum]

You are correct about .001 bore increases the bore .00314. However skirt clearance is built into the piston. Example for a 4.00' bore. A .002" clearance piston would measure 3.998". A .006" clearance piston would measure 3.994". Tight ring gap really isn't a large concern. Sealing the ring to the wall and piston is where the power is. You could run a .030" gap and it really wouldn't phase it. To small a gap will kill it.
Doug

in my experience.... proper skirt clearance is not always built into the piston. therefore it is sometimes necessary to increase bore size slightly,because excess friction and bore/skirt scuffing is where the power is very much lost. i would never assemble an engine with .030 ring gaps unless it was intended to run very high amounts of boost or nitrous.

 

[dvw]

What piston doesn't have skirt clearance built in? Have you run large gaps in an N/A motor? Tested the results? Static leakdown may not show as good. But on track performace shows no change.
Doug

 

[fourgearsavoy]

No way.....That takes material off way too fast. I have enough trouble with the hand crank file!

Exactly! The hand crank deal works great but if you don't pay close attention you will have some wide gaps.

 

[Geoff 2]

Skirt clearance is built into the piston. Just as rod & m/brg etc clearances are built into the shells.

 

[DanielSlocum]

old school trw forged pistons for sure. also,factory type cast pistons often only have .001-.0015 clearance built in. forged pistons should have at least .004 skirt clearance because they expand as they heat up. high compression ratios and/or any type of boost will require even more than that. less than .002 skirt clearance with a factory type cast piston is not a good idea for a hard running street engine imho. if anyone wants to run em any tighter than i suggest then go to it. if not,then measure those piston skirts before final cylinder hone as i do.
an old saying about engine building: a little too loose nobody will know; a little too tight then everybody will know. regarding wide ring gaps:im simply going by what has been done with high performance engine builds for many decades which is to keep the gaps wide enough to not have the ends butt together and score the cylinders once they get good and hot; while at the same time not so wide as to create any more blow bye than necessary. thats all folks.

 

[Roger63]

old school trw forged pistons for sure. also,factory type cast pistons often only have .001-.0015 clearance built in. forged pistons should have at least .004 skirt clearance because they expand as they heat up. high compression ratios and/or any type of boost will require even more than that. less than .002 skirt clearance with a factory type cast piston is not a good idea for a hard running street engine imho. if anyone wants to run em any tighter than i suggest then go to it. if not,then measure those piston skirts before final cylinder hone as i do.
an old saying about engine building: a little too loose nobody will know; a little too tight then everybody will know. regarding wide ring gaps:im simply going by what has been done with high performance engine builds for many decades which is to keep the gaps wide enough to not have the ends butt together and score the cylinders once they get good and hot; while at the same time not so wide as to create any more blow bye than necessary. thats all folks.

I measured my clearance on skirts and all but 1 are .005 with 1 @ .004

 

[DanielSlocum]

I measured my clearance on skirts and all but 1 are .005 with 1 @ .004

you are good on skirt clearance. .004 is pretty loose for main bearing clearance. are you sure some are that loose? iv never ran more than .003. maybe a couple at .004 wont hurt and maybe someone else here knows for sure? perhaps running high volume and/or high pressure and/or thick oil will get it done for you with a couple of mains at .004? im very interested in other opinions on that. good luck.

 

[Roger63]

you are good on skirt clearance. .004 is pretty loose for main bearing clearance. are you sure some are that loose? iv never ran more than .003. maybe a couple at .004 wont hurt and maybe someone else here knows for sure? perhaps running high volume and/or high pressure and/or thick oil will get it done for you with a couple of mains at .004? im very interested in other opinions on that. good luck.

They were on the borderline of .003 so I called them .004. I did it with plastigauge

 

[HawkRod]

you are good on skirt clearance. .004 is pretty loose for main bearing clearance. are you sure some are that loose? iv never ran more than .003. maybe a couple at .004 wont hurt and maybe someone else here knows for sure? perhaps running high volume and/or high pressure and/or thick oil will get it done for you with a couple of mains at .004? im very interested in other opinions on that. good luck.

I just measured my main bearing clearance (see my post: HawkRod is doing the 2023 Hot Rod Power Tour! ). By the way, a few posts before this you can see pictures of me doing my file to fit rings.

Anyway, my crank was a little bit under, and I measured a max clearance of .0042; too much for a street engine. I am getting .001" over bearings to compensate. I would recommend @Roger63 do the same.

 

[Roger63]

I just measured my main bearing clearance (see my post: HawkRod is doing the 2023 Hot Rod Power Tour! ). By the way, a few posts before this you can see pictures of me doing my file to fit rings.

Anyway, my crank was a little bit under, and I measured a max clearance of .0042; too much for a street engine. I am getting .001" over bearings to compensate. I would recommend @Roger63 do the same.

So I rechecked every main this time using green plastigauge everyone was .003 should I replace or run.

 

[HawkRod]

So I rechecked every main this time using green plastigauge everyone was .003 should I replace or run.

Ideal is in the .0025" range for a street engine. A lot of race engines will go looser than that, but they also typically run heavier oil and there are other factors too.

Others may want to chime in here too, but I'd say if you know they are in the very low 3s (like .0030" to no more than .0033"), I'd run it like that. (As a matter of fact, that is exactly what I will be doing with my 500 stroker. My largest clearance should be .0032".) The problem with plastigauge is it is hard to get a refined number. To get my clearances, I am using a very high dollar Sunnen gauge double checked with a verified micrometer, and then I use the same verified micrometer on the crank. So my clearance values are pretty precise.

So bottom line, my opinion is: If you are confident you have very low 3s, run it as is. However, if the clearance values are mid 3s, you are better off getting .001 over main bearings which will put you in the sweet spot.

 

[Roger63]

Ideal is in the .0025" range for a street engine. A lot of race engines will go looser than that, but they also typically run heavier oil and there are other factors too.

Others may want to chime in here too, but I'd say if you know they are in the very low 3s (like .0030" to no more than .0033"), I'd run it like that. (As a matter of fact, that is exactly what I will be doing with my 500 stroker. My largest clearance should be .0032".) The problem with plastigauge is it is hard to get a refined number. To get my clearances, I am using a very high dollar Sunnen gauge double checked with a verified micrometer, and then I use the same verified micrometer on the crank. So my clearance values are pretty precise.

So bottom line, my opinion is: If you are confident you have very low 3s, run it as is. However, if the clearance values are mid 3s, you are better off getting .001 over main bearings which will put you in the sweet spot.

I just don't have those specialized tools available to me so I have to use plastigauge.