Don Terrill’s Speed-Talk

I recently send some forged JE'S away for a dry film skirt coating by Tech line coatings, they call TLML.
It was done to aid in anti scuff. Pistons went away at 84.42mm and came back around 84.46mm. Given my nominal bore size is 84.5mm that only leaves a piston to bore clearance of 0.04mm or exactly half the JE recommended amount.
Do I run them this way and assume they bed into the bore? Or should I scotchbrite the coating down a little?
I'm concerned they may scuff the bores even more now when they get up to temperature.
Who's used this particular coating before? Thoughts please.

I guess it would depend on if JE called that coating an abradable coating like line2line or something else entirely.

AC sports wrote: ↑Mon Apr 15, 2019 12:30 pm I recently send some forged JE'S away for a dry film skirt coating by Tech line coatings, they call TLML.
It was done to aid in anti scuff. Pistons went away at 84.42mm and came back around 84.46mm. Given my nominal bore size is 84.5mm that only leaves a piston to bore clearance of 0.04mm or exactly half the JE recommended amount.
Do I run them this way and assume they bed into the bore? Or should I scotchbrite the coating down a little?
I'm concerned they may scuff the bores even more now when they get up to temperature.
Who's used this particular coating before? Thoughts please.

What did Tech line say?

That's only about .0015" in our crazy measuring system. On your small 3.30" (84mm bore) I would say on a short stroke that even a forged piston that size might need only a .0055" to .006". So depending on what JE might say they want .007" you will probably be fine depending on what your clearance is set up for now especially as some of that coating will wear off in critical areas pretty quickly. What ifs your measured clearance now with the coating?

Tech Line used to recommend coating the pistons after the block was honed to size. No allowance was given to the coating.

Remember, most pistons are "cam ground" to non-round, non-cylindrical shapes to compensate for thermal expansion; the widest part is usually down at the bottom of the skirt perpendicular to the pin. On engines that came back through my shop, there was always metal showing there, but the rest of the skirt often showed no sign of contact.

Whole idea of piston skirt coatings are that they minimize clearances - skirt coatings are usually lubricants so they won't seize.

FWIW.. for some higher HP factory snowmobile engines (3 HP per ci), the coating is not part of the clearance.

I had a set of pistons that were to loose to use in a bore we had to cleanup with a failure. Keith black said they would apply a coating that would be nearly as wide as bore and would wear into where it had to be.No fitting necessary, they fit tight initially.

That's the premise of Line2Line's (and at least one piston supplier's) 'abradable' coatings: https://www.line2linecoatings.com/

Wiseco make now for Harley Davidson what they call 'Armor X Plating' 'Armor Glide' Black edition pistons,these are installed size to size of the bore,they make their own clearance after being run,,,it took a few calls from me back to the distributor before i could be convinced to do the bores as recommended.
i wanted to add just a fraction,that's all! ,they said i could if it made me happy but wasn't necessary.
The engines i have used these in are very quiet and display no issues at all,they have been in service for over 12-18 months now.
you can be confident the manufacturers recommended installation guide will give good results.

Cheers.

Is there any evidence that skirt coatings have any significant effect on heat transfer from piston to wall?

SELECTION DATA PRODUCT DESCRIPTION: TLML is a thermally cured Dry Film Lubricant. TLML has been designed to provide high pressure lubrication to surfaces subject to sliding, flexing, rotating or oscillating motion. TLML makes use of several design characteristics in managing friction. The coating applies in a thin film that is based on a resin system that allows the coating to be burnished to a film less than .0003” in thickness, allowing extremely tight clearances to be utilized. The coating may be pre-burnished prior to installation/assembly of a part or will burnish during operation. The coating is also designed to aid in the more even transfer of heat, reducing hot spots. This is particularly beneficial in parts that flex such as a spring. The low 300°F cure temperature allows for the coating to be used on a variety of substrates. TLML may be used in conjunction with TLMB when a physical buildup of a part is desired. Top coating with TLML increases lubrication at the surface, while the TLMB creates a hard, durable barrier to wear. TLML may be used on both wet and non-wetted surfaces. The use of a “wet” lubricant such as oil in conjunction with TLML will increase the load that the oil can be exposed to before the liquid will be squeezed away from the part. RECOMMENDED USES: TLML is recommended for use on any component where heat and friction are an issue. Gears, shafts, hubs, cams, splined shafts, wear blocks, springs, bearings (Note: for Babbitt type bearings use DFL-1), pistons, cylinder walls, rotors and more. NOT RECOMMENDED FOR: Substrates that cannot handle the cure temperature. TLML provides NO corrosion resistance.

https://techlinecoatings.com/wp-content ... -Sheet.pdf