Topic: Piston coatings and tolerances--how tight is too tight?

[jessezm]

A few questions for those with experience using Tech Line DFL coatings on their piston skirts:

I thought I read that these coatings don't need to be considered when setting your tolerances, but I do notice that my pistons are a little more snug in the cylinders now.  I can still get a .0015 feeler gauge in at the top of the bore, but at the bottom of the barrels it's very tight.  I can still squeeze the gauge in there, but not without a bit of effort...  Should I assume this is right where I want to be, or is there any danger that this is too tight?

I had the domes coated with ceramic coat and the skirts done with DFL, as I've heard good reviews about these products and their ability to keep heat where it's supposed to be.  Since my motor is not stock (466cc kit, cam, ported heads, bored carbs, headers), I figured it couldn't hurt.  But I just want to make sure I'm right about that!

I'd be very interested in folks' experiences with these coatings.  Thanks!

[crazypj]

Using a feeler gauge means the clearance is more than 0.0015" so don't worry (unless it's a big overbore?)

I use around 0.0015"~0.0017" using stock cast pistons without coatings, even on 2~3mm overbore

[jessezm]

It's a 3.5mm overbore (from 51mm to 54.5 mm), which is as big as you can go on these without re-sleeving.  My machinist said he bored to .0015", but it did seem a tiny bit looser than that before I had the pistons coated.  I think I was able to squeeze in a .002" feeler gauge at the top of the bore before.  I definitely can't now, though.

[Retro Rocket]

Just going to subscribe here as i am very interested in this. I posed the exact same question about piston coatings to Mike Rieck, i have some 1000cc forged pistons going into a billet aluminum block with Nikasil bores and Mike told me no to do the skirts, only the tops as it would be too tight in the bores.....

[KJ790]

The coatings commonly used on piston skirts can be applied at a variety of thicknesses. If applied very thin they shouldn't affect your piston to cylinder clearance a significant amount, but it all depends on how it was applied. A before and after measurement is the only way to know exactly how thick it is. Right now you should use a micrometer and a bore gauge to measure your piston diameter with the coating and your cylinder diameter. Subtract the piston diameter from the bore diameter and you will have your piston to cylinder clearance.

[crazypj]

Forged pistons need more clearance as they expand further than cast
I would put it together and run it in for 300~500 miles.
 If it's a 'race' motor expected to take full throttle withing 20 mins, have cylinders honed another half thou (0.0005")

[Retro Rocket]

Forged pistons need more clearance as they expand further than cast
I would put it together and run it in for 300~500 miles.
 If it's a 'race' motor expected to take full throttle withing 20 mins, have cylinders honed another half thou (0.0005")

Its not a race engine , just a hot street, i think i'll just coat the piston tops..... Hoping to run it in on a dyno...

[rudi]

Hy guys, I am using the coating on the Piston Skirts on my Honda CB 400/460 ccm³ since many Km/Miles. I did not raise the Piston /Cylinder Running Clearance after the Coating. The first 3000 Km/ 1900 mls. the Engine is not reaching under all conditions his full max Rpm. but from this point on the power increase smoothly . The Engine Noise is much quiete than on uncoatet Pistons. Piston Type : Yoshimura 54 mm , Camshaft : Yosh. ST 1   Engine is Blue printed .
Sorry for my bad English , I am an German Honda Owner

[jessezm]

Awesome info, thanks!  I'm going to run it as is and keep things mild while I break in the engine.

I wish I had a good set of inside and outside mics but I just have my cheap HF digital caliper that won't give me the info I need.  I may take the block down to the machine shop and have them measure the bores and pistons for me, but the fit looks and feels pretty good.

One thing I noticed, though, is that the bores are a little out of round.  I can slip a .002" feeler in on the piston-pin side of the skirt, but the .0015" gauge barely fits at 90 degrees to that.  If I spin the piston 90 degrees it's the same, which tells me it's the bores and not the pistons.  Same in all 4 cylinders.  Is .0005" out-of-round going to be an issue at all with the rings sealing, or should I expect this to even out as I break in the top end?

[crazypj]

Bore gauge and outside micrometer are more accurate, even a cheap bore gauge will work if you only use it as a comparator (although 0.0001" is a lot more accurate, it costs at least $220.00)
You can get one for less than $60.00 but still need to do several engines to justify cost
http://www.victornet.com/subdepartments/Inside-Measuring-Tools/110.html
BTW<
that site has best prices on a lot of measuring equipment, good budget micrometers (VIS, Made in Poland, as accurate as $150.00 mic, just not as 'pretty' or known name)

[jessezm]

yeah that's the problem--I've bid on a couple sets of good gauges on ebay but the price always got too high to rationalize, but at this point I just really want a set! 

I did just break down and take the cylinders to the machine shop, and everything checked out fine, so it looks like I'm good to go.