Piston skirt lubrication experiment

Drilled, lightened and oil grooved piston skirt lubrication experiments below.

When I started thinking about doing this some people said that I shouldn't or couldn't - so of course I went ahead and did it.

The drilled Hepolites below went well for 60,000 miles until the ring lands loosened and it began to rattle and smoke like a pig. There was .001" bore wear near the top.



Another bore life increasing experiment with Hepolite pistons. Oil from the oil collector groove & oil scraper rings is allowed to channel down the very shallow X grooves (.005" deep) and coat the skirt with oil on the upstroke. I've considered making this modification available but I have to hand craft the grooves by hand (with a guide) and its difficult. I would probably leave out the short vertical groove.



Earlier photo of my latest lightweight drilled pistons - more lube to the bore wall through the drilled skirts. These pistons now have 20,000 miles on them and show 0 leakage on a leak down tester with gapless rings. No measurable bore wear so far. Lightened to 170 grams bare.

Holey Smokes Batman! Your oil grooves are not that similar to the tested patterns research but with your hi rpm sensing power calibration ya might be able to detect less friction. My take on the oil supply texture is horizontal orientation of more shallow basins so did my new mower piston by handy chain saw file leaving tapered boarders. i will mimic your holey pistons in my factory combat soon plus some filing. btw have you matured enough not touch test wet paint signs?

you are a brave fella drilling huge holes in the piston skirt

When i was racing Karts back in the 90`s we used to drill boost ports on the inlet side of Suzuki RM 250 pistons to aid small end lubrication / inlet duration
if you ran them more than a couple of hours they would spit the pistons out the exhaust port

drilling holes that size will cause stress risers , me thinks you will need a good supply of cylinders and heads to keep experimenting

You got that right about Jimmy being an experimental hi rpm isk taker but he's long drilled-tested all types of Norton pistons successfully so what he's pointing out here is not the lightening oil passing holes but the oiil retaining-spreading-friction reducing oil grooves.

should work

the Original Norton 16H hepolite pistons had concentric oil grooves around the piston skirt
to aid cooling

I started drilling my pistons in my racing days since 1984 and have never had a problem and my motors were revved into the valve float range with racing valve springs.

Sometimes you have to take courage and go an extra step to beat the competition.

Jimmy I'd sure like a review or your logic-reasoning-for picking this distinct sharp edged X groove design rather than what was found best in the research inspiring us to slice and dice perfectly good components. I don't have the article reference handy at the moment to share. As ya see by the posts on this your purpose for doing this is being missed.

Interesting, thanks, Jim.

Reed valve Yamahas of the '70s came stock with two big holes in the back of their cast pistons to increase intake area/time; they never failed. We then would carve the entire center out of the back of the piston, leaving two legs on each side of center. Even with that, they rarely failed, engines ran stronger, but ring and cylinder life was compromised.

xbacksideslider said:

Interesting, thanks, Jim.

Reed valve Yamahas of the '70s came stock with two big holes in the back of their cast pistons to increase intake area/time; they never failed. We then would carve the entire center out of the back of the piston, leaving two legs on each side of center. Even with that, they rarely failed, engines ran stronger, but ring and cylinder life was compromised.

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Sorry I will have to disagree, I had a Yamaha where a the tuning mod was to cut the centre section on the piston out to give the inlet to the crankcases more volume, these pistons only had a limited life span compared with an unmodified one, I had 2 engine blow ups because of this,
( Pistons disintegrating ) but then, I knew the risks.

Take a look at an Original Manx 350/500 piston, the skirts are left intact, but the sides are cut away as much as possible, it may be the best way to go but I advise don’t cut/drill out a standard piston skirt as these Manx pistons are specially designed and made for this purpose

Here's Jimmy's inspiration source that's nothing to do with any piston holes.
http://www.federalmogul.com/en-US/Media ... cript2.pdf

As engine speed increases, the piston skirt friction
of AV304 decreases whereas the friction of the
other profiles increases. This is because AV304
has better lubrication of the load bearing surface.

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To answer Hobots question. My intention is to smear oil around the cylinder wall by bringing it down from the oil accumulator groove with the X channels instead of directing it back to the inside of the pistons through the holes in the oil accumulator ring groove as is usually done. I've read the Federalmogul piston article about recessed areas in the skirt and I think its great. But I don't have the means to produce and test those recessed areas and they still depend on other areas of the skirt to maintain clearance - and those other areas that contact the cylinder wall are still subject to wear.

As for holes weakening the skirts - I've already tested it for many years on the street and track. They have never failed and thats the bottom line for me. The drilled pistons are lighter, rev higher and most importantly - they reduce the stress on the crank and cases and everyone else has plenty of crank and case failures. You've got to keep in mind what's more important - having your crank or cases break or taking a chance on lighter pistons which have never broken throughout (my own) testing and experience - and yes there are tuners who have drilled the skirts on Manxs and G50s and thats where I got the idea in the first place. I'm not talking about some two stroke pistons. I'm talking about 4 stroke pistons that have been tested is the real world and have passed with flying colors as in 60,000 miles of testing and RPMs over 8000.

Racing is all about cutting edge and development. And those improvements get passed on to street bikes. We all love our Nortons and I like to improve on them. I'm not telling anyone to drill their pistons. I'm just showing what I've done. Its led to reduced reciprocating weight - the improvement has been dramatic and has encouraged further developments in piston design/lightening.

jseng1 said:

Drilled, lightened and oil grooved piston skirt lubrication experiments below....... I've considered making this modification available but I have to hand craft the grooves by hand (with a guide) and its difficult. I would probably leave out the short vertical groove........Earlier photo of my latest lightweight drilled pistons - more lube to the bore wall through the drilled skirts. These pistons now have 20,000 miles on them and show 0 leakage on a leak down tester with gapless rings. No measurable bore wear so far. Lightened to 170 grams bare.

Click to expand...

Thanks for posting this info Jim. I was wondering about the barrels on your latest lightened and drilled pistons....are they the OEM cast Iron ones (wondering about the gap/clearance) and if so do they have the boretech coating? Is this from your wideline street bike (as I was noticing the lightening on the topside of your crankshaft counterweight and wondering if it was OEM cast Iron too---going up to 8000 and all, just curious )......and also is this with your joe hunt modified magneto fired engine? Again thanks for posting this, Cj

cjandme said:

jseng1 said:

Drilled, lightened and oil grooved piston skirt lubrication experiments below....... I've considered making this modification available but I have to hand craft the grooves by hand (with a guide) and its difficult. I would probably leave out the short vertical groove........Earlier photo of my latest lightweight drilled pistons - more lube to the bore wall through the drilled skirts. These pistons now have 20,000 miles on them and show 0 leakage on a leak down tester with gapless rings. No measurable bore wear so far. Lightened to 170 grams bare.

Click to expand...

Thanks for posting this info Jim. I was wondering about the barrels on your latest lightened and drilled pistons....are they the OEM cast Iron ones (wondering about the gap/clearance) and if so do they have the boretech coating? Is this from your wideline street bike (as I was noticing the lightening on the topside of your crankshaft counterweight and wondering if it was OEM cast Iron too---going up to 8000 and all, just curious )......and also is this with your joe hunt modified magneto fired engine? Again thanks for posting this, Cj

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Yes this is a modified Commando motor in my stripped down Cafe racer with Maney alum cylinders with regular cast iron sleeves. The clearance is .00055" and that tightens up to about .001" after the the pistons heat up. Forged pistons expand a lot - they are a little noisy when cold and become quiet when the motor warms up. You can see the burnish marks here and there on the skirts from the minimal running clearance when hot. No the cylinders do not have the Bore tech treatment. I didn't know about it 6 years ago when the motor was built. Bore tech is something I recommend to customers who want their Commandos to last forever and for racers who thrash their motors beyond reason. Its not a bad idea for a hopped up Commando street racer but personally I have not found Bore Tech to be necessary on a street bike. It certainly wouldn't hurt and its not too expensive - actually seems like a no-brainer if you want to upgrade our soft cast iron cylinders toward today's Nikasil standards. In either case, hundreds of sets of the lightweight pistons are being ridden on the street and that's a lot of happy customers. After 20,000 miles a leak down test with gapless rings shows no measurable leakage and I haven't found any measurable bore wear yet. See the leak down test below:

[video]https://youtu.be/3S50Lvo9c-0[/video]

OEM Cast iron flywheel but the Diameter has been turned down and the crankshaft has been lightened as explained in my race manual and this is an important modification that helps prevent the flywheel from exploding because the smaller diameter means there is less centrifugal force pulling the flywheel apart. Going beyond 8000 RPM is something I do to test durability of JS rods, lightweight pistons, radiused cam kit, beehive valve springs etc and is not recommended as common practice to say the least. The modified Hunt mag is old technology, is not as precise as EI and can give you the occasional high RPM fart (it has points after all). But its reliable, gives 1st revolution spark and easy starting and I can run without a battery which makes for a cleaner looking bike.

You may be asking yourself - so do the drilled pistons offer more cylinder wall lubrication and increase bore life? Results look good but are inconclusive and have not been confirmed. My personal opinion is yes but I can't prove anything. But at least I'm working on it.

Drilled piston skirts used to be commonly done for better lube.
This is a stock 1920s piston, don't quite recall if its Excelsior, HD or Indian.
(they all had about the same scheme).



BTW, this piston is iron, has 3 compression rings and no oil ring (pre recirculating oiling systems)
and weighs an amazing 888 grams.
Note the bottom of the skirt groove for improved oil distribution too.
These engines used total loss oiling though, with minimal lube,
so probably needed all the tricks they could find to improve things ?

Rohan said:

Drilled piston skirts used to be commonly done for better lube.
This is a stock 1920s piston, don't quite recall if its Excelsior, HD or Indian.
(they all had about the same scheme).

BTW, this piston is iron, has 3 compression rings and no oil ring (pre recirculating oiling systems)
and weighs an amazing 888 grams.
Note the bottom of the skirt groove for improved oil distribution too.
These engines used total loss oiling though, with minimal lube,
so probably needed all the tricks they could find to improve things ?

Click to expand...

Those iron pistons must have been limited to low RPMs.

One way to prove if drilled or channeled pistons provide more cylinder lube and increase bore life is to modify just one piston and compare the wear rates of the two cylinders. This would need to be done on a high RPM, high maintenance race bike that sees a lot of wear and frequent tear downs so you don't have to wait years for results. If anyone's interested - contact me.

One.
I think the ancient use of holes in pistons was to give bore splash oil an escape route not to over oil ring and smoke. A side effect was lass mass.
Two. Jimmy Trixie Combat is a willing test subject, perfectly polished well used cam being send to cryo along with chain blades and couple head studs comnoz can torque to destruction, so >>>

Two.
As I-we already have baseline burnish marks- bore wear on standard can type .040" pistons - I'd like to try your X grooves in one side and my chainsaw filed other side and see what looks like in a few years or less if something else goes wrong. I modified my horizontal lawn mower 'reservoirs' leaving a more center proud area d/t thots of gravity weight bearing oil layer might not support well. Mower is Bore Tech'd but not Trixie. Will put in new rings of normal gap only for new base line as current ones still smokeless normal. Would be kick ass for a few more hp if less hi rpm friction.

1979 Cycle World article on oil control piston holes
https://books.google.com/books?id=i8_7w ... e&q&f=true

jseng1 said:

Those iron pistons must have been limited to low RPMs.

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A Big Valve Excelsior (1000cc v-twin) was the first bike to break the 100 mph mark in a oval dirt or board track race, in 1912.
Don't know if these particular pistons were used, but there was no alternative to iron pistons back then, that I am aware.

The race engines don't look much different to the road bikes either, just the chassis was shortened slightly and surplus weight items stripped off.
Indian and HD and a number of other makers all had similar models, and bouts of winningness.
OHC and 8 valve motors appeared back about then too in these competitions.

Alloy pistons didn't generally appear until the 1920s, mid 1920s. And reliable alloy pistons a bit later even....
Iron pistons were stll in use into the 1950s.

I notice that Honda Fireblade pistons look as though they are Teflon coated.

I've seen quoted that F1 pistons have 20+ different coatings/processes applied to them.

Be interesting to know more....