Piston question

Hi  
I was searching on Google for a piston for my D1 and came across this post
(cant post the url but it was on the bsabantam forum)
bantam-technical-questions/d1-piston

where someone said

"I did notice, on fitting it, that the gudgeon pin slid smoothly into the piston without warming the piston first which I have always had to do before on other bikes. I queried this with a local two stroke expert who commented that this is standard modern practice, it reduces skirt scuffing."

doesn't seem right to me at all, wonder if anyone knowledgeable in two strokes would like to comment.

Thanks

Martin,
I have pistons for Aprilia, Honda, Kawasaki, and Yamaha, both cast and forged, sitting here on my desk, with pin diameters of 14,15 and 16mm, all have hand push gudgeon pin fit. The advice you had was correct, so you should have no problems!

Any more queries, just ask!

Trevor

Trevor Amos wrote:

 

Hi Trevor thanks for your reply
I have never worked on what you could call a modern engine
but i am still surprised that they have a sliding fit gudgeon pin
are there any oil holes in these pistons for the pin, hardened steel that could move inside alloy with 2stroke level of lubrication still seems a recipe for disaster to me

on racing Bantams do you use plan or needle roller small end bearings

Thanks

Martin,
None of the afore mentioned pistons have any lubrication drill ways in them, I personally have never had any issues with that.
Caged needle rollers every time for the small end and big end bearings. Modern equipment is now in the "fit and forget" category so reliable is the design and manufacturing technology of today. The traditional bronze bush would cause serious overheating and incur more friction losses, particularly at the rpm most 125s can rev up to.
If you post your location I`m sure there must be someone within reasonable proximity of you who could invite you to visit and perhaps have a peek inside a modern race engine?

Trevor

I have posted the following article, written by Derek Betts, as it is relevant to the questions ask by Martin, I found it informative, authoritative and full of historical anecdotes from someone with personal experience of the industry!

I worked for Wellworthy/ AEPP/AE piston products and then Goetze, till the Mid 90's as a piston Manufacturing production and then Appliction's engineer.


some four strokes do have interference fits on the pistons, there are no cir-clips in these pistons, and some were clamped across the rod small end to maintain position, this is generally old hat, and on much older slow speed marine engines, some automotive manufacturers after the war just copied the technology blindly of Germans who;s scientist's who had big budgets for research
they became aware of the reason's mainly through failure's


we at AE formed a technical tie up with ART (Honda's aftermarket company) they use to insert G/pin's with tight fits, we educated that the interference would distort the skirt spoiling the very special oval COTT's machining of the skirts and lands.
In very old power generation engines this was not a problem, as generally back there was no forces throwing the piston pin forwards, as in a straight four cylinder car engine when the car braked hard, this put the pins hard against the wall of the cylinder, this was not ideal, and first combated by greater interference on the g/pin, then some went to clamping /with a weight penalty, then square cir-clip's, to round. Back then technology was in its infancy, there was less silicon and copper in the pistons, and pistons expanded/distorted more, hence the need for the interference, and more bore clearance was required, however as time progressed tolerances and clearances came down with efficiency and economy being a requirment, and skirt profiles became much more important, so hence the g/pin was installed in light hand push fit with circlips to control vehicle directional loads and thrusts. I make reference to the world class high speed designer and piston applications engineer who resite all this to me, when I joined the company (DAVID SPAIGHT who was my teacher and my friend). Universally older engine's like power generation and some automotive aftermarket companies still make them an interference fit, because thats ok and is the application for it, however for an efficient economical engine to perform you cannot have the pin as an interference, a lot of it is only copied from others, and generally they don't know or really understand the reasons why, most aftermarket companies just copy blindly, to make a buc, they purchase the metal to make piston's from the nearest spec they can get their hands on, at wood lane the old Aeroplane and Aluminium engine Casting Company ( the merlin spitfire casting were cast on our site I found the core boxes to them, that's another story, we had our own smelting and recycling plant, and often made some very exotic aluminium
In 1994 piston research was nearing its demise to future materials and technology's that are no longer in any domain but behind closed doors, these are generally thermo plastics, sintered ceramic and some caltic carbon's, with lose fit g/pins made from coated plastics and titanium, aluminium is still used in pistons because its cheap and easy to work with, its been around for over 100 years, its such an unforgiving and fatigue prone material, especially when the materials and technology 's are about for far stronger and lighter piston's, Technology has certainly been suppressed, by the movement of manufacturing to the far east, sadly Wellworth/AEPP/Goetze are no more, they all shipped their manufacturing to the far east. There are no budgets for metallurgy and product development, that seems to be confined to the F1 industry.


Circlips were introduced for this reason, first square, then a move away from sq due to thrust loads, to round cirlips, that should never have ears again these are an after thought.

Derek Betts

Hi Trevor
thanks for posting that
very interesting and hit the nail on the head so to speak
"slow speed marine engines"
is where i spent most of my engineering apprenticeship
so time to teach old dog new tricks
must also admit first time i was shown a modern Honda (CBR600) 4stroke piston
i thought the bottom 2/3rds of it must have broken off

Martin,
By way of a post-script, here are a few more lines from Derek relating to this topic; again he asked me to post on his behalf as he is still not allowed to personally do so??

"The reason for the slide fit is to promote lubrication, too tight a fit leaves no clearance for lubricating oil and then overheating occurs. I ran bronze small-end bushes for years ,right up to 1989. I never had any major blow-ups caused through piston or cage breakages with rollers coming out and up through the transfer ports.
I worked for Hepolite, Hepolite and Grandage, Welworthy came in later as did AMAC, all were then rolled into AE Piston products group. We then took the AE Goetze name, they made the racing pistons and rings for Yamaha"!

Derek.