Yamaha piston cracking

Hi eveyone,
I have a question to ask. With reference to John's race diary he states, on more than one occation, Yamaha pistons cracking,
something I never had occation to deal with. It appears they all cracked at the same point this being at the back of the transfer
cutouts by the piston pin holes. A couple of years ago Ron at Fahron gave me 6 used 54 yam pistons, on inspection all but 2 had
cracked in the same place so I ask, has anyone else experienced this, can anyone come up with a reason, too much bore
clearance I can understand but we ran max 3 thou.
On a different note you will see Bridges the younger out on his dads Bantam once I get the barrel back from Fahron that I sent
to him last August2011 for a liner to be fitted.

Regards,
Les.

Maybe no use to you Les but on Icarus-2 -- my second Bantam -- I had two (Standard BSA Bantam) pistons "stretch" and then show cracks on the periphery at the piston pin (gudgeon pin) horizontal axis because -- I think -- of the higher revs I got the Icarus-2 engine to rev to... 11,000 rpm with the rev counter needle swinging about ... This meant that with the long-stroke the piston speed was excessive relative the material of the standard BSA piston material and method of manufacture.

Such engine speed was never intended for STANDARD BSA Bantam pistons.It was obvious the `top half´ of the piston was trying to tear itself off from the bottom part at the gudgeon pin centre-line when decelerating to TDC .....

Silly suggestion: do you know if the pistons you speak of were for a lower revving engine???



Cheers!

Which Yamaha pistons are you using Les?

Hi JB,
Thanks for your reply, the pistons we used were the Yamaha TZ250 race engines as were the ones Ron sent me so they
were built for high rev motors, I have to say when I first built the motor in 1978 we had no problems as did the one I built for
Mike Bridges about 1973ish we could run a full season with the occational seal and ring change so why? I don't know but I
would like to.

Thanks,
Les.

There are lots of different TZ250 pistons Les starting with TZ250A to F 54mm G 54.25 H 56mm then on
to V-twins and Starting at A again? I'll assume your using TZ250A-F 53.96 ,97 and 98 graded pistons.
The answer is yes they crack in the places in say, problem is 2nd hand piston are nearly worn out before
you put them in and because they have come out of a cylinder with very big holes in they will be barrelled
that is worn like a barrel - keg of beer shape. Ring Ron Phillips I think he knows where you can buy new
replacements in pairs and you'll have to say you own a TZ? Good Luck... Mike

Are they vertical*** or horizontal cracks Les?

*** vertical as in in line with the stroke...!!

On the incidences I spoke of the cracks were horizontal across the diametre at the gudgeon pin centre.... That is why I said Icarus-2 problem was material and manufacture of Standard BSA being crud for higher engine speeds.

If they are vertical cracks the first thought strikes that it is too much skirt clearance allowing piston slap. Second thought suggests that that notion is crap because the effect of piston slap would result in a compressive stress in the area of the transfer cut-outs.

.How wide are the transfer cut-outs? Are they shaped with a relief from the "roof"-edge up to the gudgeon-pin bore? How big are the gudgeon pin bores? and on the Yam (these cracked pistons came from) is the small-end a plain bush or needle roller??

Cheers!

Hi Les! I found the picture of a TZ piston with the transfer cut-outs in a constant curve as opposed to any other 2-stroke pistons I know which have rectangular apertures. If that is the type then it presents an interesting exercise in Rectangular cut-outs versus Curved ... That is, relative stress created from the longest (middle) part of the skirt against teh cross-sectional area resisting the stress....

I´d still like to know whether the cracks are vertical on the centre-line to the gudgeon pin hole -- and the size of the gudgeon pin bore??

Cheers!

Mike and JB,
For what ever reason I'm having big probs with this new set up re Plusnet, I had sent a post to you both with regards yam
pistons but for some reason it was rejected, I'm not to sure if this will get to the forum if it does I will try again later, in the
mean'time I wish you well.
Les.

`S alright Les -- me too -- I feel really frustrated with my bluddy PC. On some websites it takes forever to get logged on and on others I am rejected (á la Facebook) as being not me -- I think somebody has used my identity and password and now the genuine ME is not acceptable.

Part of my job for umpteen years was the actioning of failure analysis -- along with engine development --(theses two go hand in hand otherwise the wheel gets reinvented and the spokes & rims break etc...etc... etc...) and there are some extraordinary reasons why bits and pieces of engines fail and hasty assumptions are made that end up being very costly. Some parts age-crack and don´t cause any bother. We used a Reserve Factor index for parts we would classify as a risk and be prepared for the cost of warranty.

Getting back into the racing world a good example would be the Ally rod in the JAP short-four Speedway engine -- a knowledgeable user would crack detect the rod, regularly -- 4 studs undone and the rod was available for inspection ...! This crack detection test would be done after a number of races otherwise the bottom half would explode and vanish á la Alf Hagon at Writtle one meeeting -- there was a bang and Alf came to a stop with barrel & head standing on a small piece of crankcase and just debris scattered on the grass...

But back to the Yam pistons -- I´d say it was a case of reduced-weight-design*** to the limit of the bone with a minor risk of complete catastrophe -- I wonder if Yamaha put out a Service Bulletin about the cracking you speak of....??

***Piston weight is the most important factor relative possible maximum engine speed -- the force attributable to deceleration as piston travels from it max velocity (At somewhere roughly half the stroke) to zero at TDC is a factor of n² and pi² which means a few hundred rpm increase increases the accelertion force enormously.

But you all know that already...

Cheers!

JB, I've posted a reply to Mike and youself but i'ts gone into your Snetterton post sorry mate.

Thanks Les -- I replied to the Snetterton one first.

Cheers!

Hi Mike, sorry I got this wrong, if you have not already read my last post re: JBs snetterton last post you will find my reply on it,
one day I'll get the hang of this contraption.
regards,
Les.

It is caused by mostly too large a piston clearance, anything more that 2 thou, and the large inlet port. The leading edge of the inlet side of the piston wears badly as it slides into and out of the inlet port on each stroke, very slightly tipping the piston and knocking the sharp edge off of the skirt, making it very shiny (you can see this on any 250 a/b/c and especialy D piston after just a few laps running) this constant battering not only kills the engines ability to be easily started in a push start but also allows the skirt to over flex, causing the piston to crack. 350 tz's had a much beefier piston, with large skids on the skirt to stop this. Only trouble was the extra weight and inflexibility made the problem much worse, and piston cracking was much more likely than on the thinner, more flexible skirt of the 250.

I ran my engines on 1.5th clearance at the tightest part of the skirt and had no problems with piston cracking, and I replaced them every 250 miles regardless of appearance. maybe that's why they didn't crack and all my theories are crap!

The only thing that broke on my engines were con rods, because of the very poor design and manufacture of the flywheels mostly. Yamaha machining has to be seen to be believed, it was so poor, and they often just exploded as cracks turned into bits of metal flying around the crankcases ... 

Keith Webb