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 Snetterton -- my bogey circuit...

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john bass


Number of posts : 1736
Age : 88
Localisation : Bensberg, Germany
Registration date : 2006-12-06

PostSubject: Snetterton -- my bogey circuit...   Sun Jun 03, 2012 4:14 am

Snetterton was my bogey circuit -- if anything would go wrong it would go wrong there. 1st road race of my life:- was with Icarus-1 with the 150cc engine (that came with the frame) so I was out in the practice and races with the 250s -- nearly finished me off psycho wise. Finished second from last in the first race -- and stopped in the second with a seized big-end bearing.

Trying to get the engine running for more than a few seconds nearly killed me -- I┤d knocked off 3stone of body weight and was so weak that the 4 times of run & bump left me so exhausted I had to lie down... Derek found out why it would start and not keep running (4 times already...) for more tahn 20 to 30seconds: he had put a Vincent oil-tank cap on the petrol tank -- no tank ventilation meant it would rapidly run out of fuel. Then when lining up for practice a breeze suddenly erupted, dust went into the carb, and the throttle stuck about 3/4s open and my mentor was screaming along with the engine revs...

What I meant to say was the track shape has been changed twice since I rode there. The long Norwich Straight followed a fast sweeping right hander which meant the engine was put under a lot of stress for longer time relative most other airfield circuits. I have been asked what gearing I used there and would be totally wrong because of the number of new twists and turns which have made the total length longer anyway. I did learn that a lot of clutch slip was necessary to keep the speed on thro┤the bends....

I had a couple of PMs on the above subject and replied but it seems those PMs never got there...

... and funny thing is that some*** of my e-mails were not answered either. I┤m wondering if any of you Computer whizz-kids (Edward!!) on here have any idea what┤s wrong with my computer???????

*** could be some were not answered on purpose!!

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Number of posts : 123
Registration date : 2011-12-15

PostSubject: yamaha piston cracking   Thu Jun 07, 2012 9:13 pm

Hi Mike and JB,
This time should be ok.

Thanks Mike for the info,yes I did use new F pistons but as time went by John used G with the extended inlet timing all new,I
mentioned Rons s/h ones as a comparison as they had cracked in the same place. I see what you mean regards large ports
in the TZ but JS1 ports were a fair bit smaller and still had the same problem with crackes in fact John writes " New 98 TZ prior
to Cadwell installed this only lasted 1 meeting as cracks have started in the usual place".

JB, the crackes are vertical just to one side of the gudgeon pin boss, the pin dia= 15.97ish, the width of the cutouts=one piston
is 28.18ish the others are 33.96ish ( I use the old Japanese "ish" as it gets me out of mini measurements) and all the pistons
have standard rectangular apertures. Not sure what you mean by relief from roof-edge to gudgeon pin bore.

Thank you both for your interest, I've missed out somthing, Mike I have found a replacement piston- Padgetts ( Batley) cannot
comment yet as just installed it but so far so good.

Les 2012.
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john bass


Number of posts : 1736
Age : 88
Localisation : Bensberg, Germany
Registration date : 2006-12-06

PostSubject: Thanks Les...   Fri Jun 08, 2012 3:53 am

Thanks Les...!

I take it from what you said the crack is to one side of the gudgeon hole and not into it... which makes it more puzzling: thrust side of piston or opposite side...??

By `relief┤ I mean that the side of the piston above the cut-out (and below the gudgeon pin hole) is scooped out or tapered. On a picture I found of a Yam piston it is definitely not so ... but it is so on the old piston I have of the ABS 250 -- maybe for transfer gas flow and transfer timing....?? The latter piston had a 12.5mm (1/2") gudgeon pin and piston dia of 65.4mm with the cut-outs at 36mm wide. Heavy (relative a Yam piston of similar dia...) but stronger relative stress distortion.

I know I am making a fuss about, " ... nothing very important ..." but the data above and your 34mm cutout in a 54mm dia piston along with a 16mm pin combine in stresses happening in a piston under repeated positive and negative stresses and strains*** which are difficult to understand. It┤s not necessary that I try to find out -- and I may never understand why -- but it is now something ═ shall have to research...


--- you┤ll find me an attentive listener.

*** the lighter weight piston, under load duress, distorts considerably....

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