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Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 24849 times)

Offline Pete12

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #60 on: Jan 01, 2018, 05:13:29 »
Special Piston Services should be able to recommend the ideal piston to bore clearance based on the piston makeup.
This project is getting very interesting.

Offline der_nanno

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #61 on: Jan 02, 2018, 06:38:41 »
Stock piston to cylinder clearances sound VERY small too me.

With regards to your upper conrod eye failure:
a) are you sure your oilsupply is good (or was the engine perhaps in a rather bad state initially)
b) have you noticed any detonation? You should see pinholes on the cylinder head and/or piston.

Especially detonation is a real killer on the small end as the pressure in the combustion chamber will go up in a rather uncontrolled fashion. I do like a bit of good old fashioned engine tuning like that...

With regards to why not a Warrior-Piston: 30% displacement increase shouldn't be a problem and in connection with the right cam the same goes for flow in the cylinder head. Truth be told, if Yamaha had built a 300cc version, that's probably all they would have done to it. Are you planning on welding up the combustion chamber to improve squish and speed up the combustion?

Cheers,
Greg
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Online JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #62 on: Jan 03, 2018, 13:34:22 »
Yepp, Special Piston Services came back with a recommended spec:  'CLEARANCE  = .0032 @ 23 MM BELOW OIL CONTROL RING you could perhaps try slightly less maybe .0025 min'

Anyone know why an Australian guy/company uses inches for bore measurement?  Then mm for measurement position?  .0032 = .08128mm, .0025" = 0.0635mm

So yeah, good call der nanno, the stock clearances are probs much too tight.  Will go with the recommendation from the piston manufacturer.

With regards to the conrod small end wear, I am positive oil delivery was good, I checked this with the check bolt and even connected in a low pressure oil gauge and got expected readings.  Not sure about detonation - no signs of it at all.  But an interesting point if this wear is not usual after 40,000kms, perhaps there was another failure elsewhere in the engine?  All other components look good and the oil did not show any weird debris.  When I bought the bike 2 years ago and changed the oil then, the valves were super loose and the oil was very dirty, so I suspect poor maintenance would have contributed. 

No plans to mess with the combustion chamber.  If I were building a race engine and it was my 10th engine build and I was trying to extract every last pinch of power from the engine I might.  But it would be too hit and miss I think - a bit of a black art if you ask me.  Looking at the chamber design it is set up like a modern swirl head so I hope it'll get a good burn rate with the new mods. 

Nice turbo build btw, great you found a carbon seal version for the draw thru!

Offline Pete12

  • Posts: 262
Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #63 on: Jan 03, 2018, 17:47:27 »
Anyone know why an Australian guy/company uses inches for bore measurement?  Then mm for measurement?
I'm Aussie and I agree it's odd but it's an old school industry standard. Bore and stroke generally measured in mm but clearances/interferences etc. generally given in inches. Never really thought about it until you mentioned it but it would be confusing for someone from outside. We haven't always been metric and a lot of people, myself included, were taught both systems.

Offline der_nanno

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #64 on: Jan 05, 2018, 07:01:07 »
I'd say a conrod this worn has either done rather 140.000km or as you mentioned suffered a very hard life with long overdrawn  oil-chainge intervalls. Maybe you would want to look into the slude trap inside the crank, if that has filled up with a lot of debris, it might restrict the amount of oil coming out of it.

Thx, finding that turbo was a bit of a lucky shot. Even luckier me found a nice (tiny) supercharger and I'll go down that route during the 2018 season. I'll open a thread, when there's a little more time on the engine I am currently building for my everyday steed, where I play around A LOT with combustion-chamber reshaping.

And yes, mixing imperial and metric numbers is one of those things you find in lot of (originally) imperial countries.
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Offline datadavid

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #65 on: Jan 07, 2018, 03:13:19 »
I'm Aussie and I agree it's odd but it's an old school industry standard. Bore and stroke generally measured in mm but clearances/interferences etc. generally given in inches. Never really thought about it until you mentioned it but it would be confusing for someone from outside. We haven't always been metric and a lot of people, myself included, were taught both systems.
Its because of the finer increments of 1/1000 inch than 1/100mm i think, but what do i know, im a welder! those measurements are too fine for me..

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #66 on: Jan 07, 2018, 13:12:06 »
Yeah it's interesting with the imperial thing.  Both my parents grew up with that system in New Zealand and remeber when the change came through the education system.  I think it is a legacy thing from the hot rodding industry in the US - it's just continued to be that with bore and stroke.  But good point about the finer increments david! 

Offline der_nanno

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #67 on: Jan 08, 2018, 02:33:47 »
Its because of the finer increments of 1/1000 inch than 1/100mm

1/1000" is actually 2.54x 1/100mm, so it's in fact 2.54 times less accurate ;-)

(But we're officially splitting fractions of hairs...  ;D )
Real freedom starts with the freedom of thought.

My Blog:
http://greasygreg.blogspot.co.at

My Mule TR1.1 build here on DTT:
http://www.dotheton.com/forum/index.php?topic=75458.0

My Turbo TR1.1 (XV1000) build here on DTT:
http://www.dotheton.com/forum/index.php?topic=72571

My XS Triple Sidecar build here on DTT:
http://www.dotheton.com/forum/index.php?topic=74211

Offline datadavid

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #68 on: Jan 08, 2018, 02:37:21 »
1/1000" is actually 2.54x 1/100mm, so it's in fact 2.54 times less accurate ;-)

(But we're officially splitting fractions of hairs...  ;D )
Tell the british that! they promised they will convert to metric though. Inch by inch!

Offline CrabsAndCylinders

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #69 on: Jan 08, 2018, 15:10:49 »
Metric is better.
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