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Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 15353 times)

Offline hooligan998

  • Posts: 221
Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #70 on: Jan 09, 2018, 00:51:54 »
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..

1/1000" = .001"
1/100mm = 0.000393701"

Which is finer?

Offline datadavid

  • Posts: 1323
Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #71 on: Jan 09, 2018, 01:01:18 »
1/1000" = .001"
1/100mm = 0.000393701"

Which is finer?
Did you bother to read what i wrote?

Offline der_nanno

  • Posts: 368
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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #72 on: Jan 09, 2018, 03:46:21 »
0.035 - 0.055mm.  Is this good?

I've had another good long thought on the subject... It really depends on the bore and material of your piston. Is the new one forged as well or is it cast? If it's forged, you definitely have to go up with the tolerances and gutt-feeling tells me, around the 0.07mm to 0.08mm clearance is roughly 1/100 of the bore or one percent differential expansion due to dissimilar materials of piston and cylinder/liner in the hottest area of the piston and should be alright for a forged piston. If it actually is a cast piston, going below one percent will prolong piston life dramatically and aiming for the higher end of the range will probably just do you fine. Maybe being a bit more gentle on the first hundred kilometres might be advisable anyhow.

Cheers,
Greg
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Offline hooligan998

  • Posts: 221
Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #73 on: Jan 10, 2018, 00:58:44 »
Did you bother to read what i wrote?

I had to read?  WTF is this?

Offline datadavid

  • Posts: 1323
Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #74 on: Jan 10, 2018, 02:49:52 »
I had to read?  WTF is this?
It was a bit of a joke. Been using both imperial and metric for 15 years so no need to try and lecture. Thanks.

Offline themotoworks

  • Posts: 67
    • the motoworks
Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #75 on: Jan 10, 2018, 07:38:16 »
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


maybe their bore gauge is in inches :)  that and I think they toy a lot with american engines down that way, or at least if australia is anything like I learned from mad max, they do

Offline stroker crazy

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #76 on: Jan 10, 2018, 08:47:41 »
if australia is anything like I learned from mad max …

Mad Max is actually a documentary!

Crazy
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Offline CrabsAndCylinders

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #77 on: Jan 10, 2018, 17:10:30 »
Mad Max is actually a documentary!

Crazy

Ha ha :)
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Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #78 on: Jan 10, 2018, 18:24:52 »
I've had another good long thought on the subject... It really depends on the bore and material of your piston. Is the new one forged as well or is it cast? If it's forged, you definitely have to go up with the tolerances and gutt-feeling tells me, around the 0.07mm to 0.08mm clearance is roughly 1/100 of the bore or one percent differential expansion due to dissimilar materials of piston and cylinder/liner in the hottest area of the piston and should be alright for a forged piston. If it actually is a cast piston, going below one percent will prolong piston life dramatically and aiming for the higher end of the range will probably just do you fine. Maybe being a bit more gentle on the first hundred kilometres might be advisable anyhow.

Cheers,
Greg

Thanks Greg.  Barrel is cast aluminium with a cast iron sleeve.  Piston is a forged blank which is then machined to spec.  Those clearances you suggest are pretty much bang on (on the looser side) of what SPS suggested - I reckon 0.07mm will do the trick.