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Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 15282 times)

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #90 on: Feb 12, 2018, 13:59:18 »
At some point between 1980 and 1982 (I think around then) the rockers were upgraded to be the ones used on the 350 quad engines - they are the same part.  The difference is that the original part is a cast item with a nitride hardened surface on the cam contact pad.  Whereas the new rockers/the quad rockers have a (through?) hardened pad on them as a separate part.  Another difference is the tappet screw and nut.  The original has an M6 x 0.75 thread while the replacement part has an M7 x 0.75 thread (first time I have encountered this).  This is why when you buy a new rocker, it comes as a set including the tappet and nut - because obviously these particular pieces are not interchangeable any more. Check out the photos for the differences.

I suspect there were either some wear issues or Yamaha wanted to consolidate parts across similar models.  The wear would have been on the cam and the rocker pads obviously, but I also suspect wear on the tappets and the valve stems was also an issue - as I am sure many of you have seen.  So going up a millimetre in diameter for increased contact surface area on the valve stem (up to M7) might have helped that.

There is also some good information on the Kedo site and other sites about the incompatibility of the different cams and rocker versions - looks like it was the same thing for the rockers in the SR500.  I am pretty sure though that it is no problem to run either rocker with an old cam, but not ok to run old rockers with any new cam?  I deduce that the new cams are harder and therefor the old rockers are not up to the job?

Offline der_nanno

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #91 on: Feb 15, 2018, 05:30:16 »
As an ex-SR500 rider, rockers, rockershafts and cams are a huge problem with these on higher mileage bikes, because the oil-supply to the head is a bit of an issue. That being said, the hardpad-rockers are good to be used with all cams. The older chromed pad rockers do have a central oiling port in the center of the pad, which can (and will) ultimately leave a little ridge on a used cam. If your new cam is nitride-treated, get some 2000-3000 grit sandpaper oil it generously and run the sandpaper over the cam-surface until it brightens up a bit. This will take off the sharp edges of the nitride coating and allow you to run older style softer rockers on SR-cams. (Generally the same rules should apply with the 250, but I only have personal experience on the 500, especially with reground and nitrided cams...)
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Offline datadavid

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #92 on: Feb 15, 2018, 16:55:57 »
Looks like a stellite pad on the late rockers to me, quite like old triumph cam followers.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #93 on: Feb 24, 2018, 11:49:24 »
That being said, the hardpad-rockers are good to be used with all cams.

Cool, I figured that was the case.

Cheers for the input guys!

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #94 on: Feb 24, 2018, 11:53:10 »
Got the prototypes machined aluminium parts back from the machinist.  They look amazing.  I think I'll bring the tach plug into production - because it fits other engines too I think, plus it fits unused speedo drive holes in wheels (if an electric speedo is used).  And will probably bring the ignition bracket into production as well.  Not so sure about the electric start plug.  Feels like not many people follow through with the conversion to this level...

Offline der_nanno

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #95 on: Feb 25, 2018, 13:38:23 »
I think the tach-plug is available from Kedo for the SR/XTs and is dead cheap, so you may want to check the market first before spending any money on having a few made up for resale. :-/
Real freedom starts with the freedom of thought.

My Blog:
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Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #96 on: Feb 26, 2018, 16:15:59 »
I think the tach-plug is available from Kedo for the SR/XTs and is dead cheap, so you may want to check the market first before spending any money on having a few made up for resale. :-/

Thanks for the heads up!  I found a few different ones online actually.  Imma buy a couple of them and see if they are any different.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #97 on: Mar 01, 2018, 15:58:17 »
Here comes a bit of a dense post about valves and cam.  I know I have mentioned the plans previously, but here are some photos to go along with it.

I got really confused by some cam specs for the SR and XT engines (same same).  See attached the data sheets from the respective workshop manuals.  Then when I check out cam regrind websites - Web camshafts and Tighe camshafts for example, the numbers just don't seem to add up.  Nor do they when looking at specs advertised for the 350 quad engine cams.  The answer to the misunderstanding is net and gross cam lift - or affective cam lift at the valve - which equals a number multiplied by the rocker ratio.  Because the SR's rocker is a somewhat whacky shape, it was impossible to calculate a ratio on paper with measuring tools - especially if you are going to consider the dynamic movement it has at the tip of the tappet on the top of the valve.  So the best way to measure is to set the head up and measure true valve lift with a dial gauge and compare that to measured cam profile difference - from the base circle compared to the tip of the lift lobe across.

I managed to measure 9.1mm of lift on the exhaust valve and the cam on the exhaust lobe has 8.25mm of lift.  Which for my calculations gives a rocker ratio of 1.1:1  But this still doesn't add up when looking at the numbers on the websites...  I double checked and then checked with the Tighe cam as well, same ratio.  The info on the Tighe cams website states a rocker ratio of 1.3:1 but perhaps that is for the SR500 and not for the SR250?  Which would put the specs off?  Either way, there will be clearance for the valves to the piston at maximum lift because these cams are designed to work with the stock piston.  I gave up figuring that out.

I also attached a screen shot of a spreadsheet I made comparing a few different cams - to see what was suitable for this engine.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #98 on: Mar 01, 2018, 16:18:16 »
Now about the valves...  A few people have swapped out the SR valves for 350 quad valves - 2mm bigger in diameter for both intake and exhaust (see attached comparison).  I have decided not to do this because of the extra weight, plus I don't think the SR head can flow that much air.  I want to move those valves up and down faster (higher rpm) in a controlled manner without getting valve float or 'bounce'.  I also found an interesting table in the 4 stroke tuning book by Graham Bell about valve size and the different valves sizes of engines according to their cylinder capacity.  I concluded that the SR is already over valved, or has correctly sized valves for the power I am after - see attached a summary of relevant cylinder capacities I made.

I was also going to copy a lot of modern 4 stroke motocross bikes and have the intake valve (which is always slightly larger and subject to less heat than the exhaust valve) made in Ti and the exhaust valve in SS to cope with the heat and wear.  After speaking with Chris from Schumann Motor Works I have decided to go with both valves in SS, but reduce the stem from 7mm to 5.5mm - thereby reducing weight but maintaining reliable valve life.  Out of curiosity I did a quick CAD exercise where I modelled an SR valve and 'weighed' it as Ti with a 7mm stem, then as SS with a 5.5mm stem and they came up super close.  Although not too accurate specifying a material in a CAD program, it gives an indication.  At least the valves will be lighter than stock!  Also, by using SS for both, the valve seats in the head can just be reground rather than replaced (would need to be something else for the Ti valve).

Offline der_nanno

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #99 on: Mar 03, 2018, 03:54:55 »
Are you sure, the pads aren't worn on your SR250 rockers? Also you stated that there are multiple types (chromed and hardfaced) of rockers for the 250. Now add some manufacturing tolerances and then some optimistic round from the cam-manufacturer and you could quite easily end up with the differences.

Are there any off-the-shelf 5.5mm valve guides available? Personally I am no big fan of Ti-Valves in vehicles that may actually end up making some miles, mainly because wear and tear is a bit hard to monitor (for me). If you start lightening the valve train, I'd also have a look at the rockers. At least on SR500s they can be lightened a bit.

Oh and excuse my ignorance, but what kind of rev-limit do you have in mind with this engine?

I really like your approach of doing proper research, way too few people seem to do it like that, when building a bike.
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