Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp

[JadusMotorcycleParts]

Over the weekend I managed to start the tear down of the top end and do a bit of a measure up.  I wanted to know the exact weight of the piston assembly so we have a baseline for the new big bore piston.  I hope that with new methods, we can achieve a bigger piston with the same weight as this one so that the balance of the engine (both the primary balance for the crankshaft and the secondary from the balancer shaft) remains the same - so not to effect the vibration levels.  If this cannot be achieved, we will taper bore the piston wrist pin to get the weight back to stock.

Then the piston producer wanted to know valve angle and wanted me to mark the valve centres on the piston so he has a reference for machining out the valve clearance pockets.  The valve angles in the photos don't show up very accurate because of the perspective, but they were both 115 degrees.

I also measured the compressed height of a used head gasket and the deck height.  Deck height seems to be zero...  So just relies on valve to piston clearance and the clearance from the head gasket height.  Eventually I will cc the head and add this to the volume of the compressed gasket.  Then I will be able to get an exact compression ratio and be able to calculate and predict a new compression ratio with the new piston and the new swept volume.

The head volume is listed in the manual as 30.4cm3, head gasket thickness as 1mm (which is correct for a compressed one) and compression ratio as 8.9:1 (for the 239cc versions) and I have no reason not to believe this, but I want to measure it anyway.  I will also use that compression ratio and these listed numbers to reverse calculate the head and gasket volume just to double check my numbers and see if there are any differences.

[JadusMotorcycleParts]

Tonight I managed to cc both the head and the piston dome.  These methods are pretty common practice but are usually done in a much more controlled, accurate way with proper equipment!  This was sort of a hack job to see if Yamaha were correct in the manual, where they state that the head volume is 30.4cc - which I didn't really understand, it just didn't add up in my calculations.  But then, after all the measuring and math, it turns out that the stated figure is actually total volume at TDC - taking into account the head volume, the gasket volume and the piston dome volume!

My rough measurements (all are written on the calculations sheet) all added to a volume at TDC of 31.7cc which is 1.3cc off what the manual says.  I was therefor happy to say that Yamaha are most likely correct and I am a little off haha, so will use this figure.  But good to know I was in the ball park. 

I calculated backwards for the different engine releases as well to double check the numbers - which added up.

With this number, I was able to calculate a new, predicted CR with the 77mm piston...  Which will roughly be 9.6:1 for a 9% increase over stock.  Is that enough?  Would 10:1 be better?  Imma look into it a little more.

In any case, I have many options to increase comp a little.  But the two most practical would be a slightly thinner solid copper gasket (will need a custom copper gasket anyway for the custom piston dims, so may as well be a little thinner) or to shave a little off the head.  Just by reducing compressed head gasket volume or actual head volume 2cc, the CR will be up to 10:1 

[JadusMotorcycleParts]

Here is a great post by a guy who did this process properly and accurately:  http://skrunkwerks.com/skrunk/measuring-compression-ratio-method-1/

Nicely documented too!

Just a little background perhaps for those CR numbers...  The North American SR was in fact a proper 250cc bike - displacing 249cc.  So with a TDC volume of 30.4 it gave a CR of 9.19, or as the specs round to, 9.2   Then all the other 239cc SR models have a CR of 8.8618, or as rounded to in the specs, 8.9 

[Eleganten]

This is going to be exciting to follow! Done by spring?


Sent from my iPhone using Tapatalk

[JadusMotorcycleParts]

This is going to be exciting to follow! Done by spring?

That's the plan!  But feels a little optimistic at this stage 

Sorry for the lack of updates.  Not so much work has been done, but a shite load of research and things are set in motion.  Lot's of tech posts to come about the engine work 

[JadusMotorcycleParts]

Finally saved the dough to buy the rims and the tires I mentioned.  Already have the spoke sets in stock so that part was easy!  Now can't decide if I should just build the wheels or if I should try make an instructional video on how to do it.  Feels like there are plenty on Youtube already?  But maybe good to have one specific to the SR?

[zap2504]

I assume you're going with the same 16" size as your previous build (that the spoke kit was made for)?

[JadusMotorcycleParts]

I assume you're going with the same 16" size as your previous build (that the spoke kit was made for)?

I actually had one off spoke sets custom made for those 16'' wheels - for the look I was going for.  Pretty idiotic of me to build a bike with 16" wheels, then not offer that option to customers.  I just figured most people would want either the scrambler or cafe racer look with these bikes - so I made the spoke kits for 18" - the spokes actually convert the standard 16" rear wheel to 18". 

In all honesty, and after comparing bikes, the way they ride, handle and feel, a good tyre on an 18" rim is much better than the 16's.  There is just too much tyre on there - rubber = heavy = slower acceleration = slower steering as well.

[CrabsAndCylinders]

A lot of guys over-tire their bikes to get the looks they want.  I think it just looks like bad engineering.

[JadusMotorcycleParts]

When pulling the head and barrel I noticed some pretty serious wear on the conrod small end and piston pin.  Piston pin = no problem, getting replaced anyways.  Conrod, kind of a bastard.  And with the planned mods, it doesn't make sense not to have a strong bottom end.  So I pulled her apart and will replace the crank pin, roller cage bearing, con rod and main bearings either side.  Tranny seems sweet and other bearings and seals are all good.