Motor rebuild/rephase

[Pony]

I would have posted this in my build thread but figured it would be more organized to start a new thread. Ill probably link this thread in my build for reference. I also used the search and came up with a lot of info but still have some questions, Ill try to make it as concise as possible.


What I have: Stock (as far as I know) 77' that is not locked up but not in running condition. The left side cylinde looks like it had some gnarly detonation going on and burned a hole in the piston. Right side cylinder had 30lbs compression. Valvetrain looks to be in good condition.

What I want out of this motor: I want it to pull strong and be able to get 100 mph out of it if I really twist on it, keeping it reliable and running pump gas. I am on a tight budget, but the motor is where I plan on spending the money. A budget, streetable, kick only, hotrod 650 (basically what everyone wants ha)

What my plans are: Split cases, rephase crank, bore the cylinder a step or 2, higher compression pistons, replace cam chain/guides, MrRiggs cam, pamco ignition, green monster coil, polish up the head, PMA charging unit.

Question #1: I have never ridden an xs650, am I wanting too much out of this motor for the mods I listed?

Question #2: I have not seen any photos showing a welded crank after it has been rephased. Is there a certain place/technique to doing this so the crank stays balanced?

Question #3 From what I have read with Hughs R&D the reliability concern with a rephase is with the valvetrain at high RMP's. Has leaving the stock valve springs in to allow them to "float" kept your motors from acheiving RPM's that will make it shit the bed or is it better run new springs and just keep it below 10,000 RPM's? Anything else I need to address with the rephase?

Question #4 For what I am looking to get out of this motor what is a recommended piston and compression ratio?

Question #5: How much should I step the bore up? The cylinders will clean up with one step, but will I need extra displacement to achieve my goal?

Question #6: Looking into a 78 and up transmission to use a Mikes 5th gear overdrive. I plan on making some trips pn this bike. With my tight budget, is this mod something that will make my life a lot better. Would try to keep this under 150$


I know that a lot of question and if ya'll could help me out with 1 or all of them it would be greatly appreciated. Thanks

 

[hotdog]

AAhh not that you'll be using it again but the PO obviously did not know much about motors...your holed piston is in back to front.....LOL

 

[retiredgentleman]

Not a good idea to rev above 6000 rpm.

$150.00 ....................you're dreaming.................$500 to $700 maybe

 

[sreimer]

Don't even bother welding your crank they crack very soon after the press fit is enough oems haven't welded them in years and I think over thirty years is testimony to that

 

[sgallaty]

Don't even bother welding your crank they crack very soon after the press fit is enough oems haven't welded them in years and I think over thirty years is testimony to that

I think the dudes rephasing them have said that once you press them out you have to weld them when you press fit them back or else they can ease out.

I don't know that I've heard of a lot of repressed rephased cranks cracking. anyone else?

 

[sreimer]

They crack exactly where the welds are. I've magnafluxed several and always the same result I've done many cranks singles to fours and don't weld any of them anymore I haven't welded a crank in 15 years and its never been an issue but do as you will your just supporting your economy for doing it

 

[Punkskalar]

Sreimer,

I love ya man, but these XS's are a MUST when it comes to welding the cranks. Even if they crack, they still hold together better than non-welded cranks. I've got cranks here that have "walked" apart just from the big-bore kits... Not a pretty sight inside the cases, and not a good thing for the rods.. But yes, I agree that the welds to tend to crack from time to time... I won't run ANY XS crank that hasn't been welded, causes too much havoc on rods and charging systems...

 

[Pony]

The $150 budget was just for the overdrive 5th and a used transmission.I think the gear is like $65 and I have seen transmissions go as low as $50 on eBay.

Thanks for the replies guys. I'm soaking all this information in so it is appreciated. Punkskalar, I know you are very experienced with rephasing. Have you had a lot of cranks fail that were welded? I'm looking to ride this bike a lot and one if the main things that attracted me to the rephase is it being a better design that also had performance gains. I was hoping that if done correctly it would make it more reliable due to less vibrations. If this is not the case I might stay away from a rephase.

As far as keeping it around 6000 rpms, I also thought with a rephase it would rev much farther with valve keepers and such letting go around 10,500.

I know this has been covered a lot, I am just trying to get advice on stickin the balance of reliability and performance. If anyone wants to weigh in in any of my other questions it would be appreciated. Thanks

 

[Punkskalar]

The $150 budget was just for the overdrive 5th and a used transmission.I think the gear is like $65 and I have seen transmissions go as low as $50 on eBay.

Thanks for the replies guys. I'm soaking all this information in so it is appreciated. Punkskalar, I know you are very experienced with rephasing. Have you had a lot of cranks fail that were welded? I'm looking to ride this bike a lot and one if the main things that attracted me to the rephase is it being a better design that also had performance gains. I was hoping that if done correctly it would make it more reliable due to less vibrations. If this is not the case I might stay away from a rephase.

As far as keeping it around 6000 rpms, I also thought with a rephase it would rev much farther with valve keepers and such letting go around 10,500.

I know this has been covered a lot, I am just trying to get advice on stickin the balance of reliability and performance. If anyone wants to weigh in in any of my other questions it would be appreciated. Thanks

I tear down my personal engine every season just to see how it is handling the abuse, and abuse it I do The first year I had the flywheels all seperating AND the center pin coming apart, because I didn't weld it... Once welded, I haven't had a single issue yet. I haven't torn that engine down this season since its still way to nice and riding is more fun

I have not had a single "failure" in a crank, except for the ones I don't weld. While the welds may crack, that in and of itself is not a failure if the crank still holds together. And while I agree to an extent with Sreimer, welding a crank will not hurt it any, so why not 6,000 RPM is way to low of an expectation for a rephased engine. I would plan on hitting 8,500 with plenty of ease and no worried about the stock valvetrain (if using a stock cam grind). You can run much higher with proper fueling (don't wanna lean out on the top end) and a strong valve train.

And I have had customers ask me not to weld their cranks, I am glad to build a crank any way a customer prefers...

Hugh

 

[Pony]

Right on. Sounds like I would defiantly want to send it off to have it welded up. I'm really torn because I would love to dive in and get it all done while it's apart but I would rather wait and pay the money to have it done right. Might have to build this engine in 2 phases and get some fall riding in and dive back in during the winter.

 

[Robbo]

was really finally ready to take the plunge on the rephased crank and cam etc but this weld crack info has freaked me out..

 

[hotdog]

Dont freak out Robbo...weld, don't weld - it depends on your riding style a great deal. If you intend to just ride around town & not flog the shit out of it then don't bother doing it if you are not comfortable - they are not welded standard you know...
If you intend to give it some hurry curry such as that mentioned above - then yes it would probably be in your best interest to weld it. Depends on the intended use of the motor, mostly cranks are welded for higher RPM work.

 

[irristeve]

Hi ya Pony Im running 277 rephase 750 kit,vm 34s etc proably done 10,000kms on it now still going strong I didnt weld crank but have no intention of going over 7500rpm,whitch is where it needs it,a std 650 should crack the ton but how long you could substain those speeds who knows,rephasing transforms the engine,not sure how much more power it gives think tourqe is the big winner,good points less viabration,electric starter works better & it will rev harder the best thing is the noise it makes,if you are going to use the carbs it came check them out,running lean is what burns holes in pistons,oil cooler is a must for fast running as keeping these old things cool is a bit tricky,make sure the cooler doesnt block air flow to head & cylinders,hope this helps Steve

 

[Punkskalar]

Don't be afraid of the weld, I've seen more cranks coming apart that weren't welded than cranks that were. I see about 3-5 cranks a week through my shop, and I'd say about 20% of them have started to seperate (not just a little, usually 3-4mm) and that was in a stock configuration.

If you ever have crank questions, feel free to email me at HughsHandBuilt@gmail.com - even if you don't intend to use my services, I'm more than happy to help you make an educated decision. I'd rather see these bikes hitting the roads, than turn someone away because I want to make a buck

There is also a great article coming out in RustyMetric (an online magazine that is debuting in just a few days) where I go into depth on how to build your own crank, and check for wear and damage. Until then, take a look here: http://hughshandbuilt.blogspot.com/2011/03/failed-cranks-why-to-properly-build-and.html

Hugh

 

[gggGary]

Minor data point; Suzuki recalled and welded the 16 valve 1100s around 1981 because the press fit cranks skewed.
I had one of those welded cranks out at 31,000 miles, no cracking. Looked like a Mig bead with the amperage WAY up.

 

[gggGary]

No expertise, but If I had weld cracking issues I would try using a high nickel content filler like welding cast iron. I assume you can't preheat or stress relieve because of the bearings? Hammer stress relief might help also?

 

[hotdog]

Gary, I was talking with an friend who used to work as an engine builder yesterday about this, he suggested preheating,peining then slow cooling.

Anyway, every ones own choice, I wont hijack the thread anymore....

P.S. My intention is to weld my crank, new Tig & all that......

 

[Punkskalar]

Gary, I was talking with an friend who used to work as an engine builder yesterday about this, he suggested preheating,peining then slow cooling.

Anyway, every ones own choice, I wont hijack the thread anymore....

P.S. My intention is to weld my crank, new Tig & all that......

Low Heat, Ni-99 filler, and just like you said

 

[mseriously]

My Dad has told me this story on occasion about my Grandpa and a crankshaft in a 1970 Fairlane 390 engine. From what he says the crankshaft was destroyed...not sure if the crank was broken in half, or if it was walking out on itself, or what...the point was always that my Grandpa took 4 or 5 days of pretty constant work pre-heating and hammering on the damn thing and welding and hammering and heating as he says and then they re-installed it.

The motor supposedly went 95,000 miles before the crank finally exploded. And that's in a farm truck getting beat to hell. I'm no expert, but what you guys are saying is right in line with the story I've heard probably 45 million times. So I'd guess in the right hands, and done correctly and with care, a welded crank is just as good as anything.

All I know is that I tore my engine and motorcycle down to nothing and have it back together, with someone else rebuilding the top end, and all I want to do now is start over, re-phase, big bore, oil cooler, re-mote oil filter, sump extension, hydraulic clutch, PMA replacement, Pamco, 8 plate clutch mod, etc.

Then ride the fucker for the rest of my life. I love this bike, and don't think a welded crank pin would kill it. Before I tore mine down I tried to kill it in 2 summers of hard riding, little to no oil, shit carbs....

Rambling done. Continue with the conversation while I slink back into the background...

 

[sgallaty]

punk, is there a way perhaps to anneal the metal on the weld?