Puttin 256 top end on 477 bottom

[da evil one]

Alright, so I got my 256 top end all polished and ready to go on one of the 477 motors I acquired. Heres where the questions start and heres what Ive got. The roller came with a 256 motor sans cam, pistons and cam tensioner. I ended up getting two 477 motors off craigslist for cheap but I already had the top end of the 256 motor almost completely polished. So once I found out the 256 top end would bolt up but the cam and pushrods/pistons would have to stay 477 on the bottom I decided to finish the polishing and just take the cam out of one of the 477 motors and put it in the 256 top end and bolt it up. Once I take the top end off the 477 motor Im going to use Im assuming everything should play nice if I just leave the pistons on the crank on the 477 bottom end and put the cam in the 256 top out of the 477 and bolt it up. Ive got the gasket kit and everything. So what I was gonna do is take the top end off the 477 motor and check the jug measurements against the 256 measurements before I put it together. I dont think any of these motors have been overbored. So heres the question: If the tolerances are the same and the rings look o.k. on the 477 pistons should I put it together and compression test it and run with it or do you guys see any problems I may be overlooking here? I dont want to oversimplify things here and what Im doing isnt really talked about in the book. Any suggestions are greatly appreciated! Thanks in advance fellas.

 

[da evil one]

dang..did i not word that correctly? evrybodys readin, nobody replyin'

 

[azman857]

I believe as long as your internals match (like 477 crank, pistons cam and camchain) and not mixed with 256 stuff, I believe your ok. Outside stuff your ok to mix and match. Anybody else verify this?

 

[Punkskalar]

you CANNOT run a 477 Bottom end with a 256 camshaft unless you swap out the cam gears. You will need both the cam drive gears from a 477 Engine, and all the cam chain, tensioners, and guides from a 477 engine.

A 256 Engine has smaller drive gears for the cam than the 477, so you will need to make sure those match and you will need a 6 bolt camchain tensioner to make it all work

PS, break that up into some paragraphs or something, its hard to read

 

[da evil one]

thanks hugh. it was you who told me that before. those were the swaps i was making. my question was more around the bore of the heads and if the exchange of top ends would be pretty simple if the cylinders have the same measurements. sorry about the big runon sentence. maybe thats why nobodys chiming it...prol dont want to read that shit...heh

 

[Punkskalar]

thanks hugh. it was you who told me that before. those were the swaps i was making. my question was more around the bore of the heads and if the exchange of top ends would be pretty simple if the cylinders have the same measurements. sorry about the big runon sentence. maybe thats why nobodys chiming it...prol dont want to read that shit...heh

No Problem man. You are set on the Bore, but remember you will need 477 Pistons to work with that 477 Crank, since the ID of the small end of the rods have different sizes and the 256 Pistons are not compatible.

I'm using 256 Cylinders on my bike without issue.

 

[sreimer]

I would use the crank, rods, pistons, and cam from the 256 motor. 8mm longer rods than 477. That is if the plating isn't worn off the small end of the rod. (longer rod = more dwell at TDC)

 

[da evil one]

well, the deal was the 256 motor i got with my roller didnt have the cam and pistons. i was gonna buy pistons and a cam for it but i ended up getting(or stealing) 2 477 motors off craigslist for 50 bucks. i had already spent a SHITLOAD of time polishing the 256 top end so thats why i decided to marry the 256 top externals with one of the 477 bottom ends and internals. so im just gonna put the cam and tensioner out of one of the 477s in the 256 top end and slide it on the 477 bottom's pistons and cases.

 

[Jack]

I would use the crank, rods, pistons, and cam from the 256 motor. 8mm longer rods than 477. That is if the plating isn't worn off the small end of the rod. (longer rod = more dwell at TDC)

you'd be better off going 140mm long rod to reap the benefits of lighter weigth pistons along with a hair longer dwell time at TDC and BDC and port the head accordingly for optimum flow ratio.

 

[da evil one]

yeah, jack. ill get right on all that. that will happen to the 750 big bore i send to hugh to rephase when i graduate college and get a big boy job.

 

[Tnainc1]

Hello All,

If you replaced the stock 256 rod/pistons with 447 rods/pistons on the 256 crank, would that be a workable arrangement? The reason I would desire to go this way is that I have a 1969 XS1 that I built in 1970 as a flattrack racer with a Trackmaster frame, Branch Flowametrics head, Tom Sifton Cam and the crank wheels have been balanced and the taper on the crank has been re-machined for to fit a crank fired magneto. My goal in addition to freshening up the internals is to go to the full 750cc allowed by the class. The 447 components seem to be more available.

Thank you in advance for your input.

Dave

 

[Dennis]

from Hugh above
No Problem man. You are set on the Bore, but remember you will need 477 Pistons to work with that 477 Crank, since the ID of the small end of the rods have different sizes and the 256 Pistons are not compatible.

 

[XSLeo]

Yes, putting the 477 rods in a 256 crank is workable. I might try to find a set of pistons that will give you the 750 cc's with the 256 rods. That way you won't loose the long rod advantage.
I think Hoo's Racing has them.
Leo

 

[GreasyC]

I'm a similar position Dave. If the 40 year old 256 rods haven't worn out yet, well I can't imagine it will be all that long before they do, and when they do they're $200 a piece to replace! The 447 rods on the other hand are $87 a piece, or for around $100 you can find a complete 447 crank that has life left. Long rods sound nice but if you go that way any future repairs are going to keep being a complete rip off.

It's especially annoying that they charge so much for the new 256 rods when I think they have to be getting both rod sizes made at the same damn place and shipped over in the same crate. There's no reason I can see that one should cost more than double the other. I think it's got to be that guys with 256 bikes are more often "collectors" and they know they can rip those guys for a few more coins than a guy with a 447 engined rat bike.

 

[Tnainc1]

Thank you (XSLeo and GreasyC) for your input. There should be plenty of life left in the 256 crank and rods. Just for your info the engine had 300 miles on it when I started in 1970. Since then it was ridden (as a 650) on the track for no more than an additional 500 miles. I got to thinking about the rod swap when I could not find the pistons I needed to complete it as a 750. But that is now a problem solved with XSLeo's tip with Hoo's Racing.

I totally agree with GreasyC, that these components have gotten ridiculously expensive.

Further upgrades to the project will include a 277 rephase, with a Pamco Ignition. This brings to to a slightly off thread question, will the crank magneto cooperate with the Pamco Ignition?

Again, thanks for everyone's input.

Dave