920cc with 270 crank

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I don’t think it’s the valve sizes per se that dictate moving the oil drain, it’s the port sizes. Opening the ports out can break into the oil way.

Although, of course, anyone taking the ports out that far really should enlarge the valves.

My understanding is that basically standard Norton valves a good for up to 750cc and are actually already too small for an 850. So, they’re really too small for a 920!

Opening out the ports that feed the valve, and the jugs that suck from the valve, is really gonna make stock valves a bottleneck IMHO.
 
The extra holes are compression releases.
You can see them on the sides of the fins in other pix.
 
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Would be nice if you could add some weight to the crank and refine the balance rather than undo all the high tech "improvements" of that unique motor. Check the dynamic balance at around 50 to 55% wet balance factor.
 
B+Bogus said:
The external oil drain is certainly non-standard, but typical of a big valve conversion.
Standard valve sizes are 1.49" and 1.302" according to the data - worth checking yours.

The additional holes into the chamber I've never seen before - twin plug installation, perhaps?

A very interesting engine!
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External oil drain feeds oil to the cam needle bearing on timing side. Wholes in chamber are for automatic pressure release valves(helps starting procedure...)
 
Fast Eddie said:
I don’t think it’s the valve sizes per se that dictate moving the oil drain, it’s the port sizes. Opening the ports out can break into the oil way.

Although, of course, anyone taking the ports out that far really should enlarge the valves.

My understanding is that basically standard Norton valves a good for up to 750cc and are actually already too small for an 850. So, they’re really too small for a 920!

Opening out the ports that feed the valve, and the jugs that suck from the valve, is really gonna make stock valves a bottleneck IMHO.
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On todo-list at some point. I want to proceed step by step and have dyno run after any bigger change just to see the real change.
 
MichaelB said:
The extra holes are compression releases.
You can see them on the sides of the fins in other pix.
Click to expand...
Correct. They are really needed due high compression and the fact that electric starter is needed to start 270 EFI engine.
 
jseng1 said:
Would be nice if you could add some weight to the crank and refine the balance rather than undo all the high tech "improvements" of that unique motor. Check the dynamic balance at around 50 to 55% wet balance factor.
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Unfortunately I don't have knowledge to start fiddling with 270 balance factors. Trial and error is not the way for me to do it. Also I'm under the impression that 270 should be hard mounted to the frame? Isolastics are designed for 360 setup I was told. Do you have first hand data about this stuff?

I don't think I'm undoing all "high tech improvements" by going back to 360.... I was told that driveability was one big goal with this project. Meaning to have correct AFR and ignition advance at all times and conditions = EFI. Original builder had no experience on EFI systems but he still wanted to learn and do it. I've got experience on that stuff and managed to finalize that goal. There are endless opportunities to improve the EFI system so this will be longer project for me as well.
 
HanaAna said:
External oil drain feeds oil to the cam needle bearing on timing side. Wholes in chamber are for automatic pressure release valves(helps starting procedure...)
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The pressure release valves are novel - I didn't know they existed!
Fast Eddie will hopefully be along soon to confirm that the latest aftermarket electric starters have no problems spinning up a high compression 920 motor.
I doubt that the same could be said of the original Prestolite setup?

Have you measured the valve sizes?
 
B+Bogus said:
The pressure release valves are novel - I didn't know they existed!
Fast Eddie will hopefully be along soon to confirm that the latest aftermarket electric starters have no problems spinning up a high compression 920 motor.
I doubt that the same could be said of the original Prestolite setup?

Have you measured the valve sizes?
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Valves are pretty much according to std figures you wrote earlier.

I'm sure the aftermarket starters are capable of turning this engine as well. But of course e-starter on this one is home made and is turning clutch which is not ideal.... should be on crank.
 
B+Bogus said:
The pressure release valves are novel - I didn't know they existed
Fast Eddie will hopefully be along soon to confirm that the latest aftermarket electric starters have no problems spinning up a high compression 920 motor.
Click to expand...

Well, I’ve seen these little decompression doofers before on big American Ultima engines (after market HD clone engines), they stay decompressed until the engine fires, then they close off. They always looked like potential trouble to me, but I’ve never used them so can’t say.

Dunno about ‘high compression’... all I can confirm is that the cNw starter kit spins over an 11:1 cr 920cc motor as if the plugs are out !

Looking at the trickness of this motor , I’d definitely suggest having the head flow and valve sizes looked at, it could be holding it back massively otherwise.

I had the RH10 head on my modified 850 ported by Comnoz and bigger valves fitted, the before and after comparison was + 9rwhp on the dyno...
 
HanaAna said:
Unfortunately I don't have knowledge to start fiddling with 270 balance factors. Trial and error is not the way for me to do it. Also I'm under the impression that 270 should be hard mounted to the frame? Isolastics are designed for 360 setup I was told. Do you have first hand data about this stuff?

I don't think I'm undoing all "high tech improvements" by going back to 360.... I was told that driveability was one big goal with this project. Meaning to have correct AFR and ignition advance at all times and conditions = EFI. Original builder had no experience on EFI systems but he still wanted to learn and do it. I've got experience on that stuff and managed to finalize that goal. There are endless opportunities to improve the EFI system so this will be longer project for me as well.
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Yes balance info on a 270 crank is sketchy but I've heard from several sources that the balance factor should be in the 50-55% range. Ducatti twin balance factors should give you a clue. Being that you have a lot of vibration around 3500 RPM and less at higher RPM indicates that the isolastics are experiencing a harmonic problem - for some reason the motor is moving around more at that RPM. I saw this happen once with a cracked racing frame - the bike began to shake badly in a narrow RPM range. When the crack was repaired the harmonic vibration dissappeared. You could try firmer isolastics or even fill the isolastic cavities with a hardening urathane rubber compound (its been done before) but there is no guarrantee it would work and I don't want to send you down an experimental rabbit hole.
 
jseng1 said:
Yes balance info on a 270 crank is sketchy but I've heard from several sources that the balance factor should be in the 50-55% range. Ducatti twin balance factors should give you a clue. Being that you have a lot of vibration around 3500 RPM and less at higher RPM indicates that the isolastics are experiencing a harmonic problem - for some reason the motor is moving around more at that RPM. I saw this happen once with a cracked racing frame - the bike began to shake badly in a narrow RPM range. When the crack was repaired the harmonic vibration dissappeared. You could try firmer isolastics or even fill the isolastic cavities with a hardening urathane rubber compound (its been done before) but there is no guarrantee it would work and I don't want to send you down an experimental rabbit hole.
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Regarding your conrods-They are Thunder Engineering rods,made in the UK with ARP big end bolts.I recognize the ethcing,each pair are weight matched. Excellant rods,have used them in my BSA A70 in sidecar racing.If you search Thunder Engineering his website will come up.
 
jseng1 said:
Yes balance info on a 270 crank is sketchy but I've heard from several sources that the balance factor should be in the 50-55% range. Ducatti twin balance factors should give you a clue. Being that you have a lot of vibration around 3500 RPM and less at higher RPM indicates that the isolastics are experiencing a harmonic problem - for some reason the motor is moving around more at that RPM. I saw this happen once with a cracked racing frame - the bike began to shake badly in a narrow RPM range. When the crack was repaired the harmonic vibration dissappeared. You could try firmer isolastics or even fill the isolastic cavities with a hardening urathane rubber compound (its been done before) but there is no guarrantee it would work and I don't want to send you down an experimental rabbit hole.
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Well I’ll be still having all the parts needed to continue 270 project later.... I just need to get another Commando! One for riding and another for experimenting.

Horrible shakes are not only at 3500rpm, it starts right from the idle. I guess this is the reason why extra balance shafts are used and needed.
 
tonybsa said:
Regarding your conrods-They are Thunder Engineering rods,made in the UK with ARP big end bolts.I recognize the ethcing,each pair are weight matched. Excellant rods,have used them in my BSA A70 in sidecar racing.If you search Thunder Engineering his website will come up.
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Excellent info! Thank you!
 
What does the rest of the Commando this engine came out of look like?

Yeah, getting another Commando would be the fastest route to a streetable motorcycle.
 
Whilst it’s a shame to undo the 270 work done by the PO; the bike currently vibrates badly and balance factors are guesswork.

Whereas, a well balanced 360 crank in good quality isolastics is remarkable smooth, no balance shafts needed at all.

So I vote ‘go 360’ and you’ll be back on the road for spring.
 
A new head and some smaller naturally aspirated carburetion might also help get it on the street.

You could use the intake ports on the old head for shot glass holders. Are those 39mm holes?

Seriously, It's all cool to look at, but really impractical for the street. I think there could be something wrong with me. I'm starting to agree with Al.
 
jseng1 said:
Would be nice if you could add some weight to the crank and refine the balance rather than undo all the high tech "improvements" of that unique motor. Check the dynamic balance at around 50 to 55% wet balance factor.
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You never know what you have got until you start to work with it. Then you find out whether it is good or has short-comings. The trick is to get a combination in which everything works together. If you are moving the power band up higher to get more top end, you probably need bigger ports, a different balance factor, a different cam and exhaust, and a gearbox to suit.
With any motor, you start at the beginning, look at what you have got - then start improving.
You can build a motor which theoretically the best, then find in practice that it is extremely difficult to ride with, so you go slower.
 
I’m not sure about this... but I’m gonna go out on a limb here...

I kinda suspect that if the OP wanted a stock Commando, then he would have, well, you know... bought a stock Commando...!! :rolleyes:

HanaAna, don’t listen to this lot, those old farts that are telling you to ‘go standard‘ are running a methanol burning Seeley and a totally none standard hot rod P11 FFS !

keep on keeping on...
 
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