920 Race Engine Teardown

olChris said:

.
" For the moment, it all goes in a box until I get a couple street Commandos finished. I've promised my grandson that we'll have one for each of us for the North Carolina Rally next year."

Now your talking and what a fantastic goal.. Your very lucky!

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The grandson is pretty lucky too!

Well that crank ain't gonna flex much Ken!

Add steel rods and lightweight pistons and you could be running a super tight squish band in that...

Who made the crank, Ken ? It looks really worthwhile. I don't think I'd ever try to up the capacity to 920, however in an 850 that crank would be great. I'm always very careful to keep the revs in check when I ride my bike - 7,000 RPM max. It spins up extremely quickly coming up through the gears, and I don't much like the thought of those heavy pistons at the ends of aluminium rods. The inertia must be horrendous. I know from experience that a Triumph 650 will cop 8,000 RPM on a fairly regular basis, so a near standard commando crank assembly should be relatively safe at 7,000 RPM. However I'm willing to be corrected on that. The 850 is an excellent motor on methanol, with a close box. I know from experience, that for my possible purposes it is quick enough.

Ken,

No Thackeray washers.....what gap do you use when shimming the rockers?

Steve

acotrel said:

Who made the crank, Ken ? It looks really worthwhile. I don't think I'd ever try to up the capacity to 920, however in an 850 that crank would be great. I'm always very careful to keep the revs in check when I ride my bike - 7,000 RPM max. It spins up extremely quickly coming up through the gears, and I don't much like the thought of those heavy pistons at the ends of aluminium rods. The inertia must be horrendous. I know from experience that a Triumph 650 will cop 8,000 RPM on a fairly regular basis, so a near standard commando crank assembly should be relatively safe at 7,000 RPM. However I'm willing to be corrected on that. The 850 is an excellent motor on methanol, with a close box. I know from experience, that for my possible purposes it is quick enough.

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Looks like a Nourish crank to me.

Hi Ken!

That cast piston, how thick is the top land and what kind of top ring is it? Just curious.

I will do my own 920 conversion and have read some really good thread about it on this forum, what i miss is how much press fit on the liner in the cylinder?

Sten

Ken, ever think of crack checking your brain cases before going all out? i worry most about dumped on my head-neck again, so will wear neck protector and duct-Kinesiology tape on major joints to save crews trouble clearing path of my scattered parts.

I agree with JRB's observation- reasoning that the crack is more a pre-existing thermal stress relief from the horrific amount of welding done all around it. May not want to race a world record attempt again with NOS combustion violence but likely still fine for ordinary fuel racing and for sure a potent street engine. This is perfect place to hobot mind for magnet drawing in hi Temp slow set JBW after the crack ground out a bit and round drill holes at either end. Also Muzzy Welding has some Al repair sticks that sets up harder-stronger than base Aluminum but does appear darker.

After seeing flat head garden tractors making as much or more power/rpm than Nortons Peel will follow their lead to robustly strap head * cylinder down to cradle. Also occurs to me to have cases cast in 7075 TS or hi grade $teel.

TC proved Norton rods are good a long time for 150 hp and upper 8grand rpm and never the first item to fail.

My cracked-bashed brain tells me Ken is "wishful" thinking about locktite helping the crank bearing and its "slight" looseness induces a hi freq sharp impacting rattle that resonated with the length and location of the crack to open up more. My psychedelic - yogic- mind control - hi fevers - deadly poisoned background > allowed me to witness what happens to a Norton spun so hard/fast it turns into an elastic cartoon character - bolts popping up/dn and seams opening/closing like rolling surf waves breaking diagonally on a beach, ejecting clouds of white jets of vaporized oil ...

Fast Eddie said:

acotrel said:

Who made the crank, Ken ? It looks really worthwhile. I don't think I'd ever try to up the capacity to 920, however in an 850 that crank would be great. I'm always very careful to keep the revs in check when I ride my bike - 7,000 RPM max. It spins up extremely quickly coming up through the gears, and I don't much like the thought of those heavy pistons at the ends of aluminium rods. The inertia must be horrendous. I know from experience that a Triumph 650 will cop 8,000 RPM on a fairly regular basis, so a near standard commando crank assembly should be relatively safe at 7,000 RPM. However I'm willing to be corrected on that. The 850 is an excellent motor on methanol, with a close box. I know from experience, that for my possible purposes it is quick enough.

Click to expand...

Looks like a Nourish crank to me.

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Quite correct. It is one of Dave's cranks. I bought it from him back in the '80s, I think. I did have it dynamically balanced for the 920, with a bit of heavy metal welded on the balancing plug, but no other mods. So far it's still standard on the journals, but I need to check it again to see if it's finally time for a regrind. It is heavier than the stock crank, but I've never found that to be a problem.

Ken

john robert bould said:

Crack appears to be a weld contraction , now the case as been normalized from continual heating/cooling ..cannot see it growing//a little external sealing to keep the oil in should do the trick?

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Note the length of the crack on the inside. I don't think that qualifies as a weld contraction. And it does connect through to the small outside crack. I could see the dye penetrant go through from inside to outside.

Also, it's right on the line where the cases typically break, so I'm not inclined to take the chance.

I'll probably weld it up, just to see how it turns out, but I don't think I'll use it again on a race engine, and it's not really very practical for a street bike, unless I want to machine it for the primary cover and alternator studs. And there's also still the issue of the loose main bearing, although it would probably be ok on a street bike with Loctite and the locating washer.

Ken

hobot said:

My cracked-bashed brain tells me Ken is "wishful" thinking about locktite helping the crank bearing and its "slight" looseness induces a hi freq sharp impacting rattle that resonated with the length and location of the crack to open up more. My psychedelic - yogic- mind control - hi fevers - deadly poisoned background > allowed me to witness what happens to a Norton spun so hard/fast it turns into an elastic cartoon character - bolts popping up/dn and seams opening/closing like rolling surf waves breaking diagonally on a beach, ejecting clouds of white jets of vaporized oil ...

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I probably wasn't clear enough about the slight looseness statement. The bearing wasn't actually loose in the case. When I took it out the Locktite was still in place and holding the bearing, and the retaining washer kept the bearing from trying to spin in the case. I did have to heat the case quite hot to get the bearing out. What I should have said was that the measured clearance between case and bearing was too much for a good interference fit, and I'd probably have to sleeve it, and still use the retainer, to feel comfortable about using it again in a race engine.

Ken

SteveA said:

Ken,

No Thackeray washers.....what gap do you use when shimming the rockers?

Steve

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Correct, no Thackeray (Thackery?) washers. No specific gap spec. I just use shims and/or spacers to center the rocker tip over the valve, and make it tight enough to not move sideways, but loose enough to rotate freely. They probably have .005" or so end play, certainly no more than .010".

On new builds, I usually turn up bronze spacers to get the right locations for each rocker, but in this case I used the shims and spacers that the original tuner had used, and never saw a need to change them.

I don't think it's a real critical area. When I first started racing a Commando, I ran several years with engines using the stock spring washers, and never had a problem with them. I use spacers now just because it seems in theory like a better way to do it, not because I ever had a problem with the stock arrangement.

Ken

lcrken said:

SteveA said:

Ken,

No Thackeray washers.....what gap do you use when shimming the rockers?

Steve

Click to expand...

Correct, no Thackeray (Thackery?) washers. No specific gap spec. I just use shims and/or spacers to center the rocker tip over the valve, and make it tight enough to not move sideways, but loose enough to rotate freely. They probably have .005" or so end play, certainly no more than .010".

On new builds, I usually turn up bronze spacers to get the right locations for each rocker, but in this case I used the shims and spacers that the original tuner had used, and never saw a need to change them.

I don't think it's a real critical area. When I first started racing a Commando, I ran several years with engines using the stock spring washers, and never had a problem with them. I use spacers now just because it seems in theory like a better way to do it, not because I ever had a problem with the stock arrangement.

Ken

Click to expand...

Yep, I spelt it correctly! 8)

Thanks Ken. Yes, as far as I can tell this is a real 'some do some don't' ....

Please tell me, anyone, what interference fit should i use for the 920 cylinder liner in my iron block?
I have found a general recommendation of 0,0025" in cast iron blocks but wants to know for sure if this is ok in this specific case?

Sten

lcrken said:

SteveA said:

Ken,

No Thackeray washers.....what gap do you use when shimming the rockers?

Steve

Click to expand...

Correct, no Thackeray (Thackery?) washers. No specific gap spec. I just use shims and/or spacers to center the rocker tip over the valve, and make it tight enough to not move sideways, but loose enough to rotate freely. They probably have .005" or so end play, certainly no more than .010".

On new builds, I usually turn up bronze spacers to get the right locations for each rocker, but in this case I used the shims and spacers that the original tuner had used, and never saw a need to change them.

I don't think it's a real critical area. When I first started racing a Commando, I ran several years with engines using the stock spring washers, and never had a problem with them. I use spacers now just because it seems in theory like a better way to do it, not because I ever had a problem with the stock arrangement.

Ken

Click to expand...

My old 500cc Triumph engine used to rev to 10,500 RPM, and I used bronze spacers and lightened rockers. However I since come to believe that the Thackeray washers might provide a certain amount of desirable friction damping. I haven't bothered to modify the valve train in my commando engine. I believe that apart from the cam followers it should be safe to 8,000 RPM, and I never use more than 7,000 RPM. So unless the springs collapse, with the standard cam I should not have a problem.

billet said:

Hi Ken!

That cast piston, how thick is the top land and what kind of top ring is it? Just curious.

I will do my own 920 conversion and have read some really good thread about it on this forum, what i miss is how much press fit on the liner in the cylinder?

Sten

Click to expand...

Hi Sten,

the top land is 0.145" (3.68 mm) thick, and the top ring is .059" (1.5 mm) wide chrome faced iron.

I use .0015" interference fit for iron sleeves in iron cylinders, for all bore sizes. Bacause the sleeve is always at higher temperature than the cylinder, the fit stays tight in use. I think .0010" would probably be enough, but .0015" gives me a little safety margin. For iron sleeves in aluminum cylinders, I use .001" per inch of diameter. For Commando cylinders that's usually .0035" - .0040" interference fit. A tighter fit in the aluminum sleeves is not useful, because the tensile strength of the aluminum won't accommodate it. It just expands to the limit of it's strength, and you are back to the looser fit.

The pistons were originally from Fair Spares, now Norvil. As I recall they are just English Ford 4-cylinder pistons with the valve notches added to suit the Commando. They use a larger pin, and the stock rods were honed to the larger size.

Ken

Thank you Ken for this useful information!

Very important for me

Sten

I use to use the spacers instead of the Thackeray washers (Triumph) because it supposedly freed up some hp at hi rpm but it sure makes assembly of the Triumph top end tougher with the rocker arms flopping around. Then I read the specs on Romero's dirt tracker and it used the Thackeray washers, if they are good enough for Gene then that is good enough for me and makes assy. easier (Tri) and possibly a quieter top end too...

Back on topic, I have an old Gerex multifire capacitive discharge ignition for my Kawi Z1, since it fired 2 dual lead coils and the Z1 has 2 pistons up/2down at the same time I should be able to adapt it easily to a dual plugged 360* twin, does anyone remember these units? I had one of the 2 pickups go bad and after a short but tiring push (540#) reinstalled the points set up but now I'm thinking it might go on a dual plugged Triumph or add an opposing magnet on the rotor and use one pickup for single plugged heads, does anyone think this would be worth pursuing? Keep in mind I have one bad pickup unit and probably no parts avialabilty or enough knowledge to repair the pickup, I guess I should be asking if the unit can be or is worth repairing first...after reading here about comnoz's multispark high energy system this old unit came to mind...not to far off topic I hope..Mark

marksterrtt said:

I use to use the spacers instead of the Thackeray washers (Triumph) because it supposedly freed up some hp at hi rpm but it sure makes assembly of the Triumph top end tougher with the rocker arms flopping around. Then I read the specs on Romero's dirt tracker and it used the Thackeray washers, if they are good enough for Gene then that is good enough for me and makes assy. easier (Tri) and possibly a quieter top end too...

Back on topic, I have an old Gerex multifire capacitive discharge ignition for my Kawi Z1, since it fired 2 dual lead coils and the Z1 has 2 pistons up/2down at the same time I should be able to adapt it easily to a dual plugged 360* twin, does anyone remember these units? I had one of the 2 pickups go bad and after a short but tiring push (540#) reinstalled the points set up but now I'm thinking it might go on a dual plugged Triumph or add an opposing magnet on the rotor and use one pickup for single plugged heads, does anyone think this would be worth pursuing? Keep in mind I have one bad pickup unit and probably no parts avialabilty or enough knowledge to repair the pickup, I guess I should be asking if the unit can be or is worth repairing first...after reading here about comnoz's multispark high energy system this old unit came to mind...not to far off topic I hope..Mark

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Definitely worth considering if it has an advance curve built into it. I'm still thinking about the Nippon Denso units used on the V-twin Honda Shadows, however getting a way to reliably trigger it from the camshaft seems it might be a problem. As I'm using methanol, I can probably afford to be a bit sloppy with the shape of the curve I use.
Is this an issue ? :
http://forums.sohc4.net/index.php?topic=78861.15;wap2

Alan, trigger it from the crank....

Ugh, sorry guys but I got 2 posts mixed up when I commented on my Gerex ignition when responding on Thackeray washers, I have also been following comnoz's combustion post but in answer to acotrel's response, I don't know about the Gerex box ignition curve but if I remember correctly it was full advanced at 2500 rpm like the points system and like the Triumph, I'll have to do some research and sorry about the thread jacking...Mark