920 Race Engine Teardown

I had to tear the 920 Commando engine down after going to Bonneville this year, because it developed a crack in the crankcase. I thought it would be a good opportunity to post details of how the iron cylinders were sleeved, for those who haven't already seen them. These are the cylinders that originally were in the factory short stroke 750 dirt track bike I bought back in the '70s from John Hateley. That's why it has the socket head set screws in the lifter tunnels. They were used to locate the adapters for the BSA radiused lifters used with the Sifton 460 cam. I still use the BSA lifters, but they are now located with the locking plate like stock lifters, and the screws are now just to seal the holes. When the short stroke 750 blew at Steamboat back in the day, I salvaged the cylinder by having it sleeved out to 81 mm bore for the 920 kit. This is a shot of the cylinder deck surface, and you can see why there is a problem getting a good ring seal around the counterbores for the through bolts. There's no support for the sleeves there. You can also see the copper o-ring seal in this picture.



This is a more detailed shot of the counterbore area. The notch in the liner lip is to clear the head of the through bolt.



This is a shot of the area where the two liner lips meet.



This is a shot of where the cylinder gets bored through and exposes the liner. I seal this area with a Locktite penetrating sealer.



This shot shows how little of the lower part of the cylinder remains.



These are the pistons I've been using in this engine for a really long time, including a lot of use as a road racer before I started running it at Bonneville. They are cast pistons that I machined for high compression years ago, and have worked great up until this year, when I started using nitrous oxide. I didn't leave enough material in the top land to stand up to the extra pressure, and the ring grooves compressed enough to lock the rings in place. I'll replace them with my forged JE pistons with thicker deck and taller top land.

If you look to the left, you can see the crack in the crankcase, circled in red. I won't know how large it really is until I get it apart and check it with the dye penetrant. I'm hoping it isn't too serious, and that I can repair it by welding. We'll see.



This is a shot of the bottom end, which I haven't got apart yet. The stock rods have worked really well all these years, but will be replaced with Carrillos for the nitrous use.



The head is a modified short stroke 750 head, that I'm planning to send to Jim Comstock to run through his flow bench, and add to his growing collection of port flow data. I'll add some pictures of it later.

Ken

thanks for that Ken, yep thats really pushin it!!!!

Ken, despite it looking so 'wrong' when you see barrels bored out for big liners, your motor would seem to show that, if executed with care, it does in fact work ok.
Few road (or road race) motors would ever get put to the same WOT treatment as a Bonneville runner. So if it works there, it should work anywhere!
Please do keep us posted, and please keep the pics coming!

Fast Eddie said:

Ken, despite it looking so 'wrong' when you see barrels bored out for big liners, your motor would seem to show that, if executed with care, it does in fact work ok.
Few road (or road race) motors would ever get put to the same WOT treatment as a Bonneville runner. So if it works there, it should work anywhere!
Please do keep us posted, and please keep the pics coming!

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+1 thanks

I wonder if the big bore would work OK if the sleeve centres were each offset 1mm or 2mm towards the left and right outside of the barrel- is it really necessary to have the rod little end central between the piston bosses ?

The through bolts which are allready very close to the sleeves will become a real problem....I had two worn 850 jugs machined in my local shop , but they were scrapped due to that , oil was seeping !!need to be an experienced engineer to do it and as said before , not two jugs are the same!

lcrken said:

This is a shot of the area where the two liner lips meet.

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Can't get much closer than that! Nice work by whoever did it.
I never realized exactly how extreme a 920 is.
Thanks for the pics.

acotrel said:

I wonder if the big bore would work OK if the sleeve centres were each offset 1mm or 2mm towards the left and right outside of the barrel- is it really necessary to have the rod little end central between the piston bosses ?

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Not possible without moving cylinder and head bolts in crankcases, cylinders, and head.

That's what Steve Maney's 1007 kit does. The 83 mm bores are offset to the outside by 1 mm each. To do so he had to also move the head and cylinder bolt positions towards the outside. Since he makes his own cylinders and crankcases, he was able to make the necessary changes in the castings and machining. He converts 850 (or 750) heads to the new bolt pattern by boring out the existing holes, fitting plugs, and drilling the new holes. The combustion chamber counterbore is also re-cut to match. The pistons have the valve cutouts moved inward from the center of the piston so they remain in the same position relative to the valves. To move the rod throws out, he simply uses a wider flywheel. His crankcase castings already have enough material in them to allow machining for the wider crankshaft.

In addition to the 1007 kit with it's 93 mm stroke crankshaft, it is also possible to build a 83 mm bore engine from the same parts, but with a standard 89 mm stroke crankshaft, which gives you a 963 cc engine with a bit higher rev limit.

Ken

acotrel said:

I wonder if the big bore would work OK if the sleeve centres were each offset 1mm or 2mm towards the left and right outside of the barrel- is it really necessary to have the rod little end central between the piston bosses ?

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There is a company in Oz, http://www.healeyfactory.com.au/dmd/components/ that makes replicas of "Works" alloy block motors for Austin Healey 3000's.

In their enthusiasm, they also made it in 3.2l and 3.8l sizes by having the c/l of the rod offset from the center of the bearing bosses as they had to do a bit of siamesing of the bores.

There used to be a vid of the big bugger being powered up, not sure if it is still there.

Goodness!

Im amazed and impressed that motor stays in one piece! It is stretched a long way further than I thought it would be reliably possible.

I'm inclined to believe that increasing the capacity to get more torque by overboring or increasing the stroke doesn't give big problems with the components unless the operating rev range is moved upwards. The internal loads increase as the square of the accelerations involved. When you overbore or increase the stroke, the increase in the loads is more linear. The same sorts of laws tend to apply when you try to overcome wind resistance at high speeds - it is a squared relationship ? I think it is much better to race a commando on a tight twisty circuit than a big one such as Daytona or Ontario, especially if you get the torque, gearing and steering characteristics right.

Nice engine, ken.

The pistons look very clean on top, is that d/t the short time running or the type of fuel and leaness of the attitude at Bonniville? Also the side of pistons show some wear and I'm studying piston wall textureing for oil pocket dragging so helpful to me. Very strange place to see a case crack with all the surrounds so beefed up.

acotrel said:

Nice engine, ken.

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Thanks, Alan. It's been a good engine for me for quite a few years, but I think adding nitrous exceeded it's capabilities. I'm putting together a couple other engines intended for nitrous that should be a bit more sturdy.

Ken

hobot said:

The pistons look very clean on top, is that d/t the short time running or the type of fuel and leaness of the attitude at Bonniville? Also the side of pistons show some wear and I'm studying piston wall textureing for oil pocket dragging so helpful to me. Very strange place to see a case crack with all the surrounds so beefed up.

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The color in the pictures is misleading, Steve. The flash makes it look lighter than it really is. In fact, the top is pretty black, except for some parts of the squish band. I've been running it just a bit on the rich side at Bonneville because of the tendency to lean out a bit at the top end of the run. The wear on the skirt is also exaggerated by the flash. There is really no significant wear there. I'll see if I can get a better picture without the flash.

It is an interesting location for the crack. I'll be interested to see how far it extends, once I get the bottom end apart.

Ken

More pictures. First ones show the combustion chamber, valves, and port details. For anyone who notices the corrosion around the exhaust valve seat, that's after effect from the salt air environment, and not something that happened while the bike was running.







Next some shots of the valve train bits.





The intake valve is from a Gold Star, and the exhaust is a Triumph item, and the rockers were lightened and polished that way when I got the original bike. That's how the head was set up by the factory tuner. The push rods are also the originals from the race bike. The BSA lifter blocks were originally aluminum, but I replaced them with the bronze ones somewhere along the way. Valve springs are RD. I've been using 36 mm Amals and 1 5/8" pipes into Axtell megaphones. Compression ratio was 10.8 for Bonneville efforts. ARD magneto set at 28 degrees on gas and 27 degrees on nitrous.

I think that's about it. I'll add some more pictures when I get the crankcases apart.

Ken

Understand the flash wash out but still able to see quenched rim and good combustion sense. The valve looks like still good meat around them, would that be something Peel's Combat 920 head could take someday?

Per piston sliding sheen, I really liked the oil spreading pockets so tried my version on Kohler 18 hp boxer twin Bore Tech + new pistons. Goverenor fell apart just prior to start up so gave up on other disasters this year. Jim also impressed me on the Norton stroke rpm friction so think there's something to pay special attention too on fragile Nortons.

Per the strange crack, likely was pre-existing below surface from prior stress prior to the weld up so a missed chink in armor that took till now to expand to surface. If just a sealing issue then just 'surface' patch over but if a structural stress riser fracture must essentially melt about all the way down and through with almost surperman magnitfied X-Ray Vision to follow the crack by blowing away the Al puddle then filling back in + plus a little extra. Hey at least it ain't near a mount flange or a seam.

Old farts are not supposed to be swishing and swashing on unstable surfaces at limb tearing speeds touching off Nitous Oxide for extra thrills!

Next installment. This is a shot of the crankshaft and rods, alongside the drive side crankcase half.



And these two shots show the crack in the crankcase from the inside, and from the outside. I think Hobot may have it right here. I suspect there was a crack there when I welded up the reinforcements, and it's just been growing slowly since. The crack is pretty large from the inside, and barely visible from the outside, and does not extend into the welded reinforcements. The red dye shows the crack.





And a couple shots of the crank and rods.





I'm debating about repairing the cases. The crack is right in the high stress area of the crankcase, and I'd have to put a lot of heat into the case to get deep enough penetration for the weld. I've got a lot of good years out of these cases, and I think it's time to retire them. The main bearing is also a bit loose in the case. I used penetrating Loctite along with a locating washer last time I rebuilt it, and they seem to have held up quite well, but if I wanted to keep using the cases, I'd probably feel obligated to sleeve the mains. All in all, I think new cases are the better choice here. I have a set of Mk 3 cases (that's what the crank fits) with a Maney drive side half that I haven't used yet, so I think I'll probably use it with this crank. 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.

Ken

hobot said:

Old farts are not supposed to be swishing and swashing on unstable surfaces at limb tearing speeds touching off Nitous Oxide for extra thrills!

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Who better? It would just be wasted on young folks, who have plenty of other ways to generate endorphins :lol:

Ken

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?

.
" 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!