Lightweight forged piston clearances measured hot

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Rohan said:
acotrel said:
Have a look at a piston out of a Jawa speedway motor . They are stressed to the max.

Is that good ???
The pistons used in two valve Jawa speedway engines were Mahle forged type. They make a standard Commando piston pale in comparison.
 
Mahle were making pistons for manx nortons, at some stage ?
That was a while back, but they were like $500 each ?

You don't hear of too many pistons failures in (stock) Commandos ?
For something built and sold as a touring/sports roadbike 40+ years ago,
thats not a bad record....
 
I must say that I am amazed just how good the 850 commando motor is. I expected it to let go the minute I started racing it. I believe the stroke and crank weight are a superb design. At 7000 RPM cast pistons should stand up forever. I would never try to increase the rev limit or fit cams where the power band extended much above 7000 RPM, however I don't know where the top of the power band is in my own motor. It seems to want to rev out forever. If max torque occurs above 7000 RPM in the 850, I suggest you have a problem.
Theoretically you could build a 75mm stroke 750cc commando motor and rev it to ten with only slightly modified valve gear and forged pistons. Then you'd have to ride the nasty piece of garbage.
 
Steven Maney tries to limit his 920's to 7200 with lighter pistons, heavy cases and stronger cranks than the desert racers 750's and TC's ole 850's exceeding 8000 grand on shifts. Friction vs fuel burn and crank whip seem to merge about 8000 so may be counter producitve beyond that. Rev limiter seems a vital device if ya knew where to set it for sure.
 
I don't think I will ever fit a rev limiter to my bike. Years ago when I used magnetos, a miss up high was the last thing you ever wanted to hear. Even with an updated ignition system, I could not bear to hear it. If you are racing, it probably means you have paid all your fees and transport costs and driven hundreds of kilometres for nothing.
 
I can relate to loss of power when that's what holding bike up as well as steering it. I've asked w/o a response how the various rev limiters available for Norton worked-felt, sharp kill for on/off jerking or dropping out some sparks to just stop making more power/rpm. Still a seized engine likely worse.
 
I used a Boyer rev limiter for a couple years and never found it to affect racing results. It was an analog circuit, not digital, and was adjusted by turning a screw, and the running the bike up to see where it cut out. Modern digital systems are much better. I set it a couple hundred rpm higher than my max desired rpm (7200), just for insurance. The only times I ever recall actually hitting that limit were a couple instances during starts, when I wasn't watching the tach, and didn't shift in time. I tried to gear the bike to just reach red line at the end of the fastest straight, so didn't really have any reason to want to over-rev it during the races. The only exception was Daytona, where I deliberately geared the bike a bit high, partly because it was at WOT for so long, but mostly because you could easily pick up a couple hundred rpm running in someone's draft. The Boyer limiter didn't seem to be all that precise, and I eventually quit using it. It was pretty abrupt when it did cut in.

The only other adjustable rev limiter I've used was one I designed and built myself back in the '70s. In prototype form it was about the size of a cereal box, and I hung it on the side of the bike. It worked well, but I only ever got to use it in one race, at Sears Point, and promptly crashed and wiped out that side of the bike, including the rev limiter. It was actually fairly expensive to build, and I decided that building another one wasn't worth the money and effort. It's only real advantage over the Boyer limiter was that I could set the rpm limit very accurately with a set of dip switches. It was also a simple cutoff circuit, with no soft limiting. I had planned to add that after I was sure it worked properly in action.

Lots better digital limiting devices available now, so no point in reinventing the wheel.

Ken
 
A few extra 100 rpm in someone's draft eh, Cool Ken. Thanx for old school limiter review and crashing on yours like all my heroes do to qualify. Part of a limiters sex appeal for Peel is putting her on limiter for drift games like stunt guys rough sounding redlined for some time. That's what the hottest flat trackers are doing a good bit of the time playing like they was jet skis.

Meanwhile back at the ranch - I think pistons in air cooled hot rods can reach 700'F and barrel only couple hundred less so why limit temp testing under bacon sizzling? Peel will have to have a cylinder thermometer to know when able to put hi heat to er. If the relative clearance stays ok, hurray!

Lightweight forged piston clearances measured hot
 
Of course anyone can test the growth rate of pistons and cylinders at 700 deg or higher. The whole point is not to overheat and seize the motor. Best thing to do is pay attention to what you hear from others - especially from those who have pushed things to destruction.

The piston below got very hot and was at its limit of tightness in the bore. It must have been at zero clearance or less when hot and maybe burnished itself in. There is no evidence of rocking and no bore wear was recorded because the piston was tight enough in the bore to prevent rocking, but not so tight as to damage the cylinder.

Lightweight forged piston clearances measured hot


The recommended clearance of JS forged pistons is .0055” to .006” for 850 and larger pistons. These clearances have worked well but anyone should go through a proper break in if they want it to come out right. Those who are in too big a rush can expect some damage or seizure if a brand new motor is run at continuous WOT under heavy load. If you can go through a proper breakin then the pistons have time to bed themselves in. You will have a better seal and everything will last longer. This is common knowledge and no new motor should be taken to its limits.
 
Many years ago we had aftermarket pistons in Australia which were cast from hi-silicon aluminium. After they had seized a few times and been filed down, they were perfect. We were never smart enough to solution heat treat them prior to use.
 
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