Hollow Tappet Adjusters

With my roadracer I always just stated out at about .020 squish and reduced it until there was contact. Then I increased it a few thousandths. Just shy of contact is where the best power is.
I usually found it ended up at about twice the piston to cylinder clearance.
That was with steel rods and full skirt pistons.

I broke a few cases but not from piston contact with the head.

SteveA said:

Norman White showed me JE pistons he had had made that are very similar to JSMs JSM made pistons and told me he had used titanium rods at one point and had got the squish down to a few thou......

Most things have been done at some point!

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Perhaps he meant within a few thou in a running engine.
The Martians could have supplied the connecting rods but there is no way you could run a on the bench 0.003" squish clearance.
A larger squish dimension that reduces to 0.003" from contact maybe.
0.003" would nearly contact on the kick starter with a short skirt piston (rock at TDC).

comnoz said:

With my roadracer I always just stated out at about .020 squish and reduced it until there was contact. Then I increased it a few thousandths. Just shy of contact is where the best power is.
I usually found it ended up at about twice the piston to cylinder clearance.
That was with steel rods and full skirt pistons.

I broke a few cases but not from piston contact with the head.

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That sounds a lot like the 2-stroke mantra that the best power is just before it seizes. My diagnosis of the 872 failure could be wrong, but the contact with the head was quite severe, and it looked to me like that was a lot of force trying to rip the top and bottom of the engine apart. It happened at a Laguna Seca race, and my friend, who was riding the bike, admitted to running it well past the red line.

Ken

lcrken said:

That sounds a lot like the 2-stroke mantra that the best power is just before it seizes. My diagnosis of the 872 failure could be wrong, but the contact with the head was quite severe, and it looked to me like that was a lot of force trying to rip the top and bottom of the engine apart. It happened at a Laguna Seca race, and my friend, who was riding the bike, admitted to running it well past the red line.

Ken

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Severe contact might do it. Never had one hit that hard.
I have had them hit hard enough to tighten up the top ring groove though.

Did you tell "your friend" it wasn't a Yamaha.

"That sounds a lot like the 2-stroke mantra that the best power is just before it seizes."

That goes for 4 strokes also.
This is why I didn't ride my Norton to Bonneville. My luck with forged pistons has not changed. I am putting it together with cast pistons this time.

7800 miles and then I came to a 15 mile long hill on a straight road. The clearance was .0065 when I assembled it.

comnoz said:

"That sounds a lot like the 2-stroke mantra that the best power is just before it seizes."

That goes for 4 strokes also.
This is why I didn't ride my Norton to Bonneville. My luck with forged pistons has not changed. I am putting it together with cast pistons this time.

7800 miles and then I came to a 15 mile long hill on a straight road. The clearance was .0065 when I assembled it.

View attachment 12155

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That's just nasty looking. And it looks like you had the pistons coated too. I'm also a big fan of cast pistons for street Nortons. For the first several years that I raced my Production Racer I used the stock Hepolite Powermax cast pistons at .004" - .0045" clearance with never a problem, except eventually the ring lands just wore out.

Ken

lcrken said:

I once rebuilt a Commando 872 cc engine with Nourish one-piece crankshaft, forged Omega pistons, and Crower titanium rods. It had been originally built with .020" squish, and in racing use the pistons had hit the head hard enough to break the crankcases. I rebuilt it with .040" squish and new cases, and it never had that problem again. It's very risky to try to push the limits of squish clearance in any Commando engine.

Ken

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Pistom and con-rod mass mass must have a big effect on the performance of a Commando engine. Probably the trick is to use the longer rods so the rock-over at TDC is slower, with the lighter pistons. Then you can rev higher and the motor will spin-up faster. On every stroke at TDC and BDC, there is a reversal of the pistons and rods, so inertia becomes an important factor. There is probably a lot of con-rod stretch, which limits how close you can run the squish band, if you are using high revs. All the more reason to work at getting more torque out of the engine, rather than top end horsepower. Also, using petrol as a fuel, limits what you can do. With race motors, heat build-up also imposes a limit. In a lot of cases, the length of the races is a factor. Some bikes go well for a few laps, then slower, others do the reverse.
With very old two strokes on methanol - they are usually fast until the crankcases heat up. And by that time the drum brakes are fading. - Good for three lap races on short circuits.
With four-strokes, a lot of guys believe that big inlet ports give more power. They probably do at high revs. But that is not a Commando engine. Other things impose a limit to the revs you can use. You have a choice - a top end motor is usually very expensive. Methanol is not. Change your class rules to allow it.

lcrken said:

That's just nasty looking. And it looks like you had the pistons coated too. I'm also a big fan of cast pistons for street Nortons. For the first several years that I raced my Production Racer I used the stock Hepolite Powermax cast pistons at .004" - .0045" clearance with never a problem, except eventually the ring lands just wore out.

Ken

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Yeah, I raced with both cast pistons and forged pistons. No problem.

But roadracing is an easy life for pistons. You never keep the power on very long since there is always another corner coming up soon. Rolling off the throttle gives them time to cool.

The only time I have had any piston problems on the roadracer was when using forged pistons at Daytona.

[unless you go back to the RD350 days, then I was always sanding out the seizure marks or dumping the chunks out of the expansion chamber]

comnoz said:

[unless you go back to the RD350 days, then I was always sanding out the seizure marks or dumping the chunks out of the expansion chamber]

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As much as I hate to see anyone lose an engine, I had to laugh upon seeing the whitish fireball out the back of an unsilenced expansion chamber when someone lunched a piston.

Nater_Potater said:

As much as I hate to see anyone lose an engine, I had to laugh upon seeing the whitish fireball out the back of an unsilenced expansion chamber when someone lunched a piston.

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Yeah, I raced an RZ350 with TZ porting in the cylinders. It worked well in the TZ but the RZ engine ran the other direction so when the piston left BDC it would attempt to push the piston out of the big exhaust port.

Every 4 to 6 races it would succeed. So, new pistons every weekend was the rule. But it won the championship.....

I raced a T250 Suzuki with chambers and larger carbs and minor porting , on methanol. It did a very fine job of blowing off a new TZ350G Yamaha on an airstrip race circuit. I never liked it, even though it was extremely fast. So I sold it. The guy who bought it won 28 races and 4 historic championships with it. Whenever he replaced the motor he maintained the porting in the next one. What I did not like, was it was blisteringly fast until it seized. I never broke a piston, but methanol runs very cold, so the performance of the motor changes as it warms-up during races.

We

Time Warp said:

Perhaps he meant within a few thou in a running engine.
The Martians could have supplied the connecting rods but there is no way you could run a on the bench 0.003" squish clearance.
A larger squish dimension that reduces to 0.003" from contact maybe.
0.003" would nearly contact on the kick starter with a short skirt piston (rock at TDC).

Click to expand...

Well you said 3 thou not me. As to what Norman actually said, it is going on 5 years ago, so I wrote 'a few' rather than try to be specific on number, but if I recall it was 7 thou (maybe 10, but like I said 5 years).

Surely, whatever figure he quoted it had to be something he could measure, and yes, piston rock makes it harder to do. But I am guessing he has a method for that too, with 40 years working on it!

7 is 'a few' in my mind when I am setting 30 plus with steel rods and iron barrels!

Remember that Norman also made his own alloy barrels at one point and was likely using those with titanium rods.

The discussion took place when I was there with two sets of barrels and JSM pistons to get them bored to suit.

I suspect that when the motor is revving high, the centre of the crank grows especially if the balance factor is not right, and the rods stretch due to the inertia of the pistons. Perhaps you need to run a larger squish clearance if you run the motor at higher revs ?

Norman also told me that when he worked for HRC they would set the cold squish to zero!

A very careful warm up routine would then allow the coefficient of expansion of the different components to generate the required tolerances.

Which I thought was both impressive and scary !!