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#853409 07/10/21 12:22 am
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Hey folks,

I have a technical question for the collective brain power here…

When looking at engine performance parts for our old bikes, I was struck by how high-performance pistons found in widely available big bore kits still look like an old soup can with the long piston skirts, when compared to modern pistons. I assume light-weight pistons means a rebalanced crank, but crank balancing doesn’t seem to be an insurmountable task. Would combustion heat management be a factor? Or the primitive oiling system?

Are there other factors inherent in our (80-year-old!!) engine design that makes this physically impossible? Or has the performance ceiling of this engine design basically been reached with “period correct” design and engineering? Or does the law of diminishing returns play into this?

Are there “modern” big bore kits out there that I simply haven’t seen yet?

Thanks!

Dale

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Hi Dale, Look at the tech articles in Vintage Bike Magazine.

Yes, basically heat management, cooling. Same with valves, seat widths.

Oil system was designed for stock motor. Modern motors with small skirt pistons most often have piston cooling oil spray nozzles & hollow passage in piston to accept & distribute cooling oil. Working at the dealership as the small skirt piston came to be in production motors to save friction thus fuel, a lot of problems came with short skirt lightweight pistons.

What about the cooling properties of cast iron air cooled cylinder? It's not good. When you get into small skirts heat management is a huge issue. Liquid cooling becomes almost a must. Aluminum cylinders cool much better with air cooling. Not to mention saving lots of weight.

Ring width is very important to cooling as well. Much piston heat is transferred to cyl. walls via rings.

Map cycle has some high performance pistons. Check them out. Still a track motor often doesn't really have the day to day durability for street use.

Amazing the power Triumph got a lot of power out of a motor, basically similar to the speed twin of 1938. Might be getting to it's limits??
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The Harris 750 had Gilardoni alloy barrels with Nicasil coated bores for a while until Mr Bloor put a stop to them. If you can get a set of these with the late UNC studs they allow 0.002" skirt clearance and have much better heat transference. I have two sets of these with the original Asso Werke pistons, you can certainly run short skirt or slipper piston. And no, I'm not selling.

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People do put modern pistons in old engines.

The pistons on general sale are like-for-like replacement parts: they fit, they work, they’re cheap and they look reassuringly like the pistons they are replacing.

To get the best from modern pistons, you may need longer rods and/or a shorter barrel. Then you still need a dome on top of the piston (what does that do to the performance of a shallow piston?), or a radically altered cylinder head, to get a high compression ratio.

Then you may have to address alloy/iron differential expansion. As said above, alloy cylinders might help. You still have the heat distortion inherent in a twin cylinder single casting, cooled only by the wind in the willows blasting against one side of it at wind speeds varying from zero to 100 mph. So are you now going to water cool your Triumph?

Last edited by triton thrasher; 07/10/21 8:43 am.

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Stroke to bore ratios have changed over the years with a move to shorter stokes and hence the shorter pistons.

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Originally Posted by kommando
Stroke to bore ratios have changed over the years with a move to shorter stokes and hence the shorter pistons.


I don’t see why longer stroke would need a longer piston.


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I guess longer in proportion to the bore?

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These bikes are getting old, and as someone told me years ago, when you tune an engine in a British bike, quite often the niceness goes out of the engine. I'd have to agree. I'd rather ride a 5ta than a t100 Daytona. You can buy a modern 250, used, for probably what you would spend on tuning bits, that would run rings around a classic.

Remember what Edward Turner said of the twins, they were at their best at 500cc, 650 was getting near the limit.

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Yes, but he was full of sh1t, witness his Bandit design which had to be completely re-worked!

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Just saying, no point over stressing old bikes with engine tuning you are not going to use. I'd prefer to do things to make the machine better overall. Better brakes, better ignition, electronic ignition, better lights. The amount of times i have had someone in traffic, tell me they could barely see my bike in the dark.

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Modern engines with very short pistons have very short strokes compared to our old things. I expect that has a lot to do with why even MAP etc don't sell hi-comp pistons that have really short skirts.


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Isn't there someone doing modern pistons for classic Nortons, Jim Schmidt or someone ?

Yup.

https://jsmotorsport.com

https://jsmotorsport.com/js-motorsport-technical-pistons-and-rods/

Last edited by bon; 07/10/21 2:28 pm.
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Originally Posted by bon
Isn't there someone doing modern pistons for classic Nortons, Jim Schmidt or someone ?

Yup.

https://jsmotorsport.com

https://jsmotorsport.com/js-motorsport-technical-pistons-and-rods/


It looks the part!

[Linked Image from jsmotorsport.com]


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Originally Posted by bon
Just saying, no point over stressing old bikes with engine tuning you are not going to use.

True that!

Last edited by triton thrasher; 07/10/21 3:13 pm.

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Originally Posted by TR7RVMan
Hi Dale, Look at the tech articles in Vintage Bike Magazine.

Yes, basically heat management, cooling. Same with valves, seat widths.

Oil system was designed for stock motor. Modern motors with small skirt pistons most often have piston cooling oil spray nozzles & hollow passage in piston to accept & distribute cooling oil. Working at the dealership as the small skirt piston came to be in production motors to save friction thus fuel, a lot of problems came with short skirt lightweight pistons.

What about the cooling properties of cast iron air cooled cylinder? It's not good. When you get into small skirts heat management is a huge issue. Liquid cooling becomes almost a must. Aluminum cylinders cool much better with air cooling. Not to mention saving lots of weight.

Ring width is very important to cooling as well. Much piston heat is transferred to cyl. walls via rings.

Map cycle has some high performance pistons. Check them out. Still a track motor often doesn't really have the day to day durability for street use.

Amazing the power Triumph got a lot of power out of a motor, basically similar to the speed twin of 1938. Might be getting to it's limits??
Don


Lots of good points here Don. I'll see what I can find in Vintage Bike Magazine. Much appreciated.

My interest in the "modern piston" discussion comes after looking at the multiple high performance bolt-on engine kits for Evo Sportsters. Check out the pistons here: https://www.1250kits.com/ Incredible power increases out of those air cooled bikes when modern technology is applied. But to your points, those are alloy cylinders, and they employ oil jet piston cooling.

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Originally Posted by triton thrasher
Originally Posted by bon
Just saying, no point over stressing old bikes with engine tuning you are not going to use.

True that!


Agreed! I'm interested in finding hard data that shows at what point in power output these engines are actually over stressed, and how to eliminate stress points. Seems like lower reciprocating mass would be less stressful than higher RM.

It's interesting JS Motorsport claims, "The combination of lighter pistons with our longer lightweight rods makes for a much smoother and reliable ride whether you are cruising down the highway or pushing it to redline on the racetrack."

Nothing wrong with smoother and more reliable, especially if there's also a little more thrust! Clearly engineering isn't my strong point...but curiosity is. :-)

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Originally Posted by triton thrasher
Originally Posted by bon
Isn't there someone doing modern pistons for classic Nortons, Jim Schmidt or someone ?

Yup.

https://jsmotorsport.com

https://jsmotorsport.com/js-motorsport-technical-pistons-and-rods/


It looks the part!

[Linked Image from jsmotorsport.com]


I have a piston out of a Ducati sports bike. Nearly 4 inches in diameter and its only half the length of that piston in the picture. Massive bores, tiny strokes and they rev like a 2 stroke !

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Originally Posted by triton thrasher
Originally Posted by bon
Just saying, no point over stressing old bikes with engine tuning you are not going to use.

True that!


Except if you are racing or hill climbing or something like that. The speed limit here on motorways is 75 mph, if you are enough of a masochist to want to subject a classic machine to motorway riding, then you don't need 120mph performance machine to ride them.

Personally i think a larger capacity engine in a lower state of tune is the most pleasant. But everyone has their own ideas.

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Originally Posted by Dale Hoover
Originally Posted by triton thrasher
Originally Posted by bon
Just saying, no point over stressing old bikes with engine tuning you are not going to use.

True that!


Agreed! I'm interested in finding hard data that shows at what point in power output these engines are actually over stressed, and how to eliminate stress points. Seems like lower reciprocating mass would be less stressful than higher RM.

It's interesting JS Motorsport claims, "The combination of lighter pistons with our longer lightweight rods makes for a much smoother and reliable ride whether you are cruising down the highway or pushing it to redline on the racetrack."

Nothing wrong with smoother and more reliable, especially if there's also a little more thrust! Clearly engineering isn't my strong point...but curiosity is. :-)


Look at modern bike engines that are as smooth as silk, docile at low speeds yet can hit speeds no old British bike can ever get near. Balancer shafts to kill crank vibes, four valves per cylinder, big bores and short strokes, variable valve timing, computerised engine management systems, electronic fuel injection, all sorts of light yet strong metals in them, machined to tolerances that the old stuff could never be.

A set of titanium conrods and light set of pistons isn't going to transform your bike into a rocketship.

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Hi, The T140 motor has dames stroke as 650. About 1/2” shorter rod. Pitons a fair amount shorter. Wrist pin closer to crown. For whatever reason pistons tend to wear faster & rattle much more at same clearance as 650. Bore is about a 1/4” larger.

I’ve ridden 650 & 750 twins back to back many times on same roads. Very interesting how the short rod with larger bore feels so different. Even when putting 750 in 650 frame. Same ignition systems, timing the same, carbs same but different jets.
Just runs different. The motor kicks different when starting. Overall the 750 had more power to my feeling of power. That’s very debatable.

The big bore kits for 650 to 750 is different. Again riding back to back with short rod 750 feel very different. They both vibrate pretty bad at 4000 up. The big bore kit just gives more power than 650 across the board. Big bore kit is very desirable on sing carb bikes as well. Maybe best of all??

Still big bore feels different from short rod 750. Short rod changes effective rod angle to crank so produces more effective leverage on crank.

Absolutely lighter rods & pistons reduces vibration. You may be surprised at how much lowering compression reduces vibration.

Probably real cure for vibration is going with MAP offset crank ,cams, ignition. That can allow for steel rods, heavier/stronger pistons etc.
The 360 crank allows good mixture with single carb. Edward Turner wanted to keep cost low so had to have single carb. Make no mistake, they knew about 180 & offset cranks back then. Costs was so important. Bikes sold well. If they changed things it could put buyers off. Many companies made vertical twins & they sold well. Until Honda of course.

Modern parts can allow a really nice build. Price is staggering for many, but some have the cash to spend. Why not spend it if you have it?

Original lighting is obviously very substandard. We all know that.
Upgraded charging system is a must to support modern lighting.
Electronic ignition has a much more desirable advance curve. Too bad no provision for vacuum advance. That works really well.
Dual plug head mod allows higher compression Modern fuel has low octane. Unless you run race gas 110 leaded, you with lower octane.

We must keep fuel in mind. The 100 mile leash of race gas drove me crazy. If you want to rideesrch further you need a backpack of Torco octane booster. The single only one that works in USA must be used full strength!

My riding friends & I kick this around all the time. Very careful research must be done with fuel in mind at all times. It is possible to get more power & still be fairly durable. Takes very, very thoughtful build.

This is fun stuff!!
I posted photos of 650 & 750 pistons on Triumph RAT site. Do advanced search with my username. Scroll down & you’ll se post.
Don


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--I asked that question in the British general forum few months ago.

Hi Bon; I understand your point but I am not a masochist and use (not now, waiting for a mint crankshaft) later T140 as a transportation. Bike can handle 115-130km/h constant speed for 2-4 hours. The problem with old bikes and speed is mainly with suspensions and tyres . Mine have new Hagons; and all new internals on the forks; nothing changed.
Regarding those new 250cc that you mention; in fact they cannot beat a later T140 and the feel on them is not good. I did not checked all of course but many.
Today I rode one of the fastest around 400cc; 44HP (one of the most powerful on that niche). Instantly fast but I did not felt those 44HP. All those are not so incredible they are just somewhat fast and possible more reliable than an old bike no matter the brand; but not so many more..
The point is on the suspensions; most have good ones and a couple have great ones. All are better than the old forks models and that is where you find confidence to open more the throttle.

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Originally Posted by bon
Originally Posted by triton thrasher
Originally Posted by bon
Just saying, no point over stressing old bikes with engine tuning you are not going to use.

True that!


Except if you are racing or hill climbing or something like that. The speed limit here on motorways is 75 mph, if you are enough of a masochist to want to subject a classic machine to motorway riding, then you don't need 120mph performance machine to ride them.

Personally i think a larger capacity engine in a lower state of tune is the most pleasant. But everyone has their own ideas.



Hi Bon,

Thanks for the input and your point of view on respecting and retaining the character of these old bikes. It is much appreciated.

But don’t misinterpret my initial inquiry as being a naïve attempt to turn a 1960s T120 into a 2020s Panigale V4. This is intended to be an academic discussion about the “why” associated with old-school piston design in the context of everything we’ve learned about engine dynamics in the last 50 years…and applying it to an engine architecture I really love.

At 67 years old, I’ve owned over a dozen pre-Hinkley Triumphs since the late 60s — including my current treasure, a wonderful bone-stock 9pt restoration 1967 Bonneville. (Okay, I cheated and installed a ’68 TLS front brake, and late ’67 Concentrics.) Love, love, LOVE that bike to pieces, for what it is.

But, maybe like others on this forum, I’ve also owned, ridden, wrenched on, and road-raced very modern liter-class Hondas, Kawasakis, Ducatis, and KTMs. My motorcycling interests aren’t trapped inside a 1960s box.

20 years ago, in an attempt to answer the “what if” question, I “modernized” my long-suffering ’73 TR7RV by building a modern hot rod with $20k worth of upgraded suspension and modern tires, high output electrics/lighting, 320mm front disk brakes, Magura controls, one-off custom billet triple clamps and rear sets, custom 1 5/8” stainless steel exhaust, Mikuni flat-slides (Bonneville head), Mega-Cycle cams, light-weight valve train, dry clutch, Buchannan-laced Akront rims, etc., etc. But alas, only 8.0:1 (basically stock) soup can pistons.

Brian Slark liked it so much he awarded it “Best Technical Interest” at Steamboat in 1999. But more than the national awards, the enjoyment for me came from conceptualizing the bike, building it, and RIDING it!! It was incredibly FUN to thrash that thing on the backroads of Colorado! (TR7RV Don is right…there is something about the character of our short-rod engines that the long-stroke 650s/750s can’t duplicate!)

I’m not trying to convince anyone to adopt my interests. Just trying to learn as much as I can about the technical limitations of these old bikes and get thoughts on why some of the engineering hasn’t progressed very far.

For some of us, as Don said, “This is fun stuff!”

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Just for the fun of it, I'm building an engine with Norton based 270 deg crank even shorter, Thunder Enginneering rods, the nicasil barrels and a head modified for bathtub combustion chambers. What will it be like, who knows, and may just end up on a race track if quick enough. Then again, I do more spannering than riding, sad, but there you go

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Me and Kevin on this site have land speed racing 650 Triumphs running in a modified production naked frame class. My bike has been on a dyno several times for tuning..You cannot compare different dynos ...but...about 58 rear wheel HP on leaded race gas...That would be 66 or so crankshaft HP. The bike starts using the kicker and has a wide spread of torque...We are not pro mechanics , just hillbillys,but both of our bikes are currently the fastest two bikes in the class.
Triumph engine limitations? depends on the power level....but no one in land speed racing runs 60 year old connecting rods and some use billet crankshafts...Depending on class rules, aftermarket aluminum cylinders..
My street 79 T140D. upgrades include Honda 750 VFR 41mm forks,320mm Ducati front disc and Brembo caliper.The engine has revised cam timing , the piston to head squish reduced to .032, Mikuni 32mm flatslide carbs, Boyer Micro Power with a fast advance curve. With 9.2 static compression it runs without detonation on 90 octane US gas..The engine makes noticeably more power than stock 750 and pulls well at all speeds. All internal engine parts except for the pushrods are Triumph production parts...
Step up to new theories of combustion efficiency by improving port velocity and chamber turbulence or dual plugs....Or just stay old school and complain about modern fuel limitations...

3709079D-98C2-46FD-A0A4-C8DC99A1C484.jpeg

79 T140D, 89 Honda 650NT ,61 A10 .On a bike you can out run the demons
"I don't know what the world may need
But a V8 engine is a good start for me
Think I'll drive to find a place, to be surly"
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Originally Posted by TinkererToo
Just for the fun of it, I'm building an engine with Norton based 270 deg crank even shorter, Thunder Enginneering rods, the nicasil barrels and a head modified for bathtub combustion chambers. What will it be like, who knows, and may just end up on a race track if quick enough. Then again, I do more spannering than riding, sad, but there you go

That sounds fun. Nicasil barrels are the way to go, did you see Schmidts teflon coated pistons ? I was watching Allen Millyard on youtube last week, he was working on his 1,000cc Kawasaki two stroke four that he built, he could see the teflon pistons through the ports and could not detect any wear at all on the coating !

There was a few guys on here years ago messing around with strokers/offset cranks, the shorter rods with longer strokes seemed to give problems. Check out "British Bikes by Brian" on youtube, he says the same.

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