AN Acquires Thruxton Club Racer

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I built a Triumph motor with a Norton crank. 840cc with a bathtub type squish. 72bhp on the same that my current Trident Hunter Commando produces 64bhp. I developed it a little further still after this so it was probably closer to 75rwhp in the end.

Then the crank broke !

Conclusion? F**k knows...!

But it seems to say that Triumph motors do have power potential, and that’s squish bands do deliver.
 
Commando ports are slightly steeper than Triumph ports, but the biggest factor which seems to hold Triumphs back is the dead space on the side of the piston crown away from the plug. That side of the crown always has much more coke on it, in motors which have high-crown pistons. Talking about squish bands and piston slap, have a look where the squish band is in a two-valve Jawa speedway motor. The slop doesn't seem to stop them from still being effective.

This dead space side with the coking is rather more a function of an inefficient (non-compact) combustion chamber where, in the case of the Triumph, a big honking piston dome blocks the flame front. Yes, tighter clearance (squish) between the piston and head can mitigate this to a point.

If you have photos of a two-valve Jawa speedway motor piston and/or combustion chamber, I would like to see it.
 
Getting appropriate squish on a true hemisphere is not so easy (precise) as one has to factor in the lateral slop movement of the piston within the bore. Have you ever rocked a piston left and right in a bore along the axis of the wrist pin? Now think about the piston dome and combustion chamber wall geometry; what happens to your squish while your piston can rock and slap forward & aft as well as side to side and you have a nearer to vertical clearance between the piston dome and combustion chamber. One side near closes up and/or hits and the other side becomes sub marginal or ineffective squish. I seem to recall the Ducati mono cylinders have a nice compromise approach with a gradual (nearer to flat) dome piston and sloped squish to match on the sides; at least it looks pretty. This brings up the question on how the HDXR750 managed squish. Maybe I am missing something here.

As for a Commando 750 being superior to a Triumph 750, though squish may certainly be a factor in this I believe it is the demonstrated superior intake port efficient flow characteristics of a Commando that make the big difference which naturally translates to greater volumetric efficiency. The "why" of this better flow is a topic of another thread. Furthermore, when Norton splayed the exhaust ports, the intake ports were rotated about the axis of their respective valve stems in order for the two ports to be parallel. This new orientation of the intake ports to the center-line of the cylinders imparts considerable swirl to the intake charge. I have not run a flow bench swirl test of this but I am stating what appears to be obvious. When one looks at the true orientation of the port to cylinder center-line, the intake flow comes in at a considerable tangent to the cylinder rather than straight in like on a Triumph.
Efficient flow and turbulence are good and are why a Commando generally can out perform a Triumph of that era.

Interesting questions... I thought the basic Command head configuratin was laid out for the first 500 twins in the 1940s. Nortons have always been noteworthy for needing much less ignition advance than their contemporary rivals, what I do not know is whether the squish band was introduced on the original head design , or if not when it was first used. Could have been to get round some other problem rather than 'slide rule ' stuff
 
There is another picture of the bike in question on the AN facebook page. This clearly shows through bolted 850 style barrels.

So if it is long stroke, which I think is yet to be confirmed, should we assume it is an 850?, perhaps with a standard RH4/RH10 head?

Or will it turn out to be more like the motor I built in late '75/early '76 from parts I sourced from a then recently redundant Wolverhampton development engineer?

That was a short stroke in all but stroke! MkIII cases and 89mm crank, standard 850 barrels, TX stamped cam, fully hemispherical short stroke head with the Omega domed pistons machined from blank with the smaller standard Commando piston pin hole! Head ported by John Baker.

I am led to believe the head still exists in Scotland, you will know you have it if it has my RAF service number stamped on it!

I understood that Thruxton did build a couple of these 850s (MKII cases) for events like Barcelona where they were not restricted to 750cc.


BTW, this post is mainly to stop the thread continuing to discuss Triumphs!
 
Interesting questions... I thought the basic Command head configuratin was laid out for the first 500 twins in the 1940s. Nortons have always been noteworthy for needing much less ignition advance than their contemporary rivals, what I do not know is whether the squish band was introduced on the original head design , or if not when it was first used. Could have been to get round some other problem rather than 'slide rule ' stuff

The recieved wisdom is that the squish arrived as a by product of offsetting the bores when increaasing from 500cc through to 850cc!, rather than by design!
 
Another point about the Thruxton Club racer is that as a project it failed it's brief, mainly because it was never available to, or more particularly, affordable to, a club racer, e.g. me!

Which is why I ended up building a Rickman Commando 850 'club racer', the frame of which I am racing today, initially built for around 20% less cash than the 'advertised' TX price!
 
The recieved wisdom is that the squish arrived as a by product of offsetting the bores when increaasing from 500cc through to 850cc!, rather than by design!
Well that is exactly my point. One can hardly consider that resulting Norton margin adequate squish (usually expressed as percent of bore area). Less ignition advance required leads to more useable power. Need for greater ignition advance - I the case of Triumph versus Norton, Big honking dome piston versus a flat piston; has nothing to do with squish and everything to do with obstruction (or lack of) for flame propagation as the alleged squish on a Norton is next to nil. Reason Norton’s generally did not need a dome is their included angle between the intake and exhaust valves was considerably less than say a Triumph of the day. Furthermore, swirl induced by Norton intake port orientation further reduces ignition advance requirements.
 
If the combustion chamber in the Thruxton racer is fully hemispherical without squish bands, how do they get the comp. ratio high enough without domed pistons ? In Triumph engines, you have that situation - no squish and obstructed flame front. If the combustion chamber was not truly hemispherical but a flatter oblate shape and the valves and inlet port were angled more vertical, I could believe it might be better than a standard 850 head.
 
Dances, the squish band in a two valve Jawa speedway motor is right up inside the combustion chamber. They use Mahle forged pistons with little bore clearance. The crown of the piston has a flat section around it, which matches the machining inside the combustion chamber. It doesn't matter much because a good two-valve Jawa is almost as fast as an ordinary four-valve, which has a totally different head.
 
If the combustion chamber in the Thruxton racer is fully hemispherical without squish bands, how do they get the comp. ratio high enough without domed pistons ? In Triumph engines, you have that situation - no squish and obstructed flame front. If the combustion chamber was not truly hemispherical but a flatter oblate shape and the valves and inlet port were angled more vertical, I could believe it might be better than a standard 850 head.

I believe the answer is within your question. I cannot say whether a Thruxton has true squish bands or not. As outlined in my earlier posts, the Norton head has a smaller included angle between the valves which yields a shallower combustion chamber which obviates the need for such a large honk of a piston dome like you would need on a Triumph which has a significantly deeper hemisphere due to the greater included angle between the intake and exhaust valves.
 
The crown of the piston has a flat section around it, which matches the machining inside the combustion chamber.

What you are describing is a horizontal quench area with some type of dome in the center; not really attempting to achieve a squish on a hemispherical surface. I have seen this done for high compression pistons for Commandos, particularly 89mm stroke Commandos.
 
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AN Acquires Thruxton Club Racer


Short Stroke Omega cast pistons, looks to me like there was plenty of squish going on here.

These are similar to the ones I used with my Short Stroke head, and I can confirm the combustion chamber really was fully hemispherical and the compression ratio was only around 10.25:1

When the motor dropped a valve we had to shave 0.040" from the head.....and 0.060" from the barrel! to get back to the compression with flat top pistons...which then hit the head at the perimeter on first build and had to be chamfered at the edge! Barrels also bored to 0.040" over to clean out scoring.

I still have the Omega pistons somewhere, one with a valve head not very neatly wedged in the centre.
 
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Nice examples. Virtually no flat squish, all lateral squish. Do you know what the actual clearance was set to?

No, sorry, I don't. I would have liked to continue using them, but they were pretty much unobtanium even in '77. And I am not sure oversize ones were ever available.
 
When I used up my last factory Omega short stroke pistons, I had JE make copies from the ones I had modified, but in .060" oversize, as allowed at that time for the AHRMA Formula 750 class.

AN Acquires Thruxton Club Racer


I also had some made with a raised pin for use with longer rods, and with a larger pin, but never used them.

AN Acquires Thruxton Club Racer


I still have a couple sets of each left, if anyone needs something like this. Now that I'm only racing in landspeed events, which don't allow the overbore, I have no real need for them, and would sell them at very reasonable prices.

Ken
 
Good stuff Ken. I scanned your threads but could not come up with anything on where you set the squish at. Your pistons show no evidence of kissing the combustion chamber walls which is always a good thing.

Can you fill us in on squish clearance and what rods (cast iron or alloy) and barrels (steel or alloy).
 
Good stuff Ken. I scanned your threads but could not come up with anything on where you set the squish at. Your pistons show no evidence of kissing the combustion chamber walls which is always a good thing.

Can you fill us in on squish clearance and what rods (cast iron or alloy) and barrels (steel or alloy).

I ran two sets of Omega pistons contoured like that, the first with stock steel short stroke rods and iron cylinders, and the second with Carrillo steel rods (after the stock ones broke!) and alloy cylinders. I just pulled out my old notes on those pistons, and in both cases the squish clearance was .040" - .045", and the piston clearance was .0045" for the timing side and .005" on the drive side. The pistons in the picture are the second set, taken when I tore the engine down after its last run at Bonneville in 2006.

Edit - Just found my notes for the last build of this engine in 2001. The pistons were new then, and had .049" vertical clearance, or .035" - .040" actual squish area clearance, and were fitted to the bores with .005" on both sides. I also replaced the original 3-piece crankshaft (which had cracked in the usual place on the drive side) with a Nourish crank. At that time, the CR was 11.9. The only mileage on the pistons as shown in the picture Steve posted was from a couple of runs at El Mirage and two trips to Bonneville.

Ken
 
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The minimum squish band clearance, I have heard of, was 30 thou of an inch in a Manx Norton. Commandos don't usually have steel con-rods. My feeling is that the size of the squish band clearance is not critical as long as it is enough and is there, however others might disagree. I would always use forged pistons in preference to cast ones, in any motor. In Japanese two strokes, they are spun cast, then forged - critical to success. It is about grain distribution in the metal - determines the strength and the way the piston expands. You are much more likely to get seizures or piston collapse with cast pistons.
 
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