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Discussion Starter #1
Hi guys,

Anybody on here with a t100 and 7:1 pistons or know sombody running them? I cant seem to find much info on the 500's. Would like to know how they perform if possible.Also Are the Harris pistons and rings good quality?


Thanks
 

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Hi,

Afaict, not a lot to know about the pistons - 7:1 was only fitted as standard to the 5TA; afaik, all the "C" variants had 9:1 pistons originally?

5TA was dropped at the end of '66, because it was becoming slow and dull? According to the subsequent parts books, 7:1 pistons remained listed for "Low grade fuel" but these wouldn't fit your bike without the corresponding '67-on head.

Triumph 500 with 9:1 c.r. isn't particularly fast for modern traffic, not sure I'd want to hamstring one with 7:1?

Any particular reason you're considering 7:1?

Regards,
 

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Discussion Starter #3
Hi,

Afaict, not a lot to know about the pistons - 7:1 was only fitted as standard to the 5TA; afaik, all the "C" variants had 9:1 pistons originally?

5TA was dropped at the end of '66, because it was becoming slow and dull? According to the subsequent parts books, 7:1 pistons remained listed for "Low grade fuel" but these wouldn't fit your bike without the corresponding '67-on head.

Triumph 500 with 9:1 c.r. isn't particularly fast for modern traffic, not sure I'd want to hamstring one with 7:1?

Any particular reason you're considering 7:1?

Regards,
Hi StuartMac,

Thanks for the reply, was considering them as a ping prevention with the crap fuel we have over here in SoCal (Not sure if the 500 has that issue) But you pretty much answered my question.
 

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Hi Rodwulf,
If you still have the points setup fitted full advance is around 2000rpm, electronic ignition units tend to have a much gentler and extended advance curve (full advance 3000+ rpm) this can help in reducing detonation. Fuel additives can make a difference too, but there is a lot of snake oil out there.
Regards
Peg
 

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Hi Rodwuldf, I consider California 91 fuel on the upper end of low grade fuel. Certainly a far cry from high test.

In my experience with lower grade fuel is high compression doesn't make more power. Putting race gas in my '73 Tiger or the '69 Bonnie was dramatic change in power. Not a trace of ping. 110 octane leaded race gas. Not street legal, but I couldn't live with the 100 mile leash.

When I take my car to Arizona I reliably get 3 more miles per gallon on the 93 they sell there. How can I explain that? I really don't know why that is. Others have same results. California fuel really is substandard in many respects, but they say it pollutes less. That is the goal mandated by the state.

Regarding the quality of LF Harris pistons & rings I have experience with a few bikes now. So far good. No oil consumption on any. None. Only have about 5k on motor so far. Long term I don't know.
Don
 

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Hi Peg, The only fuel additive that works in California is Torco. It's the same ingredient in Torco race fuel. California legal. The Euro & AU additives that work are generally outlawed in USA.

As Peg stated the advance curve of electronic is way better. Boyer full advance 3500 rpm. Plus it doesn't bounce timing at lower rpm . Triumph full advance on 650 anyway is 2000. If worn at all the AAU oscillates the timing as the fly weights vibrate or the like. You can see this with the timing ligth. This oscillation stops at full advance of course. I shortened my AAU springs to very good effect, but that's another subject. I don't have the formula worked out yet. Just trial/error.
Don
 

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Discussion Starter #7
Thanks for the replys Peg,Don,. I actually just fitted a pazon recently so I am good to go in the ignition department.
 

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Hi Don,

advance curve of electronic is way better. Boyer full advance 3500 rpm.
Just an aside but uh-uh:-

. all current Boyer-Bransden e.i. certainly for Triumph twins with points-in-timing-cover, full advance is 4,000 rpm. (Mk.4, MicroDigital, MicroPower, all instruction no. 23);

. although the Mk.4 destructions say they're also for the Mk.3, the ones that came with the Mk.3 on my T100 say full advance is 5,000 rpm.;

. regrettably, I can't say what the full-advance rpm. is for older MicroDigital or MicroPower.

Hth.

Regards,
 

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I use L F Harris rings on my T120r but it is overbored and uses T140 rings. No oil burning and when i did change the rings during other work, they were worn but still not using oil. I bought another set of Harris rings and again, no oil burning. It does use a little as can be expected in normal use.
I fitted some 7 to 1 rings in a T90 model a few years ago after it had cracked a piston through detonation. The owner did not notice much difference but he was a slowish rider. That bike was ultra reliable due to its low power and only needed oil changes for the next 10 years of ownership. Not a lot of miles though. It did run very well with lower grade fuel in the UK
 

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Hi StuartMac,

Thanks for the reply, was considering them as a ping prevention with the crap fuel we have over here in SoCal (Not sure if the 500 has that issue) But you pretty much answered my question.

Here is my experience. Maybe it’s helpful..
Make sure timing is less than 35 deg total and retard your cam timing once you check where it is actually at. There is more power lost from timing retard than from cam retard.
No need for 7:1 pistons
Timing your cams can be found in A. Graham Bells book 4 Stroke Performance Tuning
Also here:

This is assuming valve clearances, fuel mix and air leaks are all in check.

Or run Sunoco 95 Optima fuel all the time

I found a couple pages in google books of Graham’s info:
View attachment 714899

 

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Hi Don,


Just an aside but uh-uh:-

. all current Boyer-Bransden e.i. certainly for Triumph twins with points-in-timing-cover, full advance is 4,000 rpm. (Mk.4, MicroDigital, MicroPower, all instruction no. 23);

. although the Mk.4 destructions say they're also for the Mk.3, the ones that came with the Mk.3 on my T100 say full advance is 5,000 rpm.;

. regrettably, I can't say what the full-advance rpm. is for older MicroDigital or MicroPower.

Hth.

Regards,
I’ve tested over a dozen right before I take them out and they are full advance at kick over. There is no retard when the circuit breaks down.
They come in hard to start with lots of kickback. I’ve never installed a new unit. Personal trust issues with the brand.


Sent from my iPad using Tapatalk
 

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Hi Denis,

I’ve tested over a dozen right before I take them out and they are full advance at kick over. There is no retard when the circuit breaks down.
Mmmm ... but we aren't talking about faulty e.i., my post that you've quoted was simply a correction to a statement in Don's post, specifically:-
Boyer full advance 3500 rpm.
Risking telling you something you know already, for any e.i. made for these old heaps, the physical setting between the fixed and moving parts of the trigger unit simply selects a point on a curve defined by the electronics (e.g. Boyer-Bransden "Transistor Box"). Plotted rpm vs. timing advance degrees, the curve shows:-

. at low rpm, relatively-large increases in advance degrees for small increases in rpm;

. at increasingly-higher rpm, increasingly-small increases in advance degrees despite large increases in rpm, to the point where the advance increase becomes difficult to see without complex instrumentation; i.e. simply using a timing light, it shouldn't 'show' any further timing advance after the "full advance" figure in a given maker's fitting instructions.

"There is no retard when the circuit breaks down" because it's the electronics do the timing advance, or retard, for a given engine rpm. If this part of the electronics fails, then the default will be the timing defined by the physical relationship between the fixed and moving parts of the trigger unit.

Aside, on the Lucas Rita, advance/retard by the electronics can be switched off deliberately; this was because the original Rita advance curve didn't suit all engines, some users wanted the electronic trigger's improvement over points but either without any advance/retard or with mechanical advance/retard.

Personal trust issues with the brand.
Ordinarily, I wouldn't have fitted B-B to my T100, having fixed too many of other people's at roadsides and campsites while having much greater reliability from Lucas Rita on my T160's. However, the T100's Mk.3 came brand-new at the right price (free ;)) and, so far, has been reliable.

Hth.

Regards,
 

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Bush bearing t100 engines dont like high compression to begin with, it stomps the bush unnecessarily hard.
To prevent pinging and hotspots the 9:1 pistons sharp edges around the crown can be radiused, this also improves combustion. Comp is lowered but not by a significant amount.
This can be done with a regular hand file, but i would recommend the pistons to be removed from the engine.
 
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