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Discussion Starter #41
Hi speedrattle, Thanks for the info. After much consideration, deliberation & stressing over this, I’ve elected to go with 7.4 pistons. They arrived in mail yesterday. I will live with the results. After putting 200 miles on ‘73 Bonnie with 7.4 on my familiar roads, canyons, steep hills, freeways, city, I’m very comfortable with my decision. It certainly has plenty of power for my needs.
Again thanks for the suggestion though.
Don
 

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sure.

it all depends on what youre looking for, don. without a doubt, the low compression motor will last longer between rebuilds and will be a well- mannered machine.
 

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Hi speedrattle, Thanks for the info. After much consideration, deliberation & stressing over this, I’ve elected to go with 7.4 pistons. They arrived in mail yesterday. I will live with the results. After putting 200 miles on ‘73 Bonnie with 7.4 on my familiar roads, canyons, steep hills, freeways, city, I’m very comfortable with my decision. It certainly has plenty of power for my needs.
Again thanks for the suggestion though.
Don
Hi Don, others undoubtedly know much better than me, but my 7.4's have resulted in a lovely motor. Loads of torque, plenty quick enough for me and an easy kick start. I can't find anything bad to say after 1500 miles on my new Harris pistons. And at 7.4:1 it would probably run on kerosene!
 

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Same experience here with the 7.4 pistons. Absolutely no loss in power, much better rideability and NO pinking! Following your thread with great interest and admire you for sharing your wealth of knowledge.
 

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Hi All, not enough to leak oily residue, but some gas seepage.??
Don
Hi Don,
It looks like you might just have dodged the bullet here, I reckon that in a few thousand more miles the gas seepage would have started burning the head and barrel. The surface of the barrel between the cylinders looks as if it is just starting to suffer. The stud marks must be though expansion, pushing a thread profile that deep into copper would take a lot of force, I can only think expansion would generatethat kind of force. The gasket should be held on place by it pressing into the machined ring grooves in the top of the barrel.

I still think the cylinder head deep machining marks are a problem, you can see pressed in grooves in the head gasket between the cylinders. That can’t be good. The explanation that the machine shop gave you was very ‘Mandy Rice-Davis’ in it’s tone.

I assume that you will use another 80nthou gasket as you already have a skimmed head, but it might be worth calculating the Compression Ratio with the Low 7.4:1 pistons and a standard (50mthou) head gasket and skimmed head. I think that might be a winning combination, the lowered head and LC pistons , might produce some really well defined squish bands at the top of the piston, these will protect somewhat against detonation and the LC pistons will not let compression ratio get too high (push rod tubes might be awkward to set though).

You asked about crankshaft balancing, I can only recommend you get it done while it is apart. As you get older you get more susceptible to vibration in the bone joints, so take the opportunity now to maximise your future riding by reducing vibration as much as possible.
Also you are using non standard pistons, not only are they going to be a different weight, but the centre of gravity is moved lower down the bore, courtesy of the low dome pistons. Your standard balance will be all over the place.
If you can find a trusted Triumph twin balancer, he should ask you a few questions and combine that with his experience to recommend a balance factor.

I have 2 at 74%, 1 at 76%, 1 at 69% (different crankshaft) and the 900 at 80%.

The three main questions are:

1- road or racing,
2- your riding style and use,
3- what frame is it fitted in.

I think it is not a question of the %, but choosing a factor that puts the worst vibration outside your normally used rev range combined with the characteristics of your frame.

You might end up with a standard balance factor, but the horrible rocking couple should be eliminated and that is a blessing in itself. I always take the opportunity to get the crank balanced as it does not come along very often, finding the right person to do it is the the dilemma.

The best big end bearings were always Vandervell tri metal , they still turn up fairly regularly on e-bay etc.
VP56 is the part number, but most available are undersize bearings.

regards
Peg.
 

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Discussion Starter #46
Hi Peg, Thanks for the info. Bike is '75 Tiger all stock. I don't know piston weights yet. Rod weights yet. I ride fairly conservatively. Only do high rpm on rare occasions. I like to cruise 62-70mph. I mostly do 62 as vibration get worse faster. A real buzz at 70 like an old Honda 305. From the 1 personal experience on '69 Bonnie I feel like dynamic is out for certain. Think of my riding as like commute riding in towns & easy cruising on rural roads with a safe speed 55-62mh, with several miles of freeway between where speed limit is 65-70, but cars go mostly 75. At 90 the vibration is just horrible. Unreal bad. Feels like it will fly apart.

I don't know if the 7.4 will be lighter than 8.6 until I get out the scale.

There is a balance shop 100 miles from me. I'm going to visit them & get a vibe on how they are. Another place 400 miles from me. I was going to drive it down for a week vacation, but not with covid. They did the '69 Bonnie. Known good shop. I'll now more after visiting shop in Sacramento
 

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Discussion Starter #47
Hi All, I checked lobe center cam timing per Kevin's method. Thanks Kevin!

Supposing I did it correctly & adding the degrees on degree wheel correctly... I got Intake 89.75deg.,
Exhaust 101.5deg.

My first attempt at this. Did several readings & get very repeatable results. I have no idea how the gear marks actually line up, as timing cover is still installed. I'll rotate motor the umpteen needed times to line up marks just to see.

This is only bike I've ever checked. Don't know what I'll do as bike was running so good. I forgot to enter Before or After on chart. I'll have to go back & retest. Still digesting all this.

Is the 89.75 plausible or did I mess up. I did the entire set up twice. Repeatable the same each time. Is that where factory would have set it? Or did they miss a tooth? That's why I want to see where gears line up.
Don
 

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Hi Don,
There is a balance shop 100 miles from me. I'm going to visit them & get a vibe on how they are. Another place 400 miles from me. I was going to drive it down for a week vacation,
Is one of those Franz & Grubb? If not, don't forget them?

Hth.

Regards,
 

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Hi Don,

Is one of those Franz & Grubb? If not, don't forget them?

Hth.

Regards,
It's interesting some of you say no loss in power with a nearly two point drop in compression ratio.. For 100 years engine builders have raised compression to gain more power.All high performance engines have higher compression than the base design. Sometimes higher compression is used to make for the loss of cylinder pressure when using long duration cams or just to boost power with the same cams.
But right here on this forum it is now proved that higher compression does not incrrese power and just makes the engine more edgy...
Me and Speedrattle are gonna get rid of the power robbing high compression pistons and get more speed with low compression...;):giggle::sneaky:
 

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Hi Truckedup. I couldn't say for certain that there was no loss of power with my 7.4 pistons, compared to what it was originally, I didn't do any back to back dyno testing etc. All I can say is that the bike rides very well, is tractable, has loads of torque, and when I give it some beans it's more than quick enough for me. I'm in my mid 60's now and my reactions are not what they were when I was commuting daily through west London on a 68 Daytona. OK, maybe it's half a second slower to 60mph than it was, but then I'm, ahem, a few pounds heavier than I was, and many pounds heavier than I'd like to be. Tractability is more important to me now than raw speed, and an easy kick-start, which is why Harris say they only make the 7.4 now. It's all us crumbling oldies wanting to ride a bike rather than win the traffic light GP.
 

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Hi Truckedup. I couldn't say for certain that there was no loss of power with my 7.4 pistons, compared to what it was originally, I didn't do any back to back dyno testing etc. All I can say is that the bike rides very well, is tractable, has loads of torque, and when I give it some beans it's more than quick enough for me. I'm in my mid 60's now and my reactions are not what they were when I was commuting daily through west London on a 68 Daytona. OK, maybe it's half a second slower to 60mph than it was, but then I'm, ahem, a few pounds heavier than I was, and many pounds heavier than I'd like to be. Tractability is more important to me now than raw speed, and an easy kick-start, which is why Harris say they only make the 7.4 now. It's all us crumbling oldies wanting to ride a bike rather than win the traffic light GP.
Oh, I understand, the lower compression engine will feel smoother to the rider who will use the throttle accordingly.....All that matters is that you are satisfied...
In theory and likely on a dyno, going from 7.4 to 9-1 compression is worth about 5 hp and an increase in torque and throttle response.And better fuel mileage..
 

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theres no doubt that lowering compression is better than retarding a spark to run on lower octane fuels while keeping horsepower, or so tuners tell me who know more than i do about stuff. with my old T120, when i got it it wouldnt run on anything less than leaded 94 octane. i retarded the spark, tried three base gaskets, MTBE, all sorts of cheap fixes. didnt help. later i would mix 91 unleaded premium with 89 leaded regular. cant get lead easily these days, but its doing okay now on unleaded regular.

don, i dont know anything about T140 cam timing. triumph did strange things about when your machine was built. 89 seems advanced in my experience, but iirc T140s sometimes were set up that way. tony has messed with it a bunch
 

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Hello Don,
Those are rather odd figures! I usually find the inlet cam around 98 deg lobe center (but not always) with standard keyway/marks. I don't use the T140 exhaust cam, so can't say for sure about that, but I'd normally be setting for circa 105 deg. Remember that each step of adjusting gives 4.8 degrees, so +/- 2.4 on your hoped-for values is the best you get. I put up my method of cam timing on the tech tips section, you can do that method with the motor fully built, so it's no big deal to change timing after the event. It's a bit tedious, but if you keep turning the crank you will get to the point where the crank pinion mark and idler wheel mark coincide (the old hunting tooth thing) you can check the timing marks on the camwheels.
HTH
M'ick
 

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Myself ans several guys on Brit Bike have checked the inlet cam centerline on perhaps a dozen T140's. The most common number was 90-93 degrees. A few were severly retarded at 107 degrees, usually the early T140's.
Every person who reset the stock intake at around 100 degrees reported noticably more mid range power and top end power and generally a better running engine.
Don, the intake at 90 degrees increases lower speed cylinder pressue making it more detonation prone..It's possible a change of just cam timing could lessen a lot of the detonation you were experiencing...
For reference, Triumph 650 intake timing was 100 degrees.
 
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Myself ans several guys on Brit Bike have checked the inlet cam centerline on perhaps a dozen T140's. The most common number was 90-93 degrees. A few were severly retarded at 107 degrees, usually the early T140's.
Every person who reset the stock intake at around 100 degrees reported noticably more mid range power and top end power and generally a better running engine.
Don, the intake at 90 degrees increases lower speed cylinder pressue making it more detonation prone..It's possible a change of just cam timing could lessen a lot of the detonation you were experiencing...
For reference, Triumph 650 intake timing was 100 degrees.
Yep,
100 degrees is what I go for on the inlet, I know you go for higher exhaust center than me, but you go a lot quicker than me!
 

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Hi Don,
I know you have not inspected your cams yet, so you don’t know if they need replacement.
I replaced the cams on my Flattracker style bike and fitted the Hyde half race cams, these are in reality only 3134 profile cams from the 650 + quietening ramps, so ‘half race’ was a bit of sales promotion, but the engine runs ever so sweetly, pulling hard from much further down the rev range and maybe losing a little at the top end.
It was a struggle to get well toleranced cams from them though, in the I had to stand in their parts dept. With a vernier and micrometer matching all of the specs. Poor QC from Hyde but got there in the end.

A little trick to try, is to measure both lobes, sometimes there is tolerance in the grind that you can’t do much about, but if you find one lobe is advanced on both opening and closing compared to the other lobe, you can twist the tappet guide block in the barrel slightly, this will move one cam follower forward and the other back, the moved forward follower will engage the cam sooner and the moved back follower later. So correcting (improving) small imbalances in cam timing between the two cylinders. If you have to compromise then try to prioritise cam opening times.
You will be pulling the tappet guide blocks anyway to change the seals, but it is a laborious process. The Patience of a Saint is required.

regards
Peg.
 
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Discussion Starter #57
Hi Peg, Thanks. I will check. Will be interesting to see. I carefully lined up tappet block with straight edge & depth gauge during last reseal.
From day one left cly plug has read leaner. Left intake guide had more wear. I don’t see oil leaking around the sleeves in head. I’ll leak test them over next few days.
Minimal time to work on bike where my brain can focus on it. Becomes frustrating, but it’s what it is right now.
Don
 

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Discussion Starter #58
Hi All, Obviously high compression makes more power. Lots more.
But you need fuel to support it.
91 E10 is a disaster for us. It’s what we have. I just want ride around. Truth be told I happy with the weak power. If it allows better use of 87 E10, I’d be extra happy.

This is only a hunch. I think with low compression we are using a little more throttle for same road speed. It takes the same cylinder pressure to move a given weight at a given speed. I suppose that’s why Bonnie’s & Tiger basically use same amount of fuel. We’ve compared in real life on several machines. The ones that do really good mpg end up with hole in piston. These are stock bikes.

Very interesting stuff!!
Don
 

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Yep,
100 degrees is what I go for on the inlet, I know you go for higher exhaust center than me, but you go a lot quicker than me!

mick, i seem to remember setting my 1972 T120 at 101/102 when i installed megacycle 1000 cams, years ago. but its been ages since i was in there.

now i may have to take the cover off and look
 

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Discussion Starter #60
Hi Peg, I just checked other lobe both cams.
Intake is within 1/4 deg, exhaust spot on the same.
Pulling timing cover now.
Don
 
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