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Discussion Starter #1
Hi all,

My 1966 Bonneville suffered a seized big end, resulting in a bent rod and a damaged crankshaft journal. I've acquired a 1971 crankshaft with rods, to swap in. Apparently the crankshafts were the same from 1963-71, part # E4646, so this should work.

However the flywheels were changed repeatedly over the years:

E4479 (1963-64)
E 5783 (1965)
E6327 (1966-67)
E7332 (1968)
E9687 (1969-70)
E12425 (1971-72)

I understand the flywheel weight was lowered for 1966, and raised again at some later point. Questions -

- Does anyone know the differences between all these varieties?

- I can keep the 1971 flywheel on the replacement crank (easy), or replace it with the flywheel from the 1966 crank (work). Which is the better option? Pros and cons?

- Are there any balance implications? Did piston weight change between 1966 and 1971? How does the weight of a new Emgo piston ( which I plan to install) compare with the old stock ones?

Thanks,

Dave
 

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Hello Dave,
I know about most of these, some-one will no doubt correct the rest! The 63-64 flywheel was heavweight with no timing slots, and 65 brought in the slot for TDC. 1966 was the start of the lightweight flywheel, still with the same balance factor as the early one @ 85%. Come 1968 was a bit of a mess. The flywheel (still lightweight) acquired a 38 degree slot for max advance timing, but the first ones also had extra slots for a re-positioned timing plug at the front of the engine, then reverting back to the rear position after DU74052. I'm not sure which "flavour" the E7332 is, I think it's the later type with just the TDC and 38 deg slots for the rear mounted timing plug. There may well be a different part number for the early 68 flywheel, but I don't know it, and I've never seen one. 1969 started with the lightweight flywheel, but quickly changed to a heavyweight flywheel, much like the 65 one, but with the 38 deg slot at NC.02256. 1971 brought in a new flywheel with a different profile, but the same balance factor and slots as the 69. The metric timing sized crank actually came in right near the end if the 71 model year, no use to you.
I'm assuming that you had the original lightweight crank in your 66 (flywheel pic attached), but no matter, any of the flywheels will fit any of the cranks, given the correct flywheel bolts, they all have (or had) the same balance factor of 85% as standard. Wisdom has it that the heavyweight cranks give less "peaky" vibration, less broken bracket etc, but with a slightly slower engine pickup - I've never really noticed a difference in response. If you wanted too, you could swap the flywheels between the 66 and 71 cranks, but I wouldn't bother. I would use the 69 onwards conrods 70-9525, they were thicker and stronger below the small end eye.
I would strongly recommend getting the crank balanced dynamically by a specialist I think the Emgo's are JCC manufacture, but I have no reference to how they are to original Triumph, as everything I have is different, original type AE Hepolite, JCC, Harris etc etc, you balance for what you have!
HTH
Mick
 

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A 71 crank might be metric one end and imperial on the other end. There was a bearing shortage. My early 71 bike had an imperial crank and when it snapped, i put in a 69 TR6C crank. Heavy flywheel. The heavy flywheel was supposed to give a better idle for the engine.
 

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Discussion Starter #4
Thanks! First-rate information. I've decided to save time and effort by leaving the 1971 flywheel on the replacement crank, and expect to enjoy the silky smoothness that will eventually result. The history suggests that Triumph eventually came to view the lighter flywheels as a failed experiment.

Pics of the cleaned-up 1971 crank, and the contents of its sludge trap:

20201113_163902.jpg

20201113_164521.jpg
 

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That picture of your crank looks like a lightweight one. The 71 crank has a much wider fllywheel. The front crank is 1971 and the rear crank is a 1969 TR6C crank.
RIMG0080.JPG
 

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Discussion Starter #6
Interesting - the seller claimed it was a 1971 crank. Based on measuring the images, both your 1969 and 1971 flywheels appear to be c. 2.3" wide - close to or the same as my replacement, which is 2.25. But mine has no bevel at the edges like these two.

It does have two slots, TDC and timing, so is 1968 or later. Both bearings are imperial, so it's not 1972. That means it could be

E7332 (1968)
E9687 (1969-70)
E12425 (1971)

If your IDs are accurate, that suggests mine is actually 1968 - since it lacks the bevels on the later two versions pictured. Which means it's lighter than 1969 on; possibly the same as 1966-67. When I remove my damaged one I can compare their weights.

The 1966 version has different widths on both the narrow and wide sections vs. the replacement, also a raised ridge in the center not found on other years.

1966 crank:

20201114_185817.jpg

replacement crank:

20201113_163946.jpg
 

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I have only ever seen the 71 cranks with bevelled flywheel. Could be the previous owner of your 71 crank had removed it from a 71 engine that had already had a different crank fitted. That one with the crack in it came from my bike and it was the original factory crank. I think Mick will know more on these crank types.
 

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Hello Dave,
That original flywheel is a lightweight one, as expected. The E12425 (71-2425) flywheel is the flat one as pictured, and was used from 71 to 80 for both 650 and 750, they were just drilled to suit the different balance factors when balanced at the factory. It is highly likely that Triumph used up the last of the E9687 bevelled flywheels at the beginning of the 71 model year as pictured by Rambo- Rambo, what engine number is your bike? For the record, I use late metric cranks modified for the imperial T/S bearing in the "big" pre-unit engines I build, most have had a much easier life.
 

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Discussion Starter #9
Excellent flywheel scholarship! It seems likely the seller was right, and the replacement is a 1971 crank - which is presumably heavier than the 1966 version, all good.

Thanks!
 

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My bike is february 1971. I noticed when replacing the 71 crank with the 69 crank, the latter was 1lb heavier. I have another one of these cranks in the garage so i could weigh it. When i fitted the 750 kit, there was no crank balancing done and the bike did not feel any worse with vibration. The gudgeon pins were much thicker metal and heavier.
 

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The "real" 750, the T140/TR7 used a 75% balance factor, wheras the 650 had 85%. Having the heavier pistons in a 650 (big bore) effectively makes the balance factor lower if you change nothing else, having a lower proportion of reciprocating weight balanced out, so that sort of ties in. I am sure that the shorter rod length of the late engines also has an effect, but all of these balance factors were empirical, depending on the resonance of the frame and where you want the least vibration to happen. As a callow youth I used a unit 650 crank in an early pre-unit bike (normally 50%), and it vibrated like a road drill! Despite that, I now use 70-72% on pre-unit engines in Tritons, depending on road or race, which works for me
 
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