HELP Pre-Unit motor build

[Rob Axel]

Soliciting ideas, suggestions, experiences with building a Pre-Unit motor
I am looking at tearing into the Pre-Unit 650 sitting on my workbench. I have MANY questions in regards to building this motor for mostly reliability with what is available today, not concerned with “originality” or period correct. I will start with the description of what I have, please excuse my ignorance if I get the nomenclature incorrect. The motor is a 56 with I believe to have the “cast Iron” head, single carb (8 bolt, 4 fins, pushrod tubes pass through the head and into the rocker boxes). If this head is solid (no cracks, or major issues) should I keep it or just go with another. I have several 9 bolt later heads and a set of jugs so swapping them isn’t much of an issue…
I would like to know is there any SIGNIFICANT benefits to “upgrading / updating these parts?
Ignition… to Magneto or not to Magneto….. well, I will disassemble it (Non- high performance), add the chip (durispark, I think)…and use it until it goes tits up and it will… I was thinking of adding the E.I. in its place (a “cup” that bolts into the magneto’s location and the E.I. sets inside)
Charging system…now comes the fun part, since there is no generator…I need a charging system (sorta). Since I will be running a Magneto(until it dies), I will only need a charging system for pretty much lights, and with LEDS now the draw is minimal. Untill I end up with the E.I set up. I have seen/ read about Alton Alt/Gen replacements….for the price, I want something with a better review…. Can anyone offer their experiences with these or others? Or about adding a stator / rotor to the primary side…
Yes, I will be adding a main seal to the primary side of the motor (Angry Monkey style), leading up to the next question…..To belt or not to belt? BNR, is a great, reliable, time tested product….no need to try to sell me on this….I will have to wait on it as it also a “big ticket item”….


ANY suggestions, Info, corrections, comments are greatly welcome!

[Lester]

I am so happy to see this thread and all of your questions. I am pretty much in the same situation and I am a total newby to PreUnits. My engine build is not going on now but in the near future. I will be following this thread closely and taking notes.

thanks
Lester

[Rob Axel]

I may have spoken a bit to soon.. the Alton gen/ alt is now made brushless and comes with a two year garrentee... and the price is $575, I think .. far cry from $800 and bad reviews in the past.

[richbob]

i did a build thread on a 58 motor
fitted a stator with the newby.now running a pazon in mag location.(mag was good)
if i was/are going to upgrade further i would use unit top end (but i push it hard)
https://www.jockeyjournal.com/forum/s...d.php?t=117561

[Rob Axel]

Thanks for the thread info.... I did a search prior to starting this ...

[choppermann]

looking forward to replies
I just bought this thing

[choppermann]

been lurking forever but now it's time to start building
the little pre-unit chopper I always wanted.

[Tony the torch]

The only problem the pre-units have with a Bob Newby belt drive is a bit of premature belt wear.

the normal cause of this is misalignment of the trans to the engine and the belt gets hot and wears on one side. most times it's worse on the rigid frame bike as on most of the rigid transmissions, the bottom mount hole gets all wallowed out.

and there is the clearance on the primary cover issue...if you want to run one. but BNR is making a new narrow pulley and belt that will fit inside.

[Rob Axel]

Chopperman... so THAT is what it looks like all together.... mine came in "some assembly required " box with the instructions missing.....
Tony, ... thank for the info, you have always been a wealth of info around here!

[M.O.Ther]

My 2¢ on a couple things:

Heads.) If you're after performance and especially if you want 2 carbs, you're way farther ahead with a unit top end. You will need the correct pushrod cover set for whatever head you have and they're not all the same.

If it's just aesthetics, then I guess go with whichever one looks best to you. I have a '55 T110 I'll be putting together with the original cast iron head because I like the look of the external pushrod drains, and the more rounded shape of the fins on the side. Mine is a 5 fin head, featured on the '54 and '55 T110, but the shape is the same. Also the cast iron head will probably be painted black while the aluminum head would probably be bead blasted so put that in your mental picturing as well.

Charging.) A stock E3L Dynamo (generator in the US) puts out enough amperage that you can run it 12v, as long as your load isn't to heavy. With LEDs that shouldn't be a problem, even with a battery and an electronic ignition. But if you don't have one, consider the cost of finding one, plus the cost of rebuilding, plus the cost of the aftermarket 12 v regulator, then compare that to the price of the Alton.

The other option is fitting a stator like ....wait a minute.

If you're motor is a 5T or a 6T (everything else came with aluminum heads for '56, generators too) then it's already set up for an alternator on the primary side. You can use later model stock Lucas replacement parts, or their aftermarket equivalents/upgrades. Much cheaper than the dynamo or it's replacement.

Then the deal becomes the primary cover, if you're going to use one, and what frame you plan to put this deal in. With a stock Triumph rigid frame with the seat post between the engine and trans you need the long primary. That means the rare '53 and '54 5T and 6T set. If you go that way learn the difference between that and the TRW one that won't work or interchange. If you a swingarm frame, hardtailed or not, it't not a problem. (see chopperman's bike above) You can use the more common alternator set from '56 up 5T and 6T and '60'-'62 TR6, T110, and T120.

[reverb]

...hello M.O.ther, there s no place for alternator in the older 5T, 6T and TR5 s

[M.O.Ther]

Originally Posted by reverb

...hello M.O.ther, there s no place for alternator in the older 5T, 6T and TR5 s

According to Roy Bacon's "Triumph Twin Restoration," the alternator first showed up on the '53 5T. That was an RM12 unit, three different versions. For 1954 the RM14 alternator came into use and was put on the 6T as well. In both cases, as well as on the TRW, the stator is contained in the outer primary cover. Once the 5T and 6T went into swingarm frames, in '55, the arrangement was changed and the stator goes onto an adapter that bolts to the inner primary. Those two models had an alternator from then 'til the end of their run.

The sportier models in the line continued to use the dynamo, the 500s (TR5 and T100) until they went to unit construction in 1960, and the 650s (TR6, T110, and eventually T120) used the dynamo until 1960 as well, when they were put into the duplex frame. After that, it's all alternators.

[richbob]

[supercrouton]

Originally Posted by Rob Axel

...the “cast Iron” head, single carb (8 bolt, 4 fins, pushrod tubes pass through the head and into the rocker boxes). If this head is solid (no cracks, or major issues) should I keep it or just go with another.

Ignition… to Magneto or not to Magneto….. well, I will disassemble it (Non- high performance), add the chip (durispark, I think)…and use it until it goes tits up and it will… I was thinking of adding the E.I. in its place (a “cup” that bolts into the magneto’s location and the E.I. sets inside)

Charging system…now comes the fun part, since there is no generator…I need a charging system (sorta). Since I will be running a Magneto(until it dies), I will only need a charging system for pretty much lights, and with LEDS now the draw is minimal. Untill I end up with the E.I set up. I have seen/ read about Alton Alt/Gen replacements….for the price, I want something with a better review…. Can anyone offer their experiences with these or others? Or about adding a stator / rotor to the primary side…

To belt or not to belt? BNR, is a great, reliable, time tested product….no need to try to sell me on this….I will have to wait on it as it also a “big ticket item”….

Iron heads work like a champ. They don't crack like all the alloy heads. They make a little more noise, but are robust and functional. I've run trouble-free for years with iron head on my daily bike. Side note- Alp just set a new Land speed record with an iron head pre unit. That speaks volumes.

Magneto is the way to go IMHO. When a K2 has been properly built, it is dead nuts reliable for many years. Less wiring, not dependent on charging/battery state, are both big advantages to me.

Stator can be done on your engine, and without battery. BUT the Alton is has its advantages too. It can allow a nice clean open primary, or if you use a full primary cover, The gennie version is MUCH better looking, and less costly to acquire. Alton do require a battery from 8-14AH (if memory serves). I have built several Alton equipted bikes, including my own '55, They have performed flawlessly. The early ones came with a crap, chinese rectifier. Now they come with a Podtronics rectifier. When I switched my early unit with a podtronics, it solved all complaints.

BNR belt drives will take the work out of a pre unit drivetrain. You'll spent more time riding less time taking the slack out of the over-stressed, tiny primary chain these bikes came with. Not to mention a FAR superior clutch action and NO primary leaks.
Best regards, Dan. (you have my number, call me if you need immediate help)

[Rob Axel]

Awesome ...thanks guys

[VitaBlixten]

I run my 53 thunderbird batteryless with an Alton and a podtronic rectifier. Just put an capacitor instead of the battery. Works great but you'll have to live with some flickering in the lights on low revs..

[reverb]

...yes M.O.ther, I know; I only stated that the OLDER ones do not have space because R Axel, did not mentioned what year...

[M.O.Ther]

Originally Posted by reverb

...yes M.O.ther, I know; I only stated that the OLDER ones do not have space because R Axel, did not mentioned what year...

Sure he did. It's kinda hidden in the first post, but he says "The motor is a 56 with I believe to have the “cast Iron” head, single carb (8 bolt, 4 fins, pushrod tubes pass through the head and into the rocker boxes)." Although he didn't say it was a 5T or 6T motor, I thought that was probably the case. (I say probably because I once bought a pieced together motor with an incorrect cast iron head on '56 TR6 cases.)

Going back and catching it on the second (maybe third) read-through is what made me stop in the middle of what I was writing and change the subject from dynamos and their replacements.

And, while you're right in saying "there s no place for alternator in the older 5T, 6T and TR5 s", "older" is kind of a vague term, especially when we're talking about a period of time from the late '30's to the early '60s. I wanted to add the specifics, not just for Rob Axel but for the people who come along later trying to figure out pre unit Triumphs.

[supercrouton]

Because there is always a possibility for mismatched years/parts, it would be good to mention that whether or not a particular pre unit engine can or cannot have an alternator installed depends on one major factor: the crank shaft. If you have the (early) spring/cush equipped crank shaft, a stator rotor is not going to fit onto it. The "cush" cranks are typically found in small bearing pre unit cases. If you'll post a pic of the timing side of the case, we can tell you whether you have small or large bearing cases. (I have seen a small bearing crank modded and installed in a set of big bearing cases, so I have learned to assume nothing).

If you have the big-bearing cases/crank (more common), going from generator to alternator is as simple as finding inner/outer alternator primary covers and a alternator ring, and installing the stator and rotor.

[Rob Axel]

Alright... I'm gonna save face and say "all the above" is Greek to me (spacifics) in regards to different pre unit motors...
I will get some photos posted in case there are any mixed matched parts with this one motor... maybe it will shine some light on someone else's basket case they have sitting on a shelf ... and give them some hope and get them to dive in and get it back in a frame....