1971 Oil In Frame Triumph Bonneville Restoration: Tire Choices

The old Bonneville needs new tires and inner tubes so I'm wrapping my head around olde fashioned, pre-metric motor-bike tyre sizes.

Those would be the Dunlop K70s that came with the Bonnieville back in the day.
Fortunately Dunlop is still looking after the bike they designed the tire for.

The '71 Triumph Bonneville came with 3.25 X 19″ front & 4.00 X 18″ rears and used Dunlop K70 tires as standard equipment.

It looks like there are modern Dunlop options for vintage SAE (non-metric) wheels at Revco where I've gotten my last two sets of tires.  The handy chart below shows metric and imperial conversions but after some digging I was able to find SAE/pre-metric modern Dunlop K70s that are the exact fit for the bike.  Using recent versions of original equipment that give me the benefit of modern rubber durability and grip while still keeping close to the originally engineering intent in Triumph's tire choice is fantastic.  I'm not trying to recreate riding in 1971, I'm trying to start with that technology and update where prudent for 21st Century use.

  Tire Charts                               Motorcycle Street Tire Size Conversion Charts

Metric 80/90 90/90 100/90 110/90 120/90 130/90 140/90	Alphanumeric - NA- MH 90 MJ 90   ML 90 MN90/MP90/MR 90 MT 90 MU 90	Inch-(Series90) 2.75 3.00/3.25   3.25 3.50 4.00/4.25 5.0 - NA-	Inch-(Series82) - NA- - NA- 3.60 4.10 4.25/82/4.40 5.10 - NA-	Front Tires: Metric 80/90 90/90 100/90 110/90 120/80 120/90 130/90 Alpha MH90 MJ90 MM90 MN90 - NA- MR90 MT90 Inch 2.50/2.75 2.75/3.00 3.25/3.50 3.75/4.00 4.25/4.50 4.25/4.50 5.00/5.10	Rear Tires: Metric 110/90 120/90 130/80 130/90 140/80 140/90 150/80 150/90 160/80 180/55 200/60 230/50 Alpha MP85 MR90 - NA- MT90 - NA- MU90 MV85 MV85 - NA- - NA- - NA- - NA- Inch 4.50/4.75 4.50/4.75 5.00/5.10 5.00/5.10 5.50/6.00 5.50/6.00 6.00/6.25 6.00/6.25 6.80/7.00 7.00/7.25 7.90/8.00 9.50
Size 120/80V16 130/90H16 130/90V16 500S16 MT90H16 3.00 130/90H16 3.00 140/90H16 140/80VB16 140/90H16 150/80V16 160/80H16 160/80H16 200/60VB16 120/80V18 120/90V18 120/90H16 130/80V18 130/70VB18 150/70VB18 140/70V18 170/60VB18 180/55VB18 230/50 X 15	Rim 2.75 3.00 3.00 3.00 3.00 3.00 3.50 3.50 3.50 3.50 4.00 4.00 5.50 2.75 2.75 2.75 3.00 3.50 4.00 4.00 4.50 5.50 7.00 to 8.00	Overall Width 4.7 5.00 5.2 5.2 5.3 5.4 5.8 5.9 6.0 6.2 6.4 6.8 7.9 4.8 4.8 4.9 5.2 5.4 5.9 6.1 6.9 7.0 9.5	Overall Diameter 23.8 25.4 25.0 26.4 25.3 25.6 25.6 24.8 25.8 25.4 26.1 26.1 25.9 25.9 26.4 26.3 26.3 25.4 26.2 25.7 26.0 26.3 25.3

Note: Measurements are based on the given rim widths. A rough rule of thumb: Each additional 1/2-inch of rim width will be approximately 1/4-inch more in each tire width.

The K70s at Revco are just over a hundred bucks a pop and Counteract Balance Beaded inner tubes are actually cheaper than name brand plain old rubber inner tubes. I've been using the Counteract beads for years to great effect so they'd be my first choice even if they weren't cheaper. All in I'm looking at about $320CAN ($250USD) for new rubber for this vintage restoration project, which considering the price of some of the other parts is pretty reasonable.

In order to rebuild the wheels I'll need to replace the bearings and clean up the brakes before putting new pads and hydraulic brake cylinders back in them. All that and getting the frame sorted out will get me back to a correct rolling chassis ready for the upgraded engine.

Somewhere at the end of all of this will be a road worthy '71 Triumph Bonneville, but it's a lot of parts to find and get in and then a lot time in the garage to get there. I'm hoping it'll be on the road for next year's riding season.  As we thaw out here in Canada I'll be out on the Tiger and GTR and not spannering so much.

Meanwhile, here are some more motorcycle tire sizing reference charts found on the interwebs:

1971 Triumph Bonneville Restoration: Gudgeons, Johnson Rods & Cylinders out! (oh my)

I've got a degree in English, but working on the old Bonneville is stressing out even my vocabulary:

That'd be your gudgeon pin right there.

Gudgeon: one who will bite at any bait or swallow anything, credulous or gullible person Mer. V. I. i. 102.  (that might apply to this project)

Definition of gudgeon

1: PIVOT sense 1, JOURNAL  (we're getting closer)

2: a socket for a rudder pintle (a rudder pintle?)

Definition of gudgeon pin: WRIST PIN

Definition of wrist pin: a stud or pin that forms a journal (as in a crosshead) for a connecting rod (that's it!)

They can make up anything! Nobody knows!  Well, you need a new Johnson Rod in here!

A few people know, George.  Consumer ignorance is expensive, but who has time

to understand the machines they depend on?

Gudgeon pins?  After the massive fight that was removing the seized cylinder head, I was bracing for misery, but I was able to pop the circlips (!) out and tap the gudgeon pins (!) through with no sweating and swearing.  Pink Floyd was playing on the computer and I think music from its era calms the old Bonneville down when I'm working on it.

The circlips popped out easily and a 3/8 inch socket extension was the perfect size to tap out the 'wrist' pins.  I thought the one on the seized side might cause more problems because it had been stuck in place with moisture but it didn't and both came out easily leaving a motor bereft of its pistons.

British Cycle Supply Co. delivered my 750cc kit quickly and without any headaches (and also for about $200 less than comparable kits out of the USA), so I got the shiny new bits out and had a look:

You'd think a 750cc head would look significantly different to the stock 650cc unit, but you have to look closely to see a difference in size.

I had a close look at the connecting rods and they appear to be in good shape and the bottom end of the motor moves freely.  I think next steps are to remove the rest of the engine from the frame, give the oil-in-frame a deep clean and then recoat it with some quality paint.  With the frame cleaned up and sorted I'll strip the rest of the motor and clean everything out to ensure nothing's whacky before I begin the great rebuild.

I could just chuck it all back together now and hope for the best, but if it ends up having other niggles rushing things at this point is just false economy.  I'm not worried about making the bike look mint, but I do want it to be dependable and that wouldn't be the case if I haven't checked it over completely after such a long (30 year?) layoff.  An engine out deep restoration was always the intention here and I'm not in any kind of rush since the other two bikes are both five by five and felt fantastic on their first rides of the year this past weekend.

The gudgeon pin itself looks to be in good shape, which is good because the head kit didn't come with new ones.  The circlips came out nicely too but the kit did come with those.  It amazes me that even specialized material like that in these cylinders can withstand the extreme forces they operate in.  This Bonnie had done at least twenty thousand miles on these cylinders and they're still in remarkable shape considering they were face to face with more than 17 controlled explosions every second (a spark plug typically fires about 17 times per second at 2000rpm!).  You'd think all that heat and violence would cause more wear than I'm seeing in the beating heart of this motor.

This is the 3/8 inch socket extension I used to tap the gudgeon pins out.  Nothing grabbed or slowed the process down and I barely had to apply any pressure.  I might have even been able to push them out had I been so inclined.  I caught the pin as it came loose and the cylinder lifted off the connecting rods easily.  Again, considering the extreme conditions the heart of every engine operates under, I'm impressed by the engineering and metallurgy that makes all this possible.

The new cylinders don't have the carbon build up from all those combustion cycles and look pretty spectacular in their pristine newness:

Here's the whole kit from British Cycle Supply.  It includes the cylinder head, cylinders, piston rings, circlips for the gudgeon pins and a head gasket.  I swiped the last one they had in stock for $688.75CAD ($535USD - told you I got a good deal).  Unfortunately I might be the last person on the planet to get a 750cc kit that cheap.  Prices will be going up on the new stock and probably even higher than that as we're in an inflationary spiral thanks to Putin overreaching and years of pandemic fueled broken supply chains; we live in interesting times.

Another tool came in that's interesting.  I was reading Practical Sportsbikes last week (highly recommended if you love bikes and getting your hands dirty on them), and editor Chris mentioned an endoscope smartphone camera he was using on a project bike to inspect the internal parts of an engine.  I found one on Amazon for under $40CAD and it came in on the weekend.  It lets you see parts of the engine that haven't seen light before.  It's an exciting thing that offers you a look into the secret life of engines.  I'm looking forward to using it as i continue to work on the Bonnie.

Through an inspection hole inside the engine - it's disco in there!

That look on my face after the first ride in a long, long time.

The 12 year old Kawasaki (left) and the 19 year old (!) Tiger (right) were keen to turn a wheel after a long hibernation.

Bison were out bisoning at Black Power Bison Co. when I was out for a ride in balmy 6°C March Sunday.

650cc Air Cooled Triumph Bonneville Exhaust and Seat Options

More 1971 Triumph Bonneville restoration project research (all prices courtesy of BritCycle):

High pipes:

721-T74X exhaust pipes - $304.51/pair.
711-709669/9670 mufflers - $486.00/pair.
2x 742-158 clamps - $16.61 ea.
70-9673 “H” connector - $72.28.
2x 742-112 clamps - $16.61 ea.

TOTAL:  $930

Plus associated bracketry and hardware, etc if needed. The one item we’ll have difficulty sourcing will be the ‘chip basket’ heatshield; our manufacturer of those long since retired.

Looking like Steve McQueen on a scrambler styled Bonneville costs extra!  Britcycle said they might have some scratched and dented options on sale, but those aren't regular stock (obviously).

Stock(ish) exhaust system:

721-T79 exhaust pipes - $289.02/pair.

712-102 Dunstall decibel replica mufflers - $330.32/pair.
70-9888 balance tube - $29.09.
2x 742-138 clamps - $16.61 ea.

TOTAL:  $683

What I'd really like to do is form my own pipes, but I don't have the space, equipment or time to do that.  For this project I think I'll use it as a learning process and get this particular Bonneville cleaned up and mechanically sorted and leave the radical customizations to a future time when I'm loaded, have lots of free time and a much bigger workshop with a full range of tooling in it.

I think stock is the way I'll go on exhausts...

As far as seats go, BritCycle has just the sort of thing I'm looking for: 

  

Custom Seat, 71on T120, Clip Type

Part Number 513-05

The only thing that might knock it out of contention is if, price and fit-wise, Corbin's customizable seat is in the ballpark.  It says they might fit a '71 but they're mainly for '72 Triumph twins and up.

Quality (made in England) newly manufactured seats specific to the oil in frame 650 twins run at about $500US ($612CAD) - I'm not sure what Britcycle's go for.  The customizable (and probably higher quality) Corbin is $618US ($788CAD).  There are cheaper options manufactured in India & China to less exacting standards to consider too.  I'll be keeping that all in mind as I juggle seat options and make a decision.

Old Bikes Tell A Story

I took the big SLR into the shop for some closeups.  These are photos from the 1971 Triumph Bonneville T120 project currently in process.

Call it patina, or scars, but the years on an old bike tell a story...

The 'spare' cylinder sleeves after some clean up.

Orange was the colour of panic in the early 80s.  The cylinder head was covered in this stuff in an attempt to seal a leaky motor during the aborted chopper phase of this bike's life.

That's the motor stamping (from Jan-Feb 1971).

Lucas! The Prince of Darkness™

My kind of still life.

Patina that tells a tale.

1971 OiF Triumph Bonneville Restoration: a seized top end

The old Triumph motor has refused to turn ever since I picked it up in the fall.  Every attempt at cycling the engine has failed so last weekend I dug into the top end, which turned out to be much more difficult than it needed to be.

Many moons ago I was putting myself through university by working as the service manager at an automotive shop.  One of our technicians, Jeff, always cheerfully described a situation where you're up against parts that don't want to move as a 'bend the fuck out of it' situation.  I'm no fan of pointless violence when it comes to mechanics, but there does come a point where you've either got to 'give 'er' (another of his favourite sayings) or give up.

Your mood when performing mechanical taks seeps into the machine.  If you're angry when you do repairs, that anger ends up in the mechanical work you're doing, which usually doesn't end well.  A Zen approach to mechanics usually creates a zen machine that doesn't emit the drama that an angry machine will.  Having said all of that, I'd pretty much emptied my swearing vocab by the time I had untangled this Triumph twin on Sunday.

Having never worked on this kind of motor before, I was lucky enough to get a spare head and cylinder sleeve when I got the bike, so rather than go in blind I disassembled the spare unit first to see how it all went together.

I continue to enjoy working on this pre-digital, very mechanical motorbike.  While it isn't as efficient and exact as a modern bike, there is something very satisfying about getting the mechanical bits lined up so that they work together.  When it's finally running it'll feel like I've rebuilt a Swiss watch.

With the practice head disassembled, I began removing the head on the bike.  It came apart as my reconnaissance suggested it would and looked mechanically sound with no discolouration or obvious wear.  The bad news came as I finally got down to the head gasket.

The right hand cylinder looked fantastic, but the left side was a mess with a thick layer of corrosion and an obviously seized-in-the-sleeve cylinder.  I'd expect to see something like this on a water cooled engine when the head gasket has failed allowing water and coolant into the cylinder, but this is an air cooled unit with no coolant in sight.  My best guess is someone left the spark plug out in a damp environment for an extended period of time letting moisture in to disastrous effect.  It's amazing what a bit of laziness or stupidity can be amplified into over time.

With a cylinder seized in the sleeve, I was left in a bind (see what I did there?).  I left it soaking in brake fluid overnight (I'd tried WD40 previously), but the next day it was just as stuck.  I applied heat, and then tried to lever the head off the cylinder to no effect, which led to that Jeff moment where I had to decide how far I'm willing to go to win (the answer is: all the way, in case you were wondering).

What followed was longer and longer breaker bars to apply more leverage, mixed with some applied heat from the propane torch.  What finally ended up working after a couple of hours of swearing and sweat was applying heat, inserting a long piece of wood to lever the head up while applying some focused violence to the cylinder top.  Millimetre by millimetre the cylinder sleeve eased up until the head finally came free, which was good because I was all out of swear words by that point.

I was rewarded with a couple of nice observations once the damned thing came off.  Firstly, the bottom end moved very smoothly for a motor that hasn't spun in 30 years.  There is no play or noise in the big end as it turns.  Secondly, the kickstarter I rebuilt the other week works perfectly, engaging and spinning the motor when applied and not interfering when left, which was satisfying.

Looking at the jammed cylinder, it looks heat seized with burn marks all over the inside and physical damage up the side.  If replacements aren't crazy, I might use the spare head I've got to get things into motion, but if it's going to cost a lot to the replace these bits I think I'm looking at a more aggressive customization option:  a 750cc big-bore kit.

Dropping $1000 into a broken motor seems extreme (it is extreme), but I've had to recalibrate my what-I'm-willing-to-pay index throughout this project; vintage ain't cheap.  If it's going to cost the better part of $500 to get the motor back to stock, why not get all new parts and get an engine upgrade in the process?

Before: sleeves like that usually slide off.  Cylinders are snug but not stuck.

After: a couple of sweaty, intense hours later.  It was a satisfying win.

I've done a number of projects now where I get a sidelined bike back on the road, the most recent of which was a '97 Honda Fireblade, but I've never had to do a complete engine rebuild.  Unlike some of my more fraught earlier mechanical work on the only bike I owned that was keeping me from the far-too-short Canadian riding season, I can take my time with the Bonnie and go all the way if it needs it and I'm up for it.  The only thing producing drag is my inherent cheapness.

I'm still intent on making sure the project pays for itself.  It doesn't have to pay me for my time, this is a hobby, but when I sell a restored machine and it balances the books in terms of purchase price and parts, I find that inherently satisfying; it means my hobby is a zero-sum game.  The Bonnie currently owes me $1500 for the bike and spares and another $500 in parts so far.  Rebuilt in running shape (but customized and I'd say pretty fugly) similar Bonnies are asking $7500.  If I can mechanically restore this bike to good running order and clean it up, a $5000 budget should see me well into the black.

Now to decide how to drop the next three grand on this thing...

Some other photos from the work this past weekend:

Seized cylinder soaking in brake fluid overnight.

First look at the head gasket after getting the cylinder sleeve off.

A disappointing first look into the heart of the Triumph.

This is the head on the bike motor.  The other head must be from an earlier machine, or it's missing parts.  Those risers with bolt holes in the top were on the '71 top end but not on the other one, which looks to be an earlier unit (Triumph built the 650 twin for many years).

I get a great deal of satisfaction from cleaning up the old parts on the wheel in the shop.  This is an exhaust clamp, lovely patina!

This is the 'practice' top end on the bench.  Unlike other top ends I've been in (my 'modern' Triumph Tiger, the Fireblade, etc), this isn't an overhead cam motor.  It uses push rods to operate the valves from down below (they're dead centre in the photo).