1952 Triumph Thunderbird 650cc

This of course then became an addiction and I acquired a 1952 650cc Triumph Thunderbird which looked complete, but in fact the guy I bought it from had obviously dismantled the engine, discovered problems and reassembled with only one piston and con rod.

I magnamiously decided no point in challenging him, and subsequently completed a full and total dismantling and began a rebuild.


It is of interest to highlight the issues that I encountered during the rebuild, and these are addressed in no particular order below;


Upon dismantling I found the cup was jammed in the pressure tube body, I elected to replace both components, it is important to ensure that assembled forks are of identical length overall.

It was a challenge compressing the spring on reassembly,  I made a compression jig to allow cap nut to be fitted. See photos below;

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Frame was undercoated then final enamel coat applied

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This is where my jig to compress springs was invaluable, refilled forks with oil (125mm) difficult to start cap nut with fine thread, established best method was with a 38mm socket, now on to wheel replacement.

I found with current bike stands very unstable decided to place lifting lugs in workshop ceiling and tie off much more stable and safe.

Modified as below which allows for unimpeded access

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Section of rear guard installed together with bike stand needed to make a jig to expand spring; 

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Upon dismantling it was obvious that the  gear operating quadrant was worn, a replacement was purchased, and installed after all gears were checked

With gearbox installed in frame I was not happy with gear selection and decided to dismantle again I established that there were no rollers for the gear control cam plate will try to source.

Sourced another gear box as I was not happy withe gear selector plate. Dismantled and decided to use the gear selector plate from the second gear box

Assembled OK requires a degree of dexterity 

See photos 

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Bought a spare gearbox for Triumph Digital StillCamera  Digital StillCamera Digital StillCamera


No problem with special tools replaced one plate

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To separate the gear box from the fixed frame, I experienced great difficulty in removing the Clutch Hub, during the attempt I mangled the special tool, mangled home made puller, used lots of gas to heat and in desperation eventually cut clutch hub with grinder, I was able to purchase a replacement.                    


With the gearbox and clutch separated, you are able to remove the primary chain case cover, which then allows easy access to engine, the cylinder head was removed and to my horror it exposed the damaged cylinder block,  I  was able to source another block

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 Union Jack motorcycles who specialises in Triumph’s.  was a very good source for all of the replacement of damaged components

Difficulty was experienced in sourcing another con-rod, but eventually located Crankshaft Rebuilds Blackburn Victoria, who  replaced both con rods with a generic type and fitted slipper bearings instead of white metal.

Crankshaft was rebalanced with crankshaft journals machined and cylinder bores honed a first class job and very helpful.

Important to mark con rod and crankshaft to ensure con rod matches crankshaft journal.

The modifications from white metal to fitting of slipper bearings on the crankshaft journals resulted in the final dimensions as follows;

Original specification for 6T

Big End 1.4360 inches

Modified 1.408 inches

Con Rod internal diameter with slipper bearings 1.409 inches

Gudgeon pin hole 0.688 inches 

Gudgeon pin 0.687 inches

Bore Diameter  2.7953 in ches

Modified  2.806 inches

Purchased Pistons 2.803 inches

Removed old crank shaft bearings with hydraulic press replaced with the following

KSM bearings made by Minamiguchi Seiko Co Ltd 

RMS8-2RS ball bearing OD 2.5 inches ID 1 inch width 3/4 inch

RMS9-2Rs ball bearing OD 213/16 inch ID 11/8 inch width 13/16 inch

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There are three issues that have not been raised before;

  1. Crankcase scavenger pipe damaged with a section missing
  2. Crankcase oil filter and plate missing
  3. Crankcase breather valve missing which is incorporated in inlet camshaft.

See photo of damaged crankcase scavenger pipe 8mm diameter steel which has been severely damaged with the bottom section missing. Experiencing difficulty sourcing replacement.

Needs to be formed to fit crankcase filter.  

Whilst searching for these components which are difficult to source I made a bracket to assist in engine reassembly

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 Finally located engine scavenger pipe and crankshaft breather valve and can start assembly.

As a matter of interest I was able to locate two but in one case had to take half a crankcase which is available to anyone

Commenced reassembly crankshaft breather fits in the inlet camshaft

Attached mounting plates for frame

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Timing of Engine and Magneto

 The timing and replacement of the camshaft and crankcase breather requires a complete understanding of the four stroke cycle, also it is necessary to have the correct tools for reinstalling timing gears

The photos below show the engine with timing gears in place also assembled valve tappets assembly

Purchased triumph valve tappet spanner as it is difficult to access nuts for adjustment customised ring spanner to facilitate easier access.

All complete all I need is assistance to place in frame

With help from my son engine is now in frame

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Moved bike to elevating work stand which will be much easier when motor is being installed 

Started assembly front guard then gear box relatively easy only need to remember to place primary chain tensioning device Digital StillCamera  Digital StillCamera Digital StillCamera  Digital StillCamera

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With engine in place commenced re assembly of clutch no problems see photos

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With the removal of the voltage regulator, dynamo and magnet,o inspection revealed it would be necessary to refurbish all of these components. A search of the web located Antique Auto Electrics Kangaroo Flat Bendigo, who were very obliging and professional. 

Magneto was rewound, poles re-magnetised, cleaned, fitted new brushes and bench tested. Dynamo had the common armature skimmed undercut and polished.

Regulator was refurbished and then bench tested with dynamo.

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Was able to source an original cloth wiring loom, now the hard part begins will require small delicate fingers; installed instruments and horn. Decided rather than have wiring loom hanging loose will proceed with installation of gearbox and defer wiring until all components are in place.

Started the wiring of nacelle but prior to that drew a wiring diagram which has enabled the wiring to proceed see diagram and the slow progress

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Completed wiring was able to locate a cloth wiring loom lots of patience required to action as per diagram


With wiring complete it is now possible to replace tank, first the tank was dressed see finished product


Then controls were fitted, battery holder, seat, oil reservoir, carburettor and new hosing

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All that is remaining is control for air mixture on AMAL carburettor, repaired exhaust’s and lock for toolbox this proved difficult but located a Dzus lock as it is called the USA

However I have taken photos with bike at this stage

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Replaced throttle and choke cables

All I am waiting on now is returned of exhaust system and choke lever from chrome plater

Exhausts received and installed.

Started bike OK but trouble with oil relief valve we believed purchased new replacement also changed oil pump,

problem still exists with pressure indicator showing no pressure

Examination revealed that crankshaft bush on timing gear casing worn to such extent 20 thous 

Had timing gear casing machined to accommodate a modified seal see photo

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Timing chain cover  replaced after timing, ensured that crank case breather connected to oil tank 

Kicked over and started runs well 

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worth the effort

9 thoughts on “

  1. What a great descrptive Blog this is and what a great restoration job has been done on all the bikes. Great Mate!!!!

  2. Jeff,
    I have a 1950 Thunderbird that I will rebuild during the winter months. I am located in the Toronto Canada area.
    Please email me as I have some questions for you.
    Thank you

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