Saturday, December 11, 2010

Triumph pre-unit engine re-build

Triumph T100 500cc engine restoration. The bike belongs to Mark H. and is undergoing a full restoration.

I truly believe the Triumph pre-unit engine is arguably the best looking motorcycle engine ever designed.
In 1937 Triumph's super team headed by Vale Page, Ed Turner shook the world with the Speed Twin and an engine that lasted till the end of 1962. Although this was the end for pre-unit engines, the design still remained consistant into the unit construction age and still to this day, modern Triumphs also sport those distinct features. That's more than 70 years!

Anyway, enough history lessons, here's the build.

The engine arrived half stripped and with most of it's parts some what arranged in small containers and bags. We began by sorting the loose parts to see if anything was missing or evidently needing imediate replacing. Next we begin to tear down the engine for inspection.

Removing the barrels exposes the worn pistons and also gives us an insight of things to come. Big ends are both worn and the mains have excessive movement with rough rotation. The pistons are standard size.

The bore has been re-sleeved in some stage of it's past life and will have to be checked. We can now remove and inspect the tappet blocks and cam followers.

Removal of the sumplate and filter gauze shows clearly the importance of  regular service.

With the engine cases apart, cams, wheels and crank removed, we now see this engine shows evidance that it has been used for racing in the past.
The smell of methanol in the sludgy oil is there, but the 'red' coating on the insides is proof as a measure to protect the alloy from the effects of methanol.

The crank is covered in a slimy coat of old thick oil and perhaps other matter. The conrods are of the white metal type meaning they have no big end shells and as mentioned, are worn. The small end bushes (gudgeon pin) have also seen better days.

The cast iron head and rocker boxes are next in line for tear down along with the oil pump, oil pressure relief valve and we will commence with the cleaning, reconditioning and rebuilding of the engine. All exposed nuts, bolts, washers and pipes are sorted and ready for plating.

We Hydro-Tech clean all our alloy parts.
With 6 months research and developement we have devised a unique and proven method involving a three way cleaning process to give us the best possible finish. See and compare these pics!

This service is available to all our customers. If anyone is interested, please contact

Barrels bored out and honed to take genuine +10 pistons. Tappet blocks checked and fitted. Gloss black hi temp paint and we're looking good.

The head was straight forward and in good condition in comparison. Valves, springs and collets replaced, head shaved, painted, assembled and ready for service.


After stripping down the crank we chemical cleaned and crack tested the parts. Re-assembly, machining and balancing and we now have a good as new (well almost) crank.

The conrods are now clean and we've modified them to take big end shells. New small end bushes installed and reamed. Conrods and pistons also balanced.

We are now ready to build this engine. Conrods torqued to crank and new main bearings installed. We placed the crank into the primary side and prepared to marry the cases.


Cams, gears and any replacement parts are cleaned and installed. Oil pump cleaned, rebuilt and checked. Cases sealed.
Piston rings are gapped and fitted to rods. We painted the original sump plate black and fitted this with new gauze and gaskets to the engine.
Full back and forth rotations to make sure all is well and we are now ready to fit the barrels.

Barrels now fitted with tappets, we move to the rocker boxes. All componenents chemically cleaned and ready.

Pushrods, tubes, gaskets and seals in place as the head and rocker boxes are torqued down. Clearances are set, inlet manifold is also fitted. Rocker inspection caps and spark plugs insure no foreign particals enter the engine.

We are waiting for the magneto and dynamo to arrive so we cannot seal the timing cover yet. Oil pressure relief valve rebuilt and checked. Timing cover polished to a satin finish and fitted. All exposed holes are covered to keep everything clean.
We are more than happy with this build and can't wait to hear it run... I love these engines!


Anonymous said...

right on

Anonymous said...

Hi, first i'd like to say that motor turned out beautiful. That conrod conversion is trick and the finish looks great. Keep up the good work.