Mechanical Wonders
Mike takes a break from bodywork, switching attention to finishing rebuilding the engine and differential.
The differential had been overhauled some months earlier and upon its return, the external surfaces had been cleaned, degreased and sprayed with black engine enamel. However, on the Stag the differential has a fabricated front extension housing that forms the central stressed member of the subframe, to support the rear suspension. This housing has an internal shaft with a splined sleeve that fits over the pinion shaft at the differential end and a bolt on flange at the front that attaches to the drive shaft. The rear outer diameter of this shaft runs in a seal in the diff’ and the shaft is supported on a sealed bearing at the front end. There are stories of this extension cracking and breaking due to the combination of loads from supporting both the differential and subframe. A major failure in this area can have a disastrous outcome, with subsequent failure of the differential front seal, loss of oil and destruction of differential gears. Information from the Stag Owners Club website and talking to local Stag owners/specialists confirmed that failures, although rare, have occurred and that the recommended modification is to weld some supporting webs between the flanges that bolt to the differential and the subframe. The extension from the project car had not been modified, so I thought it silly not to make the necessary changes now, so after stripping and blasting the housing, four webs were cut from ¼” plate and welded in place. Minimal weld was applied and the work was
completed over a period of time, to minimise the risk of heat input distorting the housing and therefore alignment of the shaft. After cooling, the welds were cleaned and external surfaces painted to match the differential.
Bearing up
A replacement sealed bearing was required for the rebuild, so the correct bearing number was obtained from the SOC website and purchased from a local bearing supplier. I ensured that the replacement was manufactured by an internationally recognised bearing manufacturer rather than a little known company from a low-cost source, where quality can be questionable. Previous experiences of premature bearing failure with products from unknown manufacturers have changed my selection process for these critical items. I am not against products from the lower cost countries, however I prefer to pay a little more and purchase from a reputable source, knowing that wherever the product is manufactured, it will be under the wing of a strict quality assurance system. The bearing is located in the housing by a circlip front and rear, so firstly the rear circlip was inserted, prior to using the press to push the bearing into the housing until it contacted the circlip. The front circlip could now be inserted, and the shaft pushed from the rear through the bore of the bearing. Finally, the washer and Nyloc nut were fitted to the end of the shaft and torqued up. Nyloc nuts should not be reused, however a half-nut was fitted and I only had full nuts available, so the full nut was machined to the half nut dimensions. After cleaning the splines – which are lubricated by the diff oil – the extension housing was bolted to the front of the differential.
Longer than expected
The block, crank and heads were sent away for rebuild in February 2013 and, while the eight month lead time was longer than anticipated, it is not exactly unusual for a classic engine in Brisbane, as I have previously experienced in excess of six months for Jaguar engines. The heads were completed in two or three months, but the Tufftriding on the crank
appeared to be responsible for the major delays, as the operator had to wait for an economic batch size before processing. It was unusual inspecting the crank, as it was jet black, including the bearing areas. The Tufftride treatment only affects a thin outer layer and therefore the crank needs to be reground to the correct size prior to treatment, otherwise the grinding would remove most or all or the hardened layer. After Tufftriding, the bearing surfaces require a quick polish with extremely fine emery cloth (what we used to call crocus paper when I did my apprenticeship) or even the reverse side of emery.
This was the first rebore for this block and, prior to the reboring, I’d sourced a set of +20 thou pistons from Rimmer Brothers. They were then used as the datum for the rebore size and all went well for seven bores, but there was an issue with the final one. The car including the engine had been stripped prior to my purchase and the block had a light rust coating over machined surfaces when I received it, so I had sprayed it with penetrating oil, followed by a coat of engine preserving oil.
Prior to sending the block for reconditioning I had checked the bores and bearing surfaces and all appeared fine, with zero rust pitting, but now one bore still showed signs of rust stain after being rebored. The minute amount of surface rust wouldn’t have penetrated this deep, the likely explanation is that from new there had been casting micro porosity in this bore, which was now showing as rust staining. The probability was that it would be fine as was, but to reduce the risk of issues later, the bore was machined and fitted with a liner. All the core plugs were removed from the block, so that it could be cleaned to ensure that the waterways were clear of the leftover casting core sand, which seems to have been one of the causes for the overheating that all but killed the reputation of the TV8.
Flat to finish
I checked the flatness of the inlet manifold flanges and found that both sides were warped by a
The manifold faces were then milled, removing a maximum of 15 thou
few thou, which would increase the chances of leakage through the gasket. The decision was taken to mill the faces, so the manifold was clamped to a faceplate on the mill table. Initially it was set up squarely with a spirit level but, as it was important to remove minimal material, a dial gauge was used to finalise an accurate setting. This also gave an accurate reading of the bow across each flange, the worst being 10 thou. The faces were then milled, removing a maximum of 15 thou, to obtain a flat flange with no corrosion marks that would provide a good sealing face. After completing the first side, the manifold was repositioned to machine the second side, which had fewer bows and only required 12 thou of material removal. The manifold was then cleaned and sprayed with the same silver paint as had been used on the carburetters, so that their appearance was similar.
The backplate was cleaned in preparation for fitting and the cam covers were cleaned and checked. It was at this stage I realised that on the right hand cam cover from the parts car the ‘Triumph’ motif was inverted when it was pressed into the cover, whereas the cover from the project car was correct. Some research confirmed that a number of cars were dispatched this way but early and late models had ‘Triumph’ pressed in the correct orientation. I decided to powdercoat the covers silver, as this would be harder wearing than paint, so they were blasted and cleaned prior to being sent for coating. After return they were again given a thorough clean in the parts washer to remove any dirt and grit, paying special attention to the wire filter under the breather vent.
When Greg Tunstall was ready to assemble the engine, he requested that I bring in the remaining components, including the new timing chains, sprockets, guides and tensioners, all of which I had purchased from Rimmers. Problems with Stag timing chains stretching are well documented, so I had specified German manufactured chains,
which are more expensive, but reportedly resist stretching better than standard chains. I had already delivered the replacement bearings, thrust washers, new oil pump and set of seals and gaskets, with exception of the head gaskets that were to be sourced locally.
Decisions, decisions…
I had spent months considering what style of water pump should be fitted; many local Stag owners have experienced the not uncommon Stag cooling issues (exacerbated by the Australian climate) and have opted for a locally manufactured Davies Craig electrically driven water pump with digital controller. From inception the Stag cooling system has been a major weakness, my research indicated that these revolved around the following areas.
Water pump is located high in the ‘vee’ of the engine, so loss of coolant tended to starve the pump and reduce water flow.
The initial 6-vane water pump did not circulate sufficient water, a 12-vane pump was introduced towards the end of production to increase water flow.
Poor casting quality and core sand not cleaned from block reduced the size of waterways within block, decreasing flow.
The cars suffered from overheating when engine was left to idle after a long high speed run, because the low engine speed results in both low coolant flow and low fan speed and airflow through the radiator.
Local owners suffered from a poor batch of replacement water pumps where the water pump driven gear was of poor quality and soon failed, leaving the impeller with no drive. It can be seen that there was a strong case for fitting an electric water pump, as this would appear to overcome most of the inherent weaknesses in the cooling. However, after discussions with Greg I decided to stick with the original water pump, given the other improvements that I was making to the cooling system.
Fitting the Stagweber header tank with low level water alarm will reduce chances of the pump ever being starved of coolant.
My car was fitted with a
12-vane pump, and as the impeller, shaft and gear were in good condition, Greg recommended rebuilding that existing pump.
Rimmers supplied a new cover for the water pump (12-vane covers differ from 6-vane) that was installed with the correct clearance between impeller and cover to get maximum flow. The manual describes the method of measuring for correct clearance and shimming the cover with gaskets of various thicknesses.
I will be replacing the old electric water temperature gauge with a new mechanical unit that has the dial calibrated in ˚ C to give an accurate read of engine temperature. The accuracy of the gauge reading will be checked prior to fitting.
I hope that the new aluminium radiator with high
It is critical to keep the correct distance between the crankshaft and camshaft sprockets
capacity electric fan will be sufficient to overcome any overheating issues at idle.
If after all this effort I experience overheating issues I will be disappointed and will fit a Davies Craig EWP.
The engine rebuild went smoothly, but I must mention the head gaskets. It is critical to keep the correct distance between the crankshaft and camshaft sprockets, so chains are correctly tensioned and the correct valve timing is maintained. Material machined from the faces of the heads during their reconditioning has to be compensated for by an increased thickness of head gasket. Greg uses a local gasket manufacturer to supply composite head gaskets that are laminated to provide the required total gasket thicknesses and then have metal sealing rings fitted to bore circumferences and oil transfer ports. New head studs and bolts were used during reassembly – I purchased nickel-plated studs from Rimmers and all cylinder head studs and bolts were installed with a generous coating of copper grease. Hopefully, I will not need to remove the heads from this engine but by taking these precautions, should it become necessary it will be easier.
A dizzying selection
Whilst preparing engine parts, a decision had to be made on the distributor specification. On all my recent classics I have replaced the old points system with a modern electronic unit that provides a better spark and does not require regular maintenance. The Stag was therefore going to have electronic ignition, but make or style was as yet undecided.
Earlier it had been noted that the parts car was fitted with a Lumenition optical system and this was a possibility. I took both distributors out of storage and placed them on the bench to assess potential for future use or modification. My first choice was the Lucas 35 D8 from the parts car with the Lumenition ignition, however a closer inspection revealed the vacuum advance diaphragm was missing, a point missed earlier when the engine was stripped. The top rotating disc of the two-part baseplate for the vacuum advance was stuck fast, so the distributor was dismantled to investigate the lack of movement. As soon as the combination baseplate was removed the reason was obvious – someone had bronze welded the two sections together, so the Lucas unit was ruled out.
Closer inspection of the distributor from the project car revealed it was a points operated Bosch unit that had been supplied by a local company, Performance Ignition Services, under the ‘Scorcher’ brand. They were contacted regarding an overhaul and fitting an aftermarket electronic kit, however the cost was reasonably high and the alternative they offered was a brand new Bosch electronic high-energy distributor. Greg confirmed this set-up worked well on a Stag, so I decided to pay the additional cost and upgrade to new unit.
The only thing I disliked about the Bosch was the method of mounting; the Lucas 35 D8 has a mounting flange with two elongated adjustment holes that align with tapped holes in the block, enabling two fixing screws to maintain a secure attachment. The Bosch distributor has a circular mounting flange with no screw holes, so requires some form of clamp to hold in the correct position. Greg used a clamp on one end that is secured by a single set screw, which is fine from an operation viewpoint, but I was unhappy with the visual appearance. I chose instead to replace this with an engineered clamping bracket manufactured from 10mm steel plate utilising two set screws.
After removing the mill scale from the flat bar, the final shape and hole position were marked on the steel. The bar was then loaded into the lathe using a four-jaw chuck and aligned so the centre hole and recess could be drilled and bored. The recess was bored to 5mm deep, which would give 0.5mm clearance between the clamp and the block when installed, after which the bar was removed from the lathe and cut to shape using a thin cutting disc on an angle grinder. The disc was also used to cut the slot to fit round the distributor and then the two holes were drilled and external corners radiused with a hand file. After a coat of paint it was fitted in place, ensuring that the correct distributor position was retained.