Hundred-mile-stone reached (part 1)

I've just come back from a lovely ride through the autumnal Chiltern hills, throughout which Bagheera performed faultlessly and was in her element chugging characterfully along the sunny backroads. The journey also brought up 100 miles since the engine was completely rebuilt following the disasterous overheat back in July.

A frankly regrettable amount of time, pain and money has been needed to reach this point. As detailed in the last post, the engine overheat caused extensive damage to the top-end of the engine; damage beyond my ability or tools to repair. After a bit of searching around, I finally got a recommendation for a good engineer/machinist called H T Howard in Slough. After a quick chat with boss Andy, I was reassured that they really knew their onions, or more importantly their vintage British bikes.

My first task was to source new engine internals. Due to the scuffing on the cylinder walls I decided to go up to the first oversize piston, officially specified by Triumph as "76mm + 0.020 inches". I kid you not. Why the piston size is the only thing on the entire bike specified in mm I have no clue.

There are a lot of people churning out pistons for classic Triumphs, but a bit of research revealed that Taiwanese company JCC make well-regarded but not excessively expensive items. I even found a supplier Grin Triumph selling them with a set of Goetze piston rings -- one of the best makes you can buy. 

Piston rings seal the small gap between the piston and the cylinder - in this case just over 0.002" (2 thousandths of an inch) all around. Without them there would be nothing to stop oil splashing up the cylinder walls and getting burnt or exhaust gasses going the other way, losing cylinder compression and pressurising the engine crankcases. The rings have a very tough job to do as the scrape up and down the cylinder walls thousands of times a minute, resisting massive gas pressures and plenty of heat. If they break, it can ruin the entire engine so it's worth getting good ones. The pistons and rings together set me back £109.

Next, the valves and valve guides needed attention. The valve guides were not damaged by the overheat but were worn well past their useful limit and were a very sloppy fit with the valve stems. This is a problem because the job of the guide is to ensure the valve closes accuately onto its seat for the best possible gas-tight seal.

Andy suggested that it is better to bore out and line the valve guides than to replace them, because removing them can often damage the cylinder head. These fancy "K-line" bronze alloy liners effectively turn the old school iron guides into modern Colsibro ones. Colsibro is a specialist alloy purpose-made for the job, being both hard wearing and low friction.

The old valves themselves were not in too bad a condition, but I decided to replace them anyway. Supplier Norman Hyde has a reputation for top-notch parts, and although many are out of my price range their valve sets are works of art at an entirely fair £95. Norman himself is an ex-engineer and designer from the Meriden Triumph factory who has been suppling parts for these bikes for as long as I have been alive. 

Shiny new bits

I dropped off the cylinder head, cylinder block, new pistons and new valves at H T Howard for them to work their magic, sneaking the job in just before their summer holidays. Their task was to rebore the cylinders perfectly straight and round to fit the new pistons with a clearance of exactly 4.5 thousands of an inch. Then they would skim the cylinder head flat and bore out and line the orignial valve guides in situ, then hone the liners to fit the new valves with a clearance of barely a thousandth of an inch. Finally the valve seats would be recut for a perfect fit on the new valves, giving that all-important gas-tight seal. All this work requires a lot of kit, such as this huge machine just to cut the valve seats.

In no time flat the job was done, and ready to pick up along with a bill for £429. It sounds like a lot, but essentially represents a brand new top-end. The work looked beautifully done. All I had to do now was put it all back together.

The freshly machined cylinder head surface and re-cut valve seats

 
Andy had advised me that 12 thou had been skimmed off the cylinder head which was quite warped, and a further 9 thou off the cylinder block, mostly to clean up the area where the head gasket. I have since learned that these head should not be skimmed at all, but rather straightened. They bend rather easily, and this is often due to a combination of carelessness and a built-in design fault in the engine.

On this engine, the four pushrods are hidden away inside chromed steel tubes, the ones you can see on the front and rear of the engine. The tubes keep the dirt out and the oil in (at least in theory), and when the head is fitted they get clamped in place between the head and the base of the barrels with a seal top and bottom.

The concept makes sense, but in practice you have to be very careful to get just the right thickness of seals. Too thin and the oil leaks out, which is annoying. Too fat and when you bolt down the cylinder head there is so much resistance from the seals it will actually bend the head upwards in the middle, which is a disaster. Judging by my old head gasket, this is exactly what some ham-fisted spannerman had done to my engine in the past.

Opinion varies but the general consensus seems to be that a gap of 0.030 - 0.040" between the cylinder block and head gasket before tightening the bolts gives enough "squish" to the seals but without bending the head. Using standard parts, I had far to much clearance so I sourced some thinner 2mm seals at the bottom of the tubes and a thicker 0.080" copper head gasket. The thicker gasket also makes up for the material machined away, preserving the original compression ratio and push rod / rocker / tappet geometry.

Before starting assemby, I washed all the parts really thoroughly in both water and paraffin; you don't want any metal particles left over from the machining work inside your engine. Then I took the opportuninty to repray the rather tired looking barrels using Simonez black enamal engine paint. Getting the paint inbetween the cooling fins is not easy, but the final result was pretty good.

While that was drying, I reassembled the cylinder head. Each valve goes into its respective guide, then the spring retainers and springs are put over the valve and finally the valve is compressed with a special tool while small metal cresents called collets are inserted into a groove in the valve stem. When the tool is removed, the spring presses the top retainer upwards and wedges the collets firmly in place thus connecting it all together. I had a slight set-back when I found the old collets were too brittle to use but a replacement set made by L F Harris was easily bought for a few quid. Harris is another big name in classic Triumphs, having bought a licence to continue making them after the factory went bust. Between 1983 and 1988 they produced many Triumph Bonnevilles and Tigers, and still make and supply many of the parts.

Over to the pistons and rings. Before assembling them I checked the ring end gaps. This is done by sliding a ring squarely into the bottom of the barrel and checking the gap between the two ends (as pictured). Too big, and gasses escape through the gap wasting energy. Too little and the ring cannot expand and the piston can sieze. Again opinion varies about what it should be, but my measurement of 0.020" was bang on where I wanted it.

Next I had to fit the piston rings to the pistons, a delicate job because the rings are made of cast iron and are very brittle. Also it is important not to bend them at all or they will not seal correctly against the cylinder walls. Special tools are available, but you can do it by hand. Carefully and squarely stretch open the ring with the thumbnails until it is just possible to slide it over the piston squarely, manouvre into position and release. Making sure of course that the correct ring is in the correct groove and they are all the right way up!

The complete pistons can now be fitted over the end of the con rods, while a a steel tube called a wrist pin or gudgeon pin is pushed through a hole in the piston, through the small end bearing and back into the piston, holding everything together. This was straightforward, the new wrist pins are an easy sliding fit in both the piston and the small end. One tricky part is fitting the fiddly little clips that locate the wrist pin. It is very easy to ping these across the workshop and lose them, or worse still drop them into the engine. Block all entry points with rags before attempting this!

I was a bit worried about the next part -- lowering the cylinder block into place over the new pistons. It is fiddly as you have to compress the sharp-edged but fragile piston rings into their grooves without breaking them or nipping your fingers. Triumph have thoughtfully bevelled the bottom lip of the barrels to make this easier. I used just a tiny smear of of engine oil (which will be explained later), and found that they went into place easily. Then I bolted down the cylinder block. Those odd-looking nuts actually fit a standard 1/2" ring spanner, but only once you have ground down the spanner to make the walls thinner!

The head gasket is solid copper and has to be softened or "annealed" before use by heating it red hot then dunking it cold water, which makes a very satisfacturing hissing noise. Once this was done and the resulting black gack cleaned off, I put the head gasket, pushrod tubes and seals into place; everything was ready for the head to be bolted back on:

The head went into place easily, with the help of some silicone grease on the pushrod tube seals. I dropped in the nine head bolts and carefully tightened them. I rarely bother using a torque wrench, prefering to work by feel, but here it is essential. The bolts are tweaked up to just 18 foot-pounds (15 for the small central one) in three stages always working from the middle outwards in a diagonal pattern. Tight enough to give a good seal on the head gasket but not so tight to distort the soft aluminium cylinder head. As a side note, I later found out that the washers originally fitted to the central four bolts are too small and were later changed to broader ones to spread out the load better.

Next I dropped the pushrods back into place. A good tip here is to fill the hollow ends with grease. When the rod is correctly located on the rounded end of the cam follower it will "suction" onto it quite strongly and resist an upward pull. Carefully placed holes in the rocker box gaskets hold the top of the pushrods in about the right place until the rocker boxes are back on. Use plenty of grease on both sides of the rocker box gaskets so they can be removed in the future without damage.

Finally, the rocker boxes are bolted back into to place and the valve clearances reset. I found they had loosened up quite a lot, meaning that the thicker head gasket had done its job of raising the head slightly and reducing the compression ratio.

The engine complete once again, all the ancillary parts have to go back on including the exhaust system, carb, oil pipes, head steady bracket and petrol tank.

While it is tempting to try to start the engine straight away, first I spend a few minutes turning it over on the kickstart with the spark plugs out to circulate oil. Once it starts to squirt out of the return line you can be happy the system is working and oil has reached all the important places. I also do a compression check -- and get a very satisfying reading of 120 - 125psi on both cylinders. This is a massive improvement and shows the machining work was well done. As the engine is run in, the compression should continue to improve.

All is good, so I initate the starting sequence. Free the clutch, turn on the fuel, tickle the carb, close the air control, switch on the ignition, find compression on the kickstart, crack open the throttle give it a good strong kick and...

...find out what happened next time!!!