1958 Ariel FH – Engine & Gearbox Fix





This tidy Huntmaster is from 1958. It is one of the last few of its breed and benefits from a modification that BSA made to the later A10 engines. To improve longevity and reliability, they made the bearing journal diameters a little larger. It had been well known among A10 owners that the bottom end of these splendid 650cc twins was its Achilles heal. The strengthening of the crank bearings not only increased the reliability of the 'cooking' A10, it also paved the way for the introduction of the Rocket Gold Star (RGS). The considerable performance boost of the RGS would have otherwise knocked out the bottom end in short order.

The Huntmaster received the crank upgrade a few months later than the BSA A10, after Ariel had run down their stocks of the older engines. That's why only a few of them benefited from it before Ariel finally ceased production of the Huntmaster.


The current UK owner bought it from a dealer who had repatriated it.


A neat but risky anti-wet-sumping tap. Sooner or later, the rider will forget it.

There was a period, a few years ago, when the supply of new pistons for Huntmasters all but dried up and the only ones that were readily available were the high-compression pistons intended for the RGS. A number of Huntmaster owners were caught out by that. Initially, they may have been pleased that their rather agricultural 'Flashes' would be spiced-up with the aid of the sporty pistons but sooner or later it would have become apparent that the new 'lumpiness' was becoming annoying. Shortly after that, the big-ends and the timing-side main bearing would get a bit rattly and the oil pressure would drop to near zero.

The high-compression pistons are a bit much for the older 'small-journal' crank, which is compounded by the fact that the volume of the combustion chambers in the iron cylinder head is significantly smaller than the combustion chambers in BSA's aluminium RGS head. The effect of this is to increase the compression still further, to the point where even the upgraded crank would groan under the strain. So when these pistons were fitted to older engines with the small-journal cranks, they didn't last long.

The previous owner of this bike lived in South Africa, from where it was repatriated to the UK by a dealer. He had refurbished the bike and commissioned the rebuilding of the bottom end of the engine by a local firm of engineers, who had done a professional job. As it happened, the rebuild was carried out during the period when only the RGS pistons were readily available, so a pair of those was incorporated. Fortunately, in this case, not only had the bike been blessed with the strengthened crank but the engine builder understood the issue with the Huntmaster's iron head and he fitted a 'compression plate' under the cylinder barrel, which brought the compression back down to a more sensible level. He also had the foresight to add a constricting gasket behind the carburettor, to limit the revs and the top-end performance during the running-in period.

By the time the new owner received the bike from the importing dealer, it was already substantially run-in but was displaying a few annoying features, which the owner entrusted to me to fix. They were mostly straightforward things except for a disturbing resonance that occurred at certain speeds. He also wanted me to look inside the engine to check that the rebuild had been done to an acceptable standard.


A nice bit of history – a dealer's plate from its previous life in Pretoria

As a part of his unusually specific briefing, the customer sent me a long list of tasks for me to do. Most of them were relatively straightforward, as you can see from the list below, except for the last one.

  • Replace the fork seal holders with new stainless steel ones;

  • Fit the new twin-leading-shoe (2LS) front brake assembly that had been supplied by the customer;

  • Check the wiring and tidy it up where necessary;

  • Check the condition of the valve gear and repair as necessary, including fitting hardened valve seats;

  • Fit an original Burgess air filter, which the owner had acquired and supplied;

  • Fix the 'wet-sumping' and upgrade the lubrication where possible;

  • Strip and powder-coat the cylinder barrel and head;

  • Adjust the ignition timing & carburettor settings;

  • Trace and fix a loud resonance that occurs at certain speeds.

The “resonance” issue was likely to be less straightforward because I had no accurate description or any real idea whether it was a mechanical issue or if it was a true resonance of a cycle part.

As the bike was apparently roadworthy and fully functioning when it was delivered, the first job, after a few pre-flight checks, was a test ride. I have several test-ride routes mapped out. They are of varying length and have varying degrees of hillyness and complexity, depending on what kind of flaws need to be identified during the test. Some routes are designed to test engine performance, some are for handling and some to isolate intermittent or reliability issues.

In this case, I used one of the shorter routes with a long, steep climb and lots of bends of varying speeds. I was looking at general performance but I was also hoping to trigger the “resonance”. I wasn't disappointed because it quickly became clear that a loud noise and associated vibration occurred consistently while accelerating in 2nd and 3rd gears. The intensity increased with throttle opening and the resultant increased load on the transmission.


The gear cluster from the Burman GB box.

Once back in the workshop, the strip-down began. The clutch and primary transmission came off first, together with both halves of the primary chain-case and various other bits to make access easier, such as the fuel tank and the battery carrier. A couple of small things came to light at this stage. Firstly, the retaining ring for the engine shock-absorber had been molested with a chisel, probably because the mechanic did not have the correct tool. Cleaning up the part with a file made it suitable for reuse with the correct tool.

Secondly, the primary chain was very tight, probably because the gearbox had moved under load as a result if inadequate tightening of the clamp bolts. No only would the tight chain make the engine feel rough, it will put a lot of unnecessary strain and wear onto the sleeve gear bush.

Then the chain sprocket came off and the inner and outer covers were removed to expose the gear cluster. I laid the gear cluster, together with its selector forks, on the work bench, where I could get a good look at the gears, shafts and dogs. The fact that the noise occurred only in 2nd and 3rd gears gave me a pretty good clue to where to look first.

Immediately obvious was the state of the two 2nd gear cogs, one on the main-shaft and the other on the layshaft. They had suffered a significant trauma, with many damaged teeth. The damage had probably been caused by a fragment of broken tooth becoming jammed between the gears. There was no evidence of the metal fragment that had caused the trauma, only the deep tell-tale marks on the teeth. That probably meant that the gearbox had already been dismantled, the offending fragment removed and the broken gear(s) replaced. Strangely though, the two damaged 2nd gear cogs had been left in there. That may have been in a bid to save cost or it may have been the result of an availability issue. I can only guess. Fortunately, the shafts were not bent.

There was, however, some evidence that the previous owner of the bike (or possibly the dealer from whom he bought it) was aware that there was still an issue with the gearbox. It had been over-filled with a prodigious quantity of very heavy gear oil, presumably in an attempt to subdue the noise. It didn't work.

I ordered two new gears and, while waiting for them to arrive, my attention refocused on the top-end. Somebody had made a decent job of rebuilding the bottom end but the same could not be said for the cylinder head. The exhaust valve seats were deeply recessed and the valve guides had been unwisely repaired (misguidedly might be a better word) by fitting K-line sleeves into them.


The traumatised gear.


All of the guides had some cracking. Cast iron is quite brittle and can not always stand the stresses involved in the process of fitting the bronze liners.

For those who are not familiar with K-line valve guide inserts, it is a way of rejuvenating worn valve guides by boring them straight (the bores of worn valve guides are usually hour-glass shaped), slipping a thin bronze tubular liner into the guides and “setting” it using a hard steel ball on the end of a rod. The liner is not actually a tube but a tubular spiral with a split along its length. It is slightly curled to make insertion easy but it is then expanded to lock into its tight-fitting cylindrical shape. The expansion is achieved by the rather brutal process of pulling the steel ball through the bore. In some cases, it works fine but in many cases, particularly if the guides are cast iron, the process cracks them.

The cracks may not be immediately visible but, over the next few thousand miles, they open up and bits of guide can be shed into the engine. I have seen the evidence of this on a number of bikes. Replacing the guides is a straightforward process and a much better long-term solution if they are worn.

This particular engine had gaping cracks in all of the guides, both above and below the head casting, and a number of fragments had been shed into the ports. All four guides needed to be replaced and the two exhaust valve seats needed to be machined out and new hardened inserts pressed in and new seats cut.


The original oil 'pot metal' oil pump.

Working my way through the customer's task-list, I removed the oil pump and checked how smoothly it would rotate. This was the original one that was fitted in the factory and it showed all of the flaws that seem to be common with them. They develop tight spots and they pass oil when they are stationary, causing the crank-case to slowly fill with oil.

The rather nasty 'pot metal' that they are made of has a tendency to swell and distort, particularly if they have ever been over-tightened, which they usually have. The resulting 'wet-sumping' can be very annoying, particularly for owners who don't ride their bikes frequently. This bike had a tap fitted in the oil feed pipe, in a somewhat risky attempt to defeat the problem. I discarded the tap, together with the pump, and replaced the pump with a particularly attractive new one from SRM. As usual, I also replaced the oil pressure regulator valve with a new one, also from SRM.


A nice new pump made from billet.


The oil filter is accessible but barely visible.

Continuing with the lubrication theme, in accordance with the owner's wishes, I set about fitting a cartridge oil filter into the oil return pipe, between the engine and the oil tank. In the case of most four-stroke Ariels, the obvious place to put one is in the void behind the gearbox. It is convenient for routing the oil pipes, it is very discrete and the adjacent lug for the sidecar fitting is a perfect mounting point for a bracket to hold the cartridge. I made a bracket from 1/4” thick, mild steel flat bar. Mild steel is a good choice for brackets of this type. It is more than strong enough and, unlike stainless steel or aluminium, it doesn't become brittle with time.

The oil filter facilitates the use of modern detergent oils, which discourage the formation of sludge and provide more protection for the moving parts.

The bike had been repainted, not to a particularly high standard but it looked quite tidy. But for some reason, the rusty old fork seal holders had been left in place and, evidenced by the oil that had escaped from them, the original seals as well. Replacing them was straightforward and the shiny new stainless steel made the bike look even tidier.



While the front wheel was out, it was a good opportunity to get on with fitting the new 2LS front brake. It had been fitted with new, over-sized linings, which were slightly too large for the drum. I made a new arbor to hold the whole assembly (less the operating linkage) securely in the lathe spindle. The diameter of the brake drum was then measured accurately with a very large vernier calliper that I keep for this very purpose. I spun the brake plate on the lathe and turned down the diameter of the shoes until it would just fit in the drum.

This method ensures that the surface of the shoes is perfectly cylindrical and has the maximum contact area with the drum, right from the outset. This way, It is possible to make even the nasty 7” SLS BSA/Ariel brakes work well.

Setting up the operating linkage on a 2LS brake is fiddly and can affect its performance significantly, so it is worth spending some time to get it right.


These 2LS brake plates are beautifully made.

I was asked by the customer to have the barrel and head powder coated in a black silk finish. Both components would benefit from being tidied up a bit but powder coating of parts that are subject to significant heat and frequent changes in temperature doesn't work at all well, for two reasons. Firstly, it forms a thick coating that insulates the metal and impedes cooling. Secondly, after a relatively short period of use, the surface becomes brittle, crazes and loses adhesion with the metal. It then progressively flakes off, which isn't a good look.

Instead, I had them blasted and coated in a special heat-resistant paint that is baked on at over 400ºC. It forms a relatively thin coating that is very durable. They also looked very attractive with a strangely metallic, charcoal grey appearance with a very slight shine.

The Burgess air filter box was missing when the current owner bought the bike but he had acquired one, which he asked me to fit. Although it was an original Ariel filter box, it turned out that it was not from a Huntmaster, which meant that some custom bracketry was required to make it fit without modifying the part.

I hate permanently modifying original parts if I can avoid it and, in this case, it was possible to get a neat fit without doing so.

It turned out that the electrical wiring was in quite good condition and mostly complied with the original colour scheme, so tidying it up was a straightforward job. There was a bit of a jumble of assorted connectors and bits of odd-coloured wire in the region of the battery. All that was required was to replace some sections with wire of the correct colour, to eliminate any unnecessary or untidy connectors and to re-route some wires to make them more discrete.




Strange pitting on the faulty new gear.

When the new gears finally arrived, one of them had a strangely pale complexion, compared to the others, but otherwise looked sound. I re-assembled the cluster and completed the assembly of the transmission. I took the bike for a test ride with greater than average optimism. Misplaced optimism, as it turned out. The noise was still there. As loud as ever. Back to the workshop to look for the cause.

The two Oillite bushes on the layshaft were possible candidates and the smaller ball bearing on the kick-start end of the main-shaft felt a little rough, so I ordered new ones of all three. More interestingly, though, I noticed that one of the brand new gears looked strangely pitted and worn on the thrust side of all of the teeth. That was after only five miles of fairly gentle test riding. It looked fine when it went in. The other new gear was undamaged.

I contacted the supplier and sent photographs of the damage. He agreed to exchange it. It turned out that it had not been properly hardened during the manufacturing process. Judging by the extent of the damage after five miles of riding, it wouldn't have been very long before it failed completely, quite possibly damaging other components in the process or even jamming the gearbox – potentially very nasty. It was really quite fortuitous that the gearbox needed to be dismantled again so that the problem was detected in time.

After re-assembly of the transmission, a further test ride revealed no sign of any noise from the gearbox. It also revealed that it was a really nice bike to ride. It performed well and handled very nicely. The new front brake was pretty effective too.