Ohv SQ bottom end 1937-58

For gasket sets click here; individual gaskets click here; pistons click here; oil, oil pipes and unions click here; for ball bearings click here.

It is a good idea to have the big-end eyes of alloy con-rods checked for ovality and it is imperative that all the oil ways are cleaned out, especially the sludge traps in the crankshaft. When rebuilding the bottom end of a Square Four or other engine with a plain timing side bearing we strongly recommend that the bushes are fitted to the crankcases and then ‘line bored’. This will ensure that they are in perfect alignment with the drive side roller main bearings, resulting in a perfect bottom end. We consider this money well spent for peace of mind and saves many hours of scraping in bushes, which is frequently required otherwise. The crankshaft end float should be .003” to .004” on the front crank and .005” to .006” on the rear. Ariels only ever listed shims to increase the end float, not to decrease it; these may occasionally be found fitted outside the timing side main bush.

The later shock absorber assembly can be fitted to earlier models.

Prices of camshafts and followers are for exchange only, THIS MEANS THAT WE WANT YOUR OLD ONE FIRST. If the follower or lobes of the cam are badly worn there will be an extra charge to cover the cost of building them up, price per lobe is part number 1750-91 (SQ4 cam = 4 lobes) price per cam follower is 1750-94. This does not apply to Square Four followers (1810-37) which cannot be reconditioned except by building up, so their price already includes this. Additionally if the camshaft is worn where it runs in a bush or bearing there will be an extra charge for reclaiming it by industrial hard chrome plating it back to size, price is given by part number RES-14; this is included in the cost of the SQ4 cam 1960-37F. This guarantees that you get a good cam. The profiles we use are close but not necessarily exact copies of the originals. We can recondition any cams not listed.

Standard size of the big end crank journal is 1.375”; the timing side journal is 1.250”; con-rod big end eye diameter is 1.5002/1.5007”.

Got questions about valve timing? Read this excellent article

Timing case screws
Individual screws may be ordered but for convenience the contents of each set are listed. Sets with a ‘Q’ suffix are in stainless steel.

Big-end nuts and con rods

There are 3 types of con-rods  used during the life of these engines, the first type, used 1937-46, are nice and light but a bit on the flimsy side and the rods are lined with white metal rather than having loose shells. They can be re-metalled but it is expensive. The original nuts were locked using split pins, were cycle thread and the hexagon was the standard size. The last year of the iron engine, 1947, saw a new con-rod with loose shells and the thread on the big-end bolt was changed from cycle to BSF; the nuts were self locking with a standard hex size that makes them rather awkward to fit a socket or spanner, the addition of the later washer, 10418-54 will help a great deal. This version of con-rod has one rib on the cap and are usually referred to as ‘MKI rods’ but as they were used until 1954 and therefore on all 3 versions of the overhead valve SQ4, this is rather misleading. Similarly, their replacement that arrived in 1955 is usually referred to as MKII rods, which is also misleading. These rods, which have 2 ribs on the cap, are stronger but heavier than the earlier ones and reverted to cycle thread for the nuts. The hexagon was reduced to aid access and washers fitted under the nuts, locking by split pins again. If these rods are fitted to earlier engines the balance weights may have to be relieved slightly to clear. As the engines were never re-balanced to allow for the heavier rods the later engines are not as smooth as the earlier ones,

The original rods are up to the job but are rather old now and failures due to fatigue, previous abuse from poor assembly techniques or engine blow-ups are causing an increasing number of failures. However most rods break, not because they are weak, but because something must give when an engine blows up. It cannot be over-stressed that an engine that has not been in regular use with regular oil changes should be stripped and the sludge traps cleaned before being used. It may well run and do so for many gentle short trips but sooner or later the bottom end will seize with expensive results. When these engines seize it not just a crank grind and new shells that are required; even at low speeds, con-rods, crankcases and cylinders can all be wrecked, which explains the rarity of crankcases and cylinders for the MKII. There is a theory to explain the relative abundance of MKI engine castings, which is that owner’s became fed up with replacing head gaskets and abandoned the bikes long before they had a chance to blow up, a problem now cured with the advent of the Otto head gaskets.

The Ariel Owner’s Club have had new con-rods made and are available from us in sets of four only as they cannot be mixed with any of the original types. They have been CNC machined from special aluminium blanks and employ the latest practices in cap location with top quality ARP bolts. The design is so good that they are as strong as the late MKII rods but only slightly heavier than the earliest white-metalled ones, so now engines can be built knowing that the rods will not break due to fatigue, they will be as smooth as the original design and they will not stretch.

The SN-3 is a locking nut, not a standard nut, the type of locking may be different to that shown on the website, but it will still be a locking nut. The original castle nuts are more expensive to make as the hex has to be milled and then the slots added but are easy to get a spanner or socket on because the hex is smaller than standard. The SN-3 nut uses a standard size hex so is more difficult to use but the addition of the proper washer eases this problem. The website calls for 4 or 8 nuts as the quantity required depends on whether they are to be used it on a SQ4 (8) or a 500 or 650 twin (4).

Shock Absorber

The complete shock absorber assembly will fit all the earlier models. All models will pull a 26 or 27 tooth sprocket in solo form and the gearing on the later models can be raised further by fitting a 40 tooth clutch sprocket instead of the standard 44. Not recommended with 12 volt lighting using the standard dynamo as the engine will not turn fast enough to keep the battery charged, more info available on request, preferably be e-mail or visit the technical pages on our web site.

Crankcase and Fittings

Timing Gear

Oil Pumps

Plunger oil pump 1937-52. The bodies of the reconditioned oil pumps have new oversize plungers fitted after boring the pump body, and are assembled with new balls, springs and new sliding block,  the gasket surface is also refaced. When fitting it is important to ensure that there is clearance between the sliding block and the cam sprocket nut so that the pump can sit flat onto the joint face of the casing.

Gear type oil pump1953-58. When we re-condition these pumps the original ‘Mazac’ body is replaced with a CNC machined steel item and the internal surfaces are ground to restore the correct clearances between the gears and the end plates as well as re-facing the gasket surface. Any additional parts required will be charged extra but we cannot accept pumps with damaged drive spindles as these are not available. When fitting a pump adjust its’ position so that the drive spindle inside the pump meshes correctly with the drive gear, not so tight that it wears and not so slack that teeth are broken, a common problem. Do not omit the additional washer under the nose of the pump to keep the pump flat on the case and if a Duplex timing chain is fitted there should be a steel packing plate fitted under the pump with a gasket either side.

Morgo gear pumps.  These modern oil pumps have proved very popular replacements for both the plunger and gear type of pumps. Whilst the standard pumps work satisfactorily when in good condition there is no doubting that the Morgo pump is an improvement. In fact the one problem that owners have reported is that so much oil is pumped round the engine that excessive pressure is experienced both within the engine and the oil tank. At low speeds or short bursts at higher speeds this is not a problem but for longer distances at higher speeds a larger breather or breathers may be required. No modifications are required to fit the pump itself.