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

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.

Part No.	Description	Per	Model	Years
0599	Balance weight bolt and nut	2	SQ 4	1937-58
1062-37U	Crankshaft, front (used), various conditions	1	SQ 4	1937-58
1063-37U	Crankshaft, rear (used), various conditions	1	SQ 4	1937-58
1090-31	Crankshaft nut except rear left	3	SQ 4	1937-58
1111-04	Con-rod – See notes above.	4	SQ4	1937-58
1111-37F	Con-rod, exchange	4	SQ4	1937-58
1131-37	Small end bush	4	SQ 4	1937-58
1162-37	Coupling gear fibre insert and 4 rivets	2	SQ 4	1937-58
1180-37	Oil seal washer	3	SQ 4	1937-58
1180-49	Crankshaft oil seal	1	SQ 4	1937-58
1185-37	Coupling gear key	2	SQ 4	1937-58
1190-37	Crankshaft sludge trap plug	4	SQ 4	1937-58
10418-54	Big end nut washer, gives a better chance of fitting a socket	8	MK II	1953-58
10419-54	Big end nut, self locking type, cycle thread, not originally used 1947-54, see SN-3	8	SQ 4	1937-58
10419-54A	Big end nut, castle type (uses S17-1 split pin), cycle thread, not originally used 1947-54, see SN-3	8	SQ 4	1937-58
K115-48/2	Shim, increases end float, fits behind 1180-37 washer .002	AR	SQ 4	1937-58
K115-48/4	Shim, increases end float, fits behind 1180-37 washer .004	AR	SQ 4	1937-58
K115-48/6	Shim, increases end float, fits behind 1180-37 washer .006	AR	SQ 4	1937-58
K138-48	Big end bolt, cycle thread (use with 10419-54 nut)	8	SQ 4	1947-58
K140-47/std	Big end shells (2 pairs) STD	2	SQ 4	1947-58
K140-47/10	Big end shells (2 pairs) -.010″	2	SQ 4	1947-58
K140-47/20	Big end shells (2 pairs) -.020″	2	SQ 4	1947-58
K140-47/30	Big end shells (2 pairs) -.030″	2	SQ 4	1947-58
K140-47/40	Big end shells (2 pairs) -.040″	2	SQ 4	1947-58
K147-53	Oil seal washer	1	SQ4	1953-58
S17-1	Split pin, big end nut	8	4G	1937-47
SN-3	Big end bolt nut BSF, see 10419-54 for cycle thread versions	4/8	SQ 4	1947-54

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.

Part No.	Description	Per	Model	Years
1180-26/104	Hardened washer	1	MK I	1949-52
1260-37	Spring, shock absorber	1	4G	1937-48
1260-50	Spring, shock absorber	1	MK I/II	1949-54
1272-48	Sliding member	1	MK I/II	1949-58
1272-49	Shock absorber kit	1	4G
1284-49	Shock absorber sleeve	1	MK I/II	1949-58
1290-26	Spring retaining nut	2	4G	1937-48
1294-49	Spring retaining castle nut	1	MK I/II	1949-58
1294-18	Securing washer, sleeve nut	1	MKI/I	1949-58
1299-28	Spring retaining nut lockwasher for 1290-26	1	4G	1937-48
1305-48	Spring retaining plate	1	MK I	1952
1315-53	Spring retaining plate	1	MK II	1953-58
1338-49B	Engine Sprocket, 21 Teeth, For when you want to climb a mountain	1	MK I/II	1949-53
1338-49	Engine Sprocket, 22 Teeth, For when you want to climb a steep hill	1	MK I/II	1949-53
1341-49	Engine sprocket, 25 teeth	1	MK I/II	1949-53
1342-53	Engine sprocket, 26 teeth	1	MK I/II	1949-53
1343-53	Engine sprocket, 27 teeth	1	MK I/II	1949-53
10474-54	Spring, shock absorber	1	MK II	1955-58
K119-48	Sliding member	1	MK I/II	1949-58
K168-48	Spring retaining plate	1	MK I	1949-52
S17-3	Spring retaining nut split pin	1	MK I/II	1949-58

Crankcase and Fittings

Part No.	Description	Per	Model	Years
045	Coupling gear cover countersunk screw	2	SQ 4	1937-58
0133	Coupling gear cover stud, long	2	SQ 4	1937-58
0134	Coupling gear cover stud, short	5	SQ 4	1937-58
0138	Oil pump stud, rear fixing	2	MK II	1953-58
0139	Oil pump stud, front fixing	1	MK II	1953-58
0184	Oil pipe adapter stud	1	SQ 4	1937-58
0191	Cylinder base stud	8	4G	1937-58
0191Q	Cylinder base stud, stainless	8	4G	1937-58
0191	Cylinder base stud	12	MK I/II	1937-58
0191Q	Cylinder base stud, stainless	12	MK I/II	1937-58
0202	Oil pump mounting stud	2	4G/MK I	1937-52
0203	Timing chain tensioner adjuster stud	1	MK I/II	1948-55
0511	Bolt, coupling gear cover to rear primary case, lower and 2 middle	3	SQ4	1937-58
0517	Bolt for oil pick up pipe in sump	1	4G/MK I/II	1937-58
0520	Sump plate bolt	4	MK I/II	1949-58
0520Q	Sump plate bolt, stainless	4	MK I/II	1949-58
0538AQ	Bolt, coupling gear cover to rear primary case, upper	1	4G/MK I/II	1937-58
0543	Crankcase tie bolt, lower, ¼ x 1 ¾	4	MK I/II	1949-58
0543Q	Crankcase tie bolt, lower, ¼ x 1 ¾, stainless	4	MK I/II	1949-58
0594	Bolt, rear fixing Magdyno rod strap	1	4G	1937-48
0604	Crankcase tie bolt, lower 5/16 X 1 7/8	3	4G	1937-48
0604Q	Crankcase tie bolt, lower 5/16 X 1 7/8, stainless	3	4G	1937-48
0604	Crankcase tie bolt, inside crankcase mouth 5/16 X 1 7/8	2	4G/MK I/II	1937-58
0604Q	Crankcase tie bolt, inside crankcase mouth 5/16 X 1 7/8 SS	2	4G/MK I/II	1937-58
0619	Crankcase tie bolt, upper rear 5/16 x 2 3/16	1	4G/MK I/II	1937-58
0619Q	Crankcase tie bolt, upper rear 5/16 x 2 3/16, stainless	1	4G/MK I/II	1937-58
0620	Crankcase tie bolt, middle rear, 5/16 x 1 ½	1	4G/MK I/II	1937-58
0620Q	Crankcase tie bolt, middle rear, 5/16 x 1 ½ SS	1	4G/MK I/II	1937-58
1018-49	Oil pipe adapter sleeve nut	1	MK I/II	1949-58
1018-49Q	Oil pipe adapter sleeve nut, stainless	1	MK I/II	1949-58
1403-37U	Crankcases (previously owned) state year	1	4G/MK I	1937-52
1410-37U	Coupling gear cover (used)	1	4G/MK I/II	1937-58
1412-37	Gasket, coupling gear cover to crankcase	1	4G/MK I/II	1937-58
1414-37	Inspection cap, front main bearing	1	4G/MK I/II	1937-58
1416-37	Sealing washer, front main bearing inspection cap	1	4G/MK I/II	1937-58
1431-37	Main bearing, un-lipped – Drive side	2	4G/MK I/II	1937-58
1431-37A	Main bearing, lipped – Drive side, unbranded. For branded see 1431-37AB	2	4G/MK I/II	1937-58
1432-37	Main bearing – Gear cover (use 1431-37 and Loctite)	1	4G/MK I/II	1937-58
The un-lipped main bearings are considerably cheaper than the original lipped main bearings. The lip is not designed to take side loading so it cannot serve any purpose and in any case the sideways movement of the crankshaft is controlled by the timing side bush and thrust washers. Using the un-lipped type does mean that the end float can no longer be reduced by positioning shims behind the inner race, which is not a good idea anyway.
1450-37	Timing side bush supplied rough machined for line boring after fitting see notes at the start of this section. (+.010” bushes also available for worn cases). There is enough white metal lining to bore to standard size!	2	4G/MK I/II	1937-55
1450-56	Timing side bush, with large lip, as above	2	MK II	1956-58
1460-49	Camshaft bush	1	4G/MK I/II	1937-58
1463-31	Camshaft bearing, unbranded. For branded see 1463-31B	1	4G/MK I/II	1937-58
1464-37	Camshaft bearing circlip	1	4G/MK I/II	1937-58
1521-49	Sump gauze	1	MK I/II	1949-58
1521-52	Sump plate, aluminium type	1	MK I/II	1949-58
1524-49	Spring for sump filter	1	MK I/II	1949-58
1526-37	Sump filter inspection cap	1	4G	1937-48
1528-48	Gasket, sump plate to crankcase	1	MK I/II	1949-58
1533-37	Oil return pipe	1	SQ4	1937-48
1533-49U	Scavenge pipe	1	SQ4	1949-52
1533-53	Oil pipe from sump	1	SQ4	1953-58
1536-37	Clip for return pipe from sump	1	4G/MK I/II	1937-58
1544-31	Dowel, between crankcases halves &  cylinder	4	4G/MK I/II	1937-58
1545-37	Dowel, coupling gear cover	2	4G/MK I/II	1937-58
1548-37	Crankcase joining bolt lock washer	2	4H/Mk I/II	1937-58
2046-37	Gasket, timing case	1	4G	1937-48
2046-49	Gasket, timing case	1	MK I/II	1949-58
2096-53	Fibre washer, case screw (listed from 1953 but will fit earlier)	11	4G/MK I/II	1937-58
2101-37	Fibre washer, breather pipe to timing case	2	4G/MK I/II	1937-58
2107-37	Breather banjo bolt in timing case	1	4G/MK I/II	1937-55
2107-56	Breather banjo bolt in timing case	1	MK II	1956-58
2151-37	Oil pressure relief valve body	1	4G	1937-48
2151-37A	Oil pressure relief valve assembly	1	4G	1937-48
2151-50	Oil pressure relief valve body, in crankshaft	1	MK I	1949-52
2151-50A	Oil pressure valve body assembly	1	MK I	1949-52
2154-37	Oil pressure regulator spring	1	4G	1937-48
2154A-50	Oil pressure relief valve spring	1	MK I	1949-52
2156-37	Oil pressure regulator adjusting screw	1	4G	1937-48
S9-1	Oil pressure relief valve ball in crankshaft	1	MK I	1949-52
S17-1	Split pin, oil pressure regulator	1	MK I	1949-52
2223-56A	Spacer between oil pump and case, duplex timing chain	1	MK II	1956-58
2247-57	Oil pressure regulator shroud, duplex chain	1	MK II	1957-58
2352-49	Dynamo strap pin	2	MK I/II	1949-58
2354-30	Dynamo strap swivel pin, plain	1	MK I/II	1949-58
2354-31	Dynamo strap swivel pin, tapped	1	MK I/II	1949-58
2355-37	Magdyno strap swivel nut, stainless	1	4G	1937-48
2356-49Q	Dynamo strap, stainless	2	MK I/II	1949-58
2357-37	Magdyno strap rod, stainless	1	4G	1937-48
2359-37	Anchor pin for dynamo strap	1	4G	1937-48
2359-49	Dynamo strap bolt	1	MK I/II	1949-58
2359-49Q	Dynamo strap bolt, stainless	1	MK I/II	1949-58
2363-31	Magdyno strap trunnion, stainless	1	4G	1937-48
4319-29	Lockwasher, Crankcase halves	2	All	1937-58
5178-37	Rocker oil feed stud 3/8″	1	4G/MK I/II	1937-58
5183-37	Rocker oil feed pipe dome nut	1	4G/MK I/II	1937-58
5183-37Q	Rocker oil feed pipe dome nut, stainless	1	4G/MK I/II	1937-58
5184-31	Fibre washer, rocker oil feed pipe union	2	4G/MK I/II	1937-58
5863-37	Oil pipe adapter with pipes	1	4G	1937-48
5863-49	Oil pipe adapter with pipes	1	MKI	1949-52
5863-53	Oil pipe adapter with pipes	1	MKII	1953-55
5863-56	Oil pipe adapter with pipes, for use with large oil tank	1	MKII	1956-58
5866-37	Gasket, oil pipe adapter to crankcase	1	4G/MK I/II	1937-58
S17-1	Oil pressure relief valve split pin in crankshaft	1	MK I	1949-52

Timing Gear

Part No.	Description	Per	Model	Years
1924-49	Dynamo sprocket, simplex chain	1	MK I/II	1949-55
1924-57	Dynamo sprocket, duplex chain	1	MK II	1956-58
1932-26	Magneto sprocket nut	1	4G	1939-47
1944-57	Half time crankshaft sprocket, duplex chain	1	MK II	1956-58
1958-37	Timing chain, simplex	1	4G/MK I/II	1937-56
1958-57	Timing chain, duplex	1	MK II	1957-58
1958-07	Kit to convert from simplex timing chain to duplex, giving a faster dynamo and better charging especially at low speeds. Not all parts are available (breather pipe) please enquire		MK I/II	1949-55
1960-37	Camshaft, ground to 1953 profile, new	1	4G/MK I/II	1937-58
1960-37F	Camshaft, ground to 1953 profile (EXCHANGE)	1	4G/MK I/II	1937-58
1960-53	Camshaft, new	1	4G/MK I/II	1937-58
1960-53F	Camshaft, exchange HCPG	1	4G/MK I/II	1937-58
1964-37	Camshaft sprocket  simplex	1	4G/MK I/II	1937-56
1964-57	Camshaft sprocket, duplex chain	1	MK II	1956-58
1972-37	Camshaft sprocket key	1	4G/MK I/II	1937-58
1976-37	Camshaft pinion nut with oil pump drive gear	1	MK I	1937-53
1976-53	Camshaft pinion nut with oil pump drive gear	1	MK II	1953-58
2016-37-600	Timing cover 600	1	MK I	1937-49
2016-37U	Tming cover, used	1	MK I	1937-49
2016-49U	Tming cover, used	1	MK I	1949-53
2016-53	Timing cover	1	MK II	1953-56
2016-53U	Timing cover, used	1	MK II	1953-56
2016-56	Cover for timing case	1	MK II	1956-58
2016-56U	Cover for timing case	1	MK II	1956-58
2110-31	Tensioner blade, timing chain	1	4G/MK I/II	1937-55
2110-56	Tensioner blade, timing chain	1	MKII	1956-58
2114-37	Tensioner blade spring	1	4G/MK I/II	1937-55
2118-31	Tensioner blade, pin	2	4G/MK I/II	1937-55
2120-31	Timing chain fibre rubbing strip	1	4G	1937-48
2122-31	Timing chain fibre rubbing strip rivet	3	4G	1937-48
180396	Dynamo sprocket nut	1	MK I/II	1950-58
S18-1	Key, half time pinion	1	4G/MK I/II	1937-58
SN-3	Tensioner blade stop nut	1	4G/MK I/II	1937-55

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.

	Set No.	03 (11/16″)	040A (1 ½”)
1937-48	CS06	11
1937-48	CS06Q	11
1949-58	CS07	9	2
1949-58	CS07Q	9	2

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.

Part No.	Description	Per	Model	Years
047	Screw, through oil pump	4	MK II	1953-58
0138	Stud, rear fixing oil pump to crankcase	2	MK II	1953-58
0139	Stud, front fixing to crankcase	1	MK II	1953-58
0202	Oil pump mounting stud	2	4G/MK I	1937-52
2210-53F	Oil pump, exchange reconditioned	1	MK II	1953-58
2212-37F	Oil pump, exchange reconditioned	1	4G/MK I	1937-52
2212-97	Morgo oil pump, recommended, ask for brochure	1	4G/MK I	1937-52
2212-98	Morgo oil pump, simplex timing chain, ask for brochure	1	MK II	1953-55
2212-99	Morgo oil pump, duplex timing chain, ask for brochure	1	MK II	1956-58
2218-37	Drive block, Dural	1	4G/MK I	1937-52
2219-29	Plunger valve plug on bottom of pump	2	4G/MK I	1929-58
2220-29	Plunger valve spring	2	4G/MK I	1929-58
2158-53	Oil pressure regulator shroud	1	MK II	1953-58
2212-53	Oil pump body only	1	MK II	1953-58
2223-56A	Spacer between oil pump and case, duplex timing chain	1	MK II	1956-58
2226-53	Nose casting for drive spindle	1	MK II	1953-58
2227-53	End plate for oil pump	1	MK II	1953-58
2228-53	Oil pump spindle	1	MK II	1953-58
2241-53	Oil pressure relief valve piston	1	MK II	1953-58
2242-53	Oil pressure relief valve spring	1	MK II	1953-58
2243-53	Oil pressure relief valve spring cup	1	MK II	1953-58
2245-53	Worm drive spindle thrust washer (FH 10611-54)	1	MK II	1953-58
2246-53	Worm drive spindle circlip (FH 10612-54)	1	MK II	1953-58
2247-57	Oil pressure regulator shroud, duplex chain	1	MK II	1957-58
S2-3	Retaining nut	2	4G/MK I	1937-52
S5-3	Shakeproof washer	2	4G/MK I	1937-52
S9-2	Plunger valve ball bearing	2	4G/MK I	1937-52
S17-3	Oil pressure relief valve split pin	1	MK II	1953-58
SP1/4	Retaining nut (Nyloc)	3	MK II	1953-58