Jowett Javelin/Jupiter Engine Overhaul: Is it Worth It?

For those fortunate enough to own a Jowett Javelin or Jupiter, the question of engine overhaul is likely to arise. These cars, particularly their distinctive flat-four engines, are exceptional and represent a unique piece of automotive engineering. While some might be daunted by the prospect, especially after hearing ill-informed comments, overhauling a Jowett engine is most definitely a worthwhile undertaking. This guide aims to demystify the process, highlighting the idiosyncrasies of this fascinating powerplant and providing the knowledge needed to approach an overhaul with confidence.

Why Overhaul a Jowett Engine?

The Jowett Javelin and Jupiter engines are unique to these models, showcasing innovative design features for their time. Introduced in 1947, the Javelin, and later the Jupiter, featured an engine that was ambitious for Jowett Cars Limited (JCL). It was one of the first mass-produced horizontally opposed engines to utilise an aluminium crankcase with wet cylinder liners. The split crankcase design and die-cast aluminium construction were also pioneering steps for JCL. While this advanced design presented a steep learning curve for the manufacturer and early mechanics, it laid the foundation for a high-performance engine capable of excellent reviews.

The perception of unreliability that sometimes surrounds these cars is often attributed to the teething problems of this advanced technology and the lack of familiarity with aluminium construction. However, with over fifty years of accumulated knowledge and the availability of detailed technical notes, these challenges can be overcome. A properly overhauled Jowett engine can offer a long, reliable, and immensely satisfying ownership experience, rewarding the owner with its characteristic exhaust note and sparkling performance.

The Cost of an Engine Overhaul

Engine overhauls, in general, are expensive, and the Jowett engine is no exception. The total cost to fully overhaul and potentially modify a Jowett engine can range from $8,000 to $9,000. While this figure may seem high, especially when compared to the current market value of some Javelin or Jupiter models, it should be viewed as an investment. As more Jowett cars are restored to full working order and demonstrate their reliability, their values are likely to increase.

It's important to note that partial overhauls can be undertaken at a minimum cost, depending on the extent of work required. However, for those planning to undertake long-distance or higher-speed running, a more comprehensive and expensive overhaul is advisable. It's also worth remembering that the cost of specialised machining and parts for a Jowett engine is comparable to, if not less than, that for many other classic vehicles.

Key Machining Costs:

Understanding Jowett Engine Types

Jowett engines evolved throughout their production life, with engineering changes being 'blended-in' rather than introduced at specific model changeovers. This can make identifying specific engine types a little complex. Generally, there are two distinct versions: the first version and the later Series III engine.

Engine identification is primarily found on a raised plinth on the front face of the LHS crankcase half. This number indicates the year of manufacture, vehicle type, production series, and serial number. For example, 'E1 PC 12782' identifies a 1951 Jowett Javelin, third production series.

Key Identification Points:

• Plinth Number: Location on LHS crankcase front.
• Production Series: Indicated by letters (e.g., P for Javelin, S for Jupiter) and subsequent letters (A, B, C, etc.).
• Series III Crankcase: Often identified by a '3' stamped on the RHS top face near the rear timing cover, and more extensive cross-bracing in the main bearing support webs. They also feature larger oil feed galleries.

Pre-Dismantling Inspection

Before commencing any dismantling, a thorough inspection of the engine is crucial. This will help identify potential problem areas and guide the subsequent inspection process. Pay close attention to any oil leaks, as these can indicate worn seals, perished gaskets, or even cracks.

Common Leakage Points:

• Timing cover joints.
• Crankcase joint.
• Filter canister sealing ring.
• Oil filter drain bolt.
• Breather valve pipe.
• Dynamo mounting bracket.
• Tappet chest covers.
• Rocker covers.
• Front crankshaft pulley seal.
• Rear crankshaft spigot seal.
• Petrol pump gasket.
• Flywheel housing to crankcase gasket.
• Push rod tubes.
• Sump gasket.

Jowett engines typically operate with high oil pressure (50-75 psi), so ensuring all sealing surfaces are clean, flat, and properly assembled is paramount to achieving a leak-free engine. Thorough cleaning of the engine before dismantling is also essential to prevent dirt from entering critical internal components.

Essential Tools for the Job

Working on a Jowett engine requires specific tools, particularly due to its use of BSF (British Standard Fine) threads. Forcing incorrect thread sizes can cause significant damage.

Key Tool Requirements:

• BSF Spanners and Sockets: Essential for all fasteners. SAE and metric tools should not be used.
• Cylinder Head Pulling Tool: Crucial to prevent damage to crankcase surfaces when removing cylinder heads.
• Engine Stand: Highly recommended for ease of access and to save your back.
• Crankcase Stand: A simple wooden block can serve this purpose.
• Starter Ring Gear Tool: A modified section of a starter ring gear can lock the crankshaft.
• Piston Ring Clamp and Pullers: Standard engine rebuilding tools.

Dismantling the Jowett Engine

Careful dismantling is vital to avoid damaging components. Pay close attention to the location of crankcase tie-bolts and studs, as access can sometimes be restricted. When removing the flywheel, use the correct threaded holes for the pusher set screws.

Important Notes During Dismantling:

• Crankcase Tie-Bolts: Note their locations, especially the hidden centre upper bolt and the rear upper bolt requiring removal of the water inlet.
• Flywheel Removal: Use the two 3/8” BSW threaded holes for set screws. High tensile 3/8” UNC set screws can also be used.
• Bronze Gear Wheel: Exercise extreme care when removing this from the crankshaft nose to avoid breaking its teeth. The large radius in the bore should face the front main bearing journal.
• Resistance: If resistance is met during dismantling, do not force components. Thoroughly examine the situation to identify the cause of the binding.

The Crankcase Set: A Critical Component

The crankcase is arguably the most critical component of the Jowett engine. It's imperative that both halves of the crankcase set are a matched pair, identified by matching numbers stamped on their upper front faces.

Key Crankcase Considerations:

• Matching Numbers: Mismatched crankcase halves can lead to misalignment of joint faces and bearing bores, causing significant problems.
• Odd-Half Crankcases: JCL sometimes saved intact halves, identified by 'OS' (oversize) markings. While acceptable in period, the associated thicker main bearing shells are now difficult to source.
• Series III Crankcases: Identified by a '3' stamp and more robust cross-bracing. They also feature larger oil feed galleries.
• Main Bearing Tunnel Bores: Must be checked for distortion caused by over-tightening tie-bolts, oil starvation, or crankshaft breakage.
• Modifications: Early crankcases can be modified to Series III specification, including enlarging oil feed galleries.
• Thread Repairs: Stripped threads are common, particularly in the front timing cover. Thread inserts (Helicoil/Recoil) or custom bushings are effective repairs.
• Cylinder Liner Protrusion: Critical for sealing. A protrusion of 0.006" to 0.008" is generally recommended with modern shims, differing from the original manual's figure due to the absence of compressible gaskets.
• Cracks: Common areas include the rear face, tappet chest, and top joint flange. Most cracks are repairable by welding, but distortion must be managed.
• Gasket Faces: Must be perfectly flat and free from burrs.
• Main Bearing Supports: Must be absolutely flat and clean to prevent oil loss.
• Thrust Bearings: Separate thrust bearings at the rear main bearing are strongly recommended for improved control of crankshaft end float.
• Camshaft Bearings: Check for wear, particularly the centre bore. Tunnel boring and hard chrome plating or oversize camshafts are potential solutions.
• Internal Balance Pipe: Correctly installed seals are vital to prevent oil smoke. Ensure clearance with the crankshaft, especially with aftermarket items.

Crankshaft Health

The crankshaft is a critical component, and its condition directly impacts engine reliability. Crankshaft breakage was not uncommon in the early days of Jowett engines.

Crankshaft Identification and Checks:

• Flat Web vs. Oval Web: Flat web shafts are prone to cracking and are not recommended. Oval web shafts, identified by fully machined webs and 'Laystall' branding, are preferred.
• Cast Iron Shafts: Australian-manufactured cast iron shafts can resemble Laystall shafts but have a sand-grain finish.
• Laystall Shafts: Differentiate between pre-1959 (non-nitrided) and post-1959 (nitrided) shafts.
• Journal Radii: A 0.100" radius at the ends of main and big end journals is essential for crankshaft longevity.
• 'Ring' Test: A simple test to detect cracks by tapping the crankshaft with a hammer. A clear ringing tone indicates soundness.
• Crack Testing: All crankshafts must be crack tested before any work.
• Grinding: Can be ground to 0.050" undersize, but the rear spigot face for the flywheel must not be ground.
• Balancing: The crankshaft, flywheel, and clutch assembly should ideally be balanced as a group.

Bearings: Main and Big End

Jowett engines initially used white metal bearings, which proved troublesome. Copper-lead bearings were introduced, improving reliability but increasing crankshaft journal wear. Subsequent modifications addressed this wear.

Bearing Options:

• ACL Tri-Metal Bearings: A reliable modification available, requiring radial grooves to be machined into the crankcase bearing supports.
• Perkins Diesel Bearings: Big end shells from a Perkins diesel engine can be machined to fit Jowett main bearings.
• Big End Bearings: ACL 4B2641AL (Hillman Avenger) or ACL 2411AL (Perkins 4-99 diesel) are suitable alternatives. Connecting rod big end bores may need honing to suit.
• Connecting Rods: Prefer later, serrated cap mating surface rods. Ensure they are bead blasted, crack tested, have true bores, are straight, weight-equalised, and that big end cap bolts are renewed.

Cylinder Liners and Pistons

The 'wet' cylinder liners require precise installation to ensure proper sealing and prevent combustion gases from entering the coolant.

Cylinder Liner Installation:

• Protrusion: Aim for 0.006" to 0.008" protrusion above the crankcase surface using shim packs.
• Sealing: Use a light smear of Loctite Master Gasket 518 on the shim's contact face. Oiled crankcase bores are acceptable, but not at the seal area.
• Liner Thickness: Later liners have a thicker wall, making them more suitable for boring oversize.
• Boring: Liners must be bored oversize while clamped in a guide, simulating their installed position.

Pistons:

• Club Pistons: Lightened pistons to Jupiter specification are available, often with solid skirts.
• Non-Genuine Pistons: Can be used but may require modification and may not be ideal for the Jowett's longer stroke.
• Original Pistons: Had split skirts and were marked 'FRONT'. Must be installed with the 'FRONT' mark facing the front of the engine.

Camshaft, Timing Chain, and Covers

Several camshaft types were used, with later vernier timing arrangements offering greater accuracy. The timing chain should be of the endless riveted type.

Timing Components:

• Camshafts: Four distinct types exist, with differences in cam lobe profiles and timing arrangements.
• Timing Chain: Use a Renold chain, or one with a securely joined link.
• Front Timing Cover: Stripped threads for the sump attachment bolts are common due to over-tightening. Metric thread repairs are recommended.

Oil Pump and Filter Systems

Jowett engines underwent several oil pump revisions to improve flow and reliability. The later pumps with adjustable relief valves and improved suction are preferred.

Oil Pump and Filter Notes:

• Oil Pump Types: Later models with extended bodies and larger suction pipes are superior.
• Oil Pump Clearance: Ensure the strainer has adequate clearance from the sump bottom.
• Oil Filter Types: Early Vokes (felt element) and later Tecalemit (paper cartridge) systems were used. Modern spin-on conversions are available.
• Filter Canister: Check for cracks and ensure the sealing ring groove is properly machined.

Cylinder Heads and Valve Train

Specialist repairers can convert cylinder heads for unleaded petrol use. Attention to detail in valve train adjustment is crucial for smooth operation.

Cylinder Head Work:

• Unleaded Conversion: Hardened valve seat inserts and modified combustion chambers are common.
• Valve Guides: Re-boring to accept liners prevents valve sticking.
• Combustion Chamber Capacity: Must be equalised for balanced running.
• Rocker Cover Gaskets: 4mm 'O' ring cord provides an effective seal.
• Cylinder Head Torquing: A revised torque of 37 lb.ft (50 Nm) is recommended, with re-tightening after the engine reaches operating temperature.
• Valve Clearance: Adjust to zero lash when cold, ensuring the push rod can just be rotated by finger and thumb.

Rear Main Oil Seal and Clutch Housing

The rear main oil seal is a critical component for preventing oil leaks. Proper installation is paramount.

Key Points:

• Seal Installation: Must be at a perfect right angle to the crankshaft. Use a suitable mandrel.
• Clutch Housing: Early housings had stress risers at rectangular openings; later Series III housings have more robust teardrop shapes. Ensure alignment with the crankcase.

Flywheel and Gaskets

A snug fit of the flywheel onto the crankshaft spigot is essential. Flywheels with separate starter ring gears require careful attention to ensure they remain securely seated.

Flywheel and Gasket Notes:

• Flywheel Fit: If loose, the crankshaft spigot may need rebuilding or the flywheel bore enlarged.
• Starter Ring Gear: Later manufactured ring gears with smaller internal diameters offer greater clamping force. Welding or grub screws can also be used for retention, but require flywheel re-balancing.
• Gaskets: Use the correct thickness and material as specified. Avoid excessive sealant, which can block oil strainers. Ensure gasket edges that meet at right angles protrude correctly to seal effectively.

Conclusion: A Rewarding Endeavour

Overhauling a Jowett Javelin or Jupiter engine is a project that requires patience, skill, and attention to detail. However, the reward is an engine that is not only reliable but also a testament to Jowett's pioneering engineering spirit. By following the guidance provided in these notes and consulting the relevant maintenance manuals, owners can ensure their classic Jowett engine performs beautifully for many years to come.