Figure 2 provides a guide to the relationship and dimensions of the various ash parts of the frame. It may not be accurate enough to use when making parts, but it should be helpful when assessing the quantity and size of timber required during a rebuild. Accurate templates should be made from old parts of the frame being rebuilt.
Plywood Components of the Frame
Ash is not the only wood used in the construction of the frame. The inner wheel arch is made of marine plywood, which is a top quality, highly durable material. It has five or more layers of good quality hardwood, bonded together with waterproof adhesive. There are no voids, which are often found in ordinary plywood and weight for weight, it is stronger than steel. Plywood resists splitting, shrinking and warping due to the laminates being bonded together with their grain laying at different angles to each other. This also endows it with excellent resistance to shearing.
Other plywood panels used in the manufacture of the frame include a removable horizontal panel in the luggage space behind the seats. When lifted out, this panel allows access to the battery, rear axle, leaf springs and handbrake cable linkage. Not all Morgans have their batteries fitted behind the seats. Some have it fitted under the bonnet. A subvertical plywood panel behind the seats, enclosing the area below the inner rear frame, is also made of plywood. It is screwed, to the leading edge of the inner rear frame and battens fixed to the rear floor panel, as shown in Figure 3. The transmission tunnel is also fixed to this panel. All the floor panels are made of plywood. If they are replaced with marine ply and bedded onto the chassis with a flexible sealant, they should never rot. The sealant should also help protect the chassis.
Ash Frame Joints
The basic skills required to renovate the frame are straightforward. There are no dovetails or complex joints involved in its construction. Most joints are simple halving joints. Others include half lap, rabbet and butt joints. The scuttle top rail is joined to the scuttle posts by a splice joint as illustrated in Figure 4B. Care has to be taken when making this joint, because the biscuit must not be too thick or thin. If it is too thick, there is danger of splitting the wood and obviously, a thin biscuit will produce a weak unstable joint. The same care applies to making the tongue and groove joint locating the marine ply wheel arch into the rear sill board.Figure 4
Figure 5 shows the basic joints found on the frame. Making them requires care, because chiseling across the grain of the wood can result in damage if not done properly. The width and depth of the joint should be marked out first with a sharp pencil. It is good practice to draw shading on the timber to be removed. To make a halving joint use a mortise gauge to mark the joint out along its length. Joining timber of equal thickness requires the depth of the joint to be half the depth of the timber. Use a tenon saw to cut down to the depth of the joint, making sure to cut on the inside of each end. The saw can be used to make several vertical cuts down to the line marked out with the mortise gauge, but make sure to stop slightly short of the mark. A broad, sharp paring chisel should be used to remove the waste wood. Use the chisel diagonally and stop short before the chisel reaches the opposite side of the wood, otherwise the chisel will tear out the grain or split the wood. It takes patience to keep alternating the side from which you use the chisel, but you must make tight accurate joints. Any play in the joints will result in a wonky frame. The joints should tap together tightly.
The joint referred to as 'groove and board' is not really a technical woodworking term, but is used here to show the way the rear inner wheel arch engages the rear wheel arch sill. Normally a router or plow plane is used to cut a groove in a length of wood. It is possible to use a narrow chisel and long saw to produce an accurate groove, but it takes a little more patience and skill.
When assembling the frame the joints should be glued and screwed. Use a urea-formaldehyde glue like Aerolite 306. It takes quite a long time to reach full strength, but is really strong. It is used in boat building and light aircraft construction. Stainless steel screws should be used, because they are stronger than brass and mild steel screws corrode, even if they are plated. Never try to screw the joints together without drilling pilot holes, otherwise the wood may split split and the joints will not be pulled together adequately. The holes should also be countersunk so that the heads do not protrude. The screws should be put in tightly at a slight angle to prevent them from coming loose. It can also help to pull up the shoulders of joints.Figure 5
The average DIY store is unlikely to keep ash. American ash will most probably be difficult to source, because since 2002, the emerald ash borer(Agrilus planipennis) has infested huge areas in the USA, killing hundreds of millions of trees. It was accidentally imported into the USA from Asia. The ash borer was discovered in Russia during 2007 and is expected to spread across Europe in the future. However, at the present time, British and European ash should be quite easy to obtain from a reputable timber merchant, but it is important to purchase good quality timber.
Coach builders have traditionally favoured ash, because it is tough, flexible and has a good strength-to-weight ratio. When purchasing ash for a rebuild project, it should be seasoned or kiln dried. Whilst a tree is growing it transports water and nutrients from the roots to the canopy above by a network of hair like tubes in its trunk. This results in the trunk possessing a high moisture content. When wood is sawn from a freshly felled tree it is described as 'green' and it possesses a typical moisture content of 50 to 70%. As the cut timber planks dry out, they lose much of their water content, which can cause them to warp and split due to shrinkage. To minimise this risk, freshly sawn timber is seasoned by carefully stacking it in sheds, allowing dry air to circulate around it. This takes a long time so alternative methods are often used to dry timber. The most commonly used method is kiln drying, which has the advantage of being quick and controllable. Using these methods reduces the moisture content to between 15 and 20%, which vastly increases the stability of timber.Figure 6
The logs are cut in various ways as illustrated in Figure 6.
Rift sawn timber is the most stable, but produces the most waste when sawn from the log. It is therefore the most expensive, but rift sawn timber resists warping and splitting, which results in less waste of the wood purchased by the customer.
Quarter sawn timber may have small splits at the end of some planks, but is still very stable and probably easier to obtain than the rift sawn product. The quality should be good enough for coach building, although there will be a little more waste if some of the planks are split at the end.
Flat sawn timber (also known as plain sawn) should be avoided, particularly for sill boards, which are exposed to damp conditions on the underside of the car. As flat sawn timbers change humidity, they tend to warp and split. Timber tends to rot quicker along splits and end grain and if ash has one disadvantage over some other hardwoods, it has a tendency to rot in poor conditions.
Examine the timber before purchasing it and make sure the grain is tight, straight and reasonably free from knots. Tight, even grained wood, is not so likely to shrink and warp as open grained timber. Kiln dried timber is best stored in a sheltered environment which has a similar humidity to which the car will spend most of its life. A dehumidifier in the garage where a Morgan is stored, will not only help preserve the frame, but will slow down the rusting process on steel panels. It also helps to keep tools in good condition. A quality hygrometer will keep you informed of the humidity in the garage. Most dehumidifiers have a drainage pipe, which avoids the chore of keep emptying the water from the drainage tank. The water that drains from a humidifier also offers the bonus of providing water for a steam iron.