Technical information

Cylinder barrel and vertical drive shaft

Re-fitting the cylinder barrel and the vertical drive shaft

With the ‘Bottom-end’ completed, the engine is ready for final assembly. Whilst it is possible to build the entire engine on the bench, it will require a stand to provide stability whilst working on it. It may be easier to install the crankcases in the frame and then continue the rebuild. A complete engine is not very easy to install.

Whichever route is taken, the piston and cylinder barrel are the next items to fit. In the case that a new piston is being used first check that there is .005" clearance at the bottom of the skirt. Less than this and the piston may be prone to seizure, more will result in clearly audible piston rattle. The piston rings need to be individually gapped at

Before fitting the piston, insert the wire circlip into the timing side of the piston and fit the piston rings. Warm the piston sufficiently to allow the gudgeon pin to become a sliding fit and push the gudgeon pin in until it protrudes very slightly from the inner face of the piston. The small end bush will have been lightly oiled before the piston is placed over the small end eye and the gudgeon pin engaged in the bush, with the piston in the correct orientation. Push the gudgeon pin fully home and fit the drive side wire circlip. When the chamfer on the end of the gudgeon pin floats into contact with a wire circlip whilst the engine is running, it makes the seating of the circlip more secure as it tries to expand the circlip into its groove. A light smear of oil is applied to the piston skirts and the piston rings in their grooves. The gaps of the rings should be rotated so that they are equi-distant around the piston.

The joint faces on the cylinder barrel and the crankcase mouth need to be clean and grease free. Paint a thin film of ‘Wellseal’ on the crankcase mouth and onto the cylinder barrel. Fit a paper gasket to the cylinder barrel and smear the bore with clean oil.

With the piston suitably supported and using a viable method used to compress the piston rings, the cylinder barrel can be placed over the piston and the skirt of the barrel engaged into the crankcase mouth. Push the cylinder barrel firmly home onto the joint faces.

The vertical clearances on the Oldham couplings in the vertical drive shaft now need to be considered, even in the case of rebuilding an engine that has been running. Remove the drive shaft from the outer cover and check the fit of Oldham couplings in their respective slots. They should be a sliding fit with no perceptible clearance. Check them also in the slots of their respective vertical bevels. It is not uncommon for wear to have taken place and the couplings may need replacement to eliminate clearance. If new couplings do not eliminate clearance, oversize ones are available that can be stoned down to fit the shaft and bevels (see spares page).

Having achieved satisfactory fits between the Oldham couplings and the vertical drive shaft, the vertical clearances need checking.

Insert a cylinder head gasket into recess on the cylinder head and place the head onto the spigot of the cylinder barrel. Fit the vertical drive shaft in place between the upper and lower vertical bevels and lightly screw two of the four cylinder head fixings down. (diagonals)

Use a small screwdriver to force all of the vertical clearances into one place and measure that gap between the Oldham coupling and the drive shaft with a feeler gauge. The clearance should be a minimum of .012" and a maximum of .025" is reasonable.

Oldham couplings are made in two thicknesses - .125" and .187" and it is usual for one of each to be present. If the desired clearance cannot be achieved using any combination of the two, it is possible to use two thick ones and turn material from one or both ends of the vertical driveshaft. The shaft is tough but can be turned with a tipped tool at low speed. Care must be taken not to reduce the length of the slot so much that the Oldham coupling stands proud from the face of the vertical shaft.

Once satisfactory clearance has been achieved, the head should be removed and the vertical drive shaft cover fitted. Veloce originally specified asbestos string for packing the glands but it is not now generally available. There seems to be no good reason for using asbestos except perhaps for its soft texture. The engine certainly does not get hot enough here to require anything more than ordinary soft parcel string. Four to five wraps are adequate, depending upon string thickness.

The lower gland nut is returned by a circlip. I have found it useful to soak the string in Wellseal and to liberally coat the outside of the vertical driveshaft cover in Wellseal. Wind the string round the cover so that the action of the tightening the gland nut will not unwind the wrappings. The Wellseal, besides providing improved sealing also assists by keeping the string in place during assembly. Assemble the lower Oldham coupling onto the driveshaft and insert into the cover. Locate the cover into the lower bevel housing and screw the gland nut fairly well home but do not tighten at this stage. It may be necessary to adjust the vertical position of the cover later. Repeat the process of the Wellseal and string at the upper end, having first dropped the gland nut into place.

The crankshaft must now be rotated until the slot in the vertical drive shaft is in line with the crankshaft and the piston is at TDC. This may require several rotations. Fit the upper Oldham coupling into the vertical drive shaft - the uppermost tongue will face ‘fore – and aft’ with the engine.

With new copper ring head gasket or the old one annealed, it is now appropriate to approach the final fitting of the cylinder head. This procedure is described in a separate article.