Main Bearing Repair – Many otherwise good plain bearing engines are handicapped by worn-out main bearings. If your previously smooth running engine starts to vibrate excessively the main bearing may need replacement.  Once wear becomes apparent it seems to progress more rapidly and eventually can be heard as sort of a rattling noise when running the engine.  If ignored the excessive vibration will rob the engine of power and can result in damage to both the engine and the airframe. Most mid size engines are set-up with will a 0.002 to 0.003 diametrical clearance between the crankshaft and the sleeve bearing.  At somewhere around 0.006 total clearance the bearing should be replaced.  If you are measuring the wear it will be more pronounced in the vertical than in the horizontal direction and more severe at the rear of the bearing than at the front.

Procedure is to turn-out the original bearing and press and/or epoxy a custom turned replacement bearing in place. After installing the replacement, the intake window is machined in place.  For engines with a window that does not lend itself to a plunge cut on the mill the bearing is slid into place from the rear and the window scribed on the bearing.  The window can then be accurately cut separate from the case.  The bearing is then replaced from the rear using a structural epoxy adhesive.  A small flange machined on the rear of the bearing absorbs any thrust loads. SAE 660 bronze material is used for all replacements.

Rod Repair/Replacement – Custom connecting rods can be made to fit most any model airplane engine using high strength aluminum alloy (generally 7075).  Bronze bushings can be inserted into new rods or replaced on worn-out lower ends.  This is a very time consuming effort and should only be considered if you cannot find a replacement rod from a parts supplier.  Shown is a rod that as made for a Veco 29 which replaced a rod cracked along the vertical axis of the rod probably due to an oversize bushing. 

Primary engine repairs include replacement of main bearings, replacement of rod lower end bearings, repair of stripped engine screw holes, custom rods, and piston refurbishment.

Piston Repairs - Sometimes cast iron pistons with cracks or missing chunks can be repaired by cutting away the damaged area of the piston.  This is possible if the damage is on the bypass port side of the piston and lower on the skirt.  Shown is a piston from a Johnson 35CS that was damaged when the lower end of the rod failed.  Cutting out the damaged portion not only provided a useable repair but served to lighten the piston and possibly improved bypass bypass flow.

Cast iron pistons that have been worn to the extent that the compression seal is lost can sometimes be restored by a high temperature heat treatment to relieve residual machining sress and with the result of very slightly growing the piston.  By treating the piston at approximately 1000 deg F a typical 29/35 cast iron piston will grow from 0.0003 as much as 0.0015 inches in diameter.  The piston must then be lapped back to proper size in with an external OD lap.

Fastener Replacement and Repairs - Stripped fastener holes can be repaired either by drilling out the stripped hole to the next fastener size and tapping or by installing a threaded insert.  Drilling out to the next size is recommended for some NOS engines that use fastener sizes which cannot be obtained.  An example is the OS Max 15/19 that uses an obsolete Japanese thread specification for which there is no direct metric or SAE replacement.  In this case it is best to tap all the holes and install 4-40 socket heads.  For the OS 35 III 5-40 screws can be used as a direct replacement.

Note that if the piston has been previously heat treated there may be no appreciable gain from this procedure.

For stripped SAE holes one can use the next size larger screw where cost is a concern and where appearance is not.  By far the best repair for most stripped holes is installing a Helicoil insert.  This provides a repair that is far stronger than the original installation and generally impossible to detect without engine disassembly.  The Helicoil is used extensively on commercial and military aircraft where threaded fasteners are used in aluminum structure.