1906 Rover

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Modifying a Bolens Lawn Tractor

Monday, March 1st, 2010

This explains the ways I have overcome various problems I have experienced with my Bolens lawn tractors


I have two Bolens lawn tractors, both bought secondhand, both hydrostatic with 42 inch cutter decks.  The first I bought 27 years ago, it was an 11hp and had only had one season’s use.  Its Briggs and Stratton engine was underpowered for the mower, as the hydrostatic unit uses quite a lot of power, and it overheated.  Nonetheless, with regular engine overhauls, I got 10 year’s use from it before I decided that an engine replacement was necessary.  I bought a scrapped 14hp Bolens, a year or two younger than the 11 hp.  It had a good Techumseh engine.  It had been scrapped because somebody had messed up the hydrostatic unit and no one had known how to repair it.  I originally planned to make one good mower out of the two and keep the non-runner for spares, but managed to do all the repairs and modifications necessary to give me two usable machines.


Lawn Tractor Modifications

Problem 1: The side discharge decks start cracking after some years of use.

Cause: There is a torsional weakness because of the opening on the discharge side.

Warning signs: Cracking of the belt guard on the right hand side, also cracking around weld at the front mounting points.

Remedy: Welding the cracks only works for a short time, they then seem to fail at increasingly frequent intervals. Fitting tubing and some flat steel has stopped further cracking (15 years now).

I also had the deck hot-dip galvanised, which slows any corrosion. In order to allow sufficient room for the extra bracing to be welded in place one of the two parallel motion bars must be bent so that it is a mirror image of the other.


Problem 2: The large roller bushes wear very quickly.

Remedy: Fitting ball bearings to the roller support arms, pushing a piece of pipe right through each roller (one piece of pipe per roller, not all rollers on one pipe), and making the rod that they originally turned on into a long bolt by welding a piece of thread to each end. When this rod is tightened the rollers will turn as one unit.

I discovered an unexpected benefit when cutting longish wet grass. The ends used to build up with layers of grass but now they self-clean. This is due to the non-differential speeding up of the inside roller as the mower turns a corner, which unwraps the tangled grass.


Problem 3: The aluminium bearing housings on the deck corrode badly and break, the centre one being most prone to do this as it carries all of the drive strain.

Remedy: A simple housing was fabricated out of two steel pipes. This is not so prone to corrosion problems and is stronger.


Problem 4: Uneven cutting height from side to side especially when turning aggressively.

Remedy: On each side the holes in the frame were enlarged to 7/8 inch.  As shown in the diagram, two bushes were welded into place as sleeves.  Replaceable bushes were pressed in to place as shown.  Two more bushes, slightly longer to allow them to stand proud of the others, were then fitted.  A 1/2 inch bolt then attaches the deck in place of the original pin. This was very successful and I now achieve a very level cut.



Problem 5: Belt tensioning screw breaks because the threaded portion that it screws into is supported on one side only.  As wear takes place there is a greater lateral force exerted on the screw tending to bend and finally break it.  The problem was exacerbated for me when I fitted a 16 hp engine.

Remedy: I turned the pulley arm into a two-sided yoke


Problem 6(a): I bought my 14hp as a non runner, the Eaton hydrostatic pack having been dismantled and not put back together properly. I decided to swap my good axle and the hydro unit from my 11hp to the 14 hp, and discovered a problem in the process. The splined coupling between the flexible drive disc and the pump was very loose. The shaft looked unworn but the hub splines of the spider were almost gone. I checked the coupling from the 14hp and found that it too was badly worn and at some time a bodged attempt to repair by welding had been made. It came off with a little grinding.

Remedy: The hub is around 2 inches long and the spider is welded to one end. The hub is splined along its whole length, the spider can be cut off and welded onto the other end as the worn part of the splines run for less than half the length of the hub it will then be as good as new. However this welding must be done accurately, I would suggest no more than a 10 thou discrepancy, otherwise the spider and propshaft will be out of balance and damage the Eaton pump. I used a lathe to check the accuracy.

Luckily, I found a very small power-take-off shaft (called a lemon shaft because of its shape) at an agricultural engineers (in the UK). These PTOs are used on small tractors to allow sliding movement between an implement and the tractor. I cut off a six inch portion of the prop shaft and welded a six inch portion of the lemon shaft in its place, allowing telescopic movement. The splined hub was then fixed in place using No. 638 Loctite (the strongest retaining compound). It has not loosened again in 15 years.


Problem 6(b): Soon afterwards, when I stripped out the non-functioning Eaton pump from the 14hp, I discovered that the bush on the charge pump had become loose in the pump unit and further damage had been caused by end thrust exerted by the welding of the hub to the splined pump shaft. The aluminium housing is weakened by the proximity of two large holes to this bush which allows the bush hole to distort. The prop shaft being out of balance had exacerbated this problem causing the bush to become loose. The hub has to slide to alleviate any end thrust exerted by movement of the engine. This engine movement and any misalignment was the cause of the wear that made the coupling fail in the first place.

Remedy: It is no good just putting a bush into the damaged hole since it would not be properly aligned with the reciprocating ball rotor on the main drive pump. A small steel bush was pushed into each of the two holes near the bush hole to help support the aluminium. The whole area around the bush hole was peaned to reduce its size then the whole unit was put onto a lathe and the bush hole turned true. It is important to do this very accurately.  New bronze steel backed gudgeon pin bushes were fitted in to the steel charge pump and the repaired aluminium body. I also fitted a lemon shaft to this unit as described above. I put this on the 11hp which I now use mainly for tractor work, rarely for mowing. The pump has to work a lot harder now (it is amazing the amount of torque you can generate before the wheels start to spin compared to a geared version) and is still working well.

As this unit had been damaged prior to me buying the mower, I also had to make a new squash plate. The original squash plate had been damaged because the hydroclamp pistons had been roughened when being removed in the incorrect way. These pistons can be removed easily using air pressure when you now how. It is also vital when disassembling this sort of unit that everything is kept scrupulously clean. Any dirt will cause damage to the cast iron rotors of the pump and motor.


Problem 7: The trunion bush on the front axle can break loose.

Remedy: This was replaced with a larger bush in place. An old gudgeon pin was used as a pivot, eliminating the wear on both the bolt and the frame.


Problem 8: The head of the top hat shaped Oilite kingpin bushes wears off over time, allowing the bushes to migrate up the axle tube.

Remedy: Bushes of the same shape were not available. Instead double length bushes, minus the shoulder, were loctited in and washers were fitted to give a larger bearing surface for the stubs to turn on. This arrangement has proved long-lived.


Problem 9: The 11hp engine was underpowered as the hydrostatic unit needs more power than a gearbox. The engine always overheated and needed cylinder head gaskets every couple of years.  The overheating caused by this overloading plus the reflected heat from the silencer eventually made the cylinder block unrepairable.

Remedy (a): Parts were no longer available. I contacted Briggs UK and, as the problem had existed from new and as a goodwill gesture, they rebuilt the engine using parts from a bench tested engine from their archives. They suggested repositioning the silencer to help reduce the temperature of the engine. I also removed part of the bonnet sides to help cooling but retained some of the side to stop hot air recirculating round the engine.

Remedy (b): I bought my second Bolens, a scrapped machine because the hydrostatic unit was damaged, for the 14hp Techumseh engine. This engine did 10 years good service before oil consumption became excessive and the price of parts for a complete overhaul was going to be high.  When at Briggs UK with my 11 hp engine I had seen their V twin on test. Briggs UK were again very helpful, telling me that the V twin would probably fit in the place of my 11hp and sending technical details for the 16hp. I was also going to need a dual shaft kit, oil filter extension, fuel pump and silencer. This engine fitted into place with very little work. The old rubber engine mountings and brackets were thrown away and the engine was mounted on a plate that was welded directly to the front axle mounting. This was an expensive option but one that I think was well worth the money.  The engine is quiet, there is no vibration, it uses less petrol, and with the extra power it is possible to use less throttle with maximum forward speed.  A much more relaxing machine to drive.


Problem 10: When the V-twin engine was fitted and started for the first time I found that the engine could not be switched off. This was because when the engine was switched off the current was directed to earth through the starter solenoid. This is an odd idea that only works on the single cylinder engines. With two pulses generated almost simultaneously by the V-twin the electrical resistance in the solenoid isn’t low enough to decay the voltage.

Remedy: A solenoid relay switch was fitted to work through the live side of the ignition switch the stop wire on the engine was connected to the solenoid relay switch so that when the ignition switch was switched off the stop wire was earthed. I have also managed to retain the safety switch under the seat so that it still operates.

A note of caution: When repairing or altering any electrical part that is connected to an engine with electronic magneto ignition an accidental connection of a live voltage into the ignition wire will destroy the ignition instantly. This is a costly error!


Problem 11: Sometimes genuine bits are over-priced or not as reliable as an available alternative.

Instance 1: The early Briggs and Stratton rectifier, connected between the alternator and the ignition switch can fail and may damage the wiring harness and/or the alternator windings.

Remedy: I fitted a higher duty diode and, most importantly, a 10 amp in-line fuse. The fuse should protect the system should the diode fail. (There is a 20 amp heat-operated overload switch fitted on the Bolens, but this can fail to protect the system.)

Instance 2: Replacement blades are very expensive. (In the UK at any rate!)

Remedy: On my 42 inch deck I have used Mountfield blades for many years (M3195), these just need the holes enlarging slightly and about 1/8th inch cut off each end. Recently I have found another blade, Oregon 91-835, which is the same as the Mountfield.

Instance 3: If the Eaton pump bushes are worn they can be replaced using engine gudgeon pin bushes.


Problem 12: Having the two machines to compare meant that any advantages that one had over the other were very nolicable.  There is no substitute for horsepower, so in many ways the 14hp was best but I found that its hydraulic lift was not very controllable.

Remedy: I swapped the hand lift from the 11hp to the 14hp. Later I made a copy of the hand lift from a combine control and fitted it to the 11hp.


A list of useful second-hand alternative parts:

  • I fitted a Deluxe seat off a New Holland combine harvester
  • Several pulleys from the New Holland combine (or from other agricultural machinery) can be  substituted for Bolens parts. Some of these pulleys can have replacement bearings fitted, which extends their working life.
  • Some vehicle air-con units have electric clutches very like the ones fitted to the Bolens, the mounting parts can be made to fit with a little care.

2 Responses to “Modifying a Bolens Lawn Tractor”

  1. Hilario Bilsborough says:

    Hello I read a bit of your site, which I found by mistake while doing a bit of online research for some of my projects. Please write lots more as it is rare that somebody has something appealing to say about this. I will be watching for more!

  2. john h says:

    Hi and help please just read your article and impressed. I am having a problem in replacing the broken drive shaft on my Bolens 1669 hydro, don’t suggest Sonnys as they are a complet waste of time. Any suggestions would be welcome. Regards John H

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