OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Victa Two Stroke Engines Power Torque conrod slop?

Howdy all!

I'm in the process of assessing the required bits for a power torque rebuild, but I've got one question which doesn't seem to be answered in the manual.

How much freeplay can you have in the conrod bearings? The conrod in the donk I have doesn't have any slop in the thrust direction (ie. inline with the piston movement in the cylinder) but has a fair amount of play if I rock the conrod on both the gudgeon pin and crankpin.

Any thoughts?


Cheers, GTG!

I have never really looked into it myself, if it feels "too loose" to me I just replace the conrod with one that "feels better"

I would dare say though that the limit would be around the 10 thou mark at a guess, measuring it could be a pain though since they are roller bearings.

Journal bearings (that is, plain bearings) to run at any speed have to rely on hydrodynamic lubrication. If there is much clearance there has to be either pressure delivery of oil to the center of the journal, or very low bearing pressures. Chevrolet achieved successful splash-lubrication of crankshaft journal bearings in six cylinder car engines as late as 1952, but to do it the bearings had to be lightly loaded and running at low engine speeds. They also used a dipper to deliver oil to the center of the bearing, at a slight pressure. There was an oil pump on most dipper bearing engines, delivering a jet of oil at 5 psi pressure, aimed at the dipper. (This is known as spit-and-hope lubrication.) The engines also had rather wide bearings by modern standards. Up to an occasional maximum speed of 3,500 or 4,000 rpm with a 6:1 compression ratio, the system was actually rather satisfactory, lasting over 100,000 miles. The side valve Briggs engines did not even have a dipper, and still lasted a while, though not by automotive standards of course. One absolute necessity, though, was to achieve hydrodynamic oil pressure inside the journal bearing, caused by the rotation of the inner in relation to the outer creating a wedge of oil. If there was any significant slack in the bearing, it was not possible to maintain this wedge of oil and the bearing failed. This is the usual fate of Briggs engines run with little or no oil: the wedge fails, the bearing fails, and the rod fails when the big end seizes. A proper hydrodynamic bearing does not have any metal to metal contact, even momentarily, except during start-up.

Ball or roller big end bearings do not need much lubrication, and there is no real hydrodynamic wedge of oil in them. They rely on rolling contact, not a wedge to support the rod without contact. Hence they can operate with some clearance. The main limit on that clearance is when the slopping up and down of the rod across the clearance actually causes Brinelling of the rod and the crankshaft journal, followed by metal being pared off and jamming up the rolling action. That is why the slack test is still relevant with rolling contact bearings, but where a hydrodynamic bearing needs a maximum clearance of only about 0.001" or so per inch of journal diameter, a rolling contact bearing can live with several thousandths. If it weren't for the load continually reversing in direction plus experiencing the "hammer blow" each time combustion occurs, rolling contact bearings could be run quite loose. Most of us have seen ballraces that have been run in bench machines, with steady unidirectional loads, until the cages wore away or the balls escaped. Unfortunately this won't work in big end bearings.

Joe, 0.010" sounds a bit too much clearance to me, but it depends on the diameter of the bearing. A bigger bearing can stand more slack.

Its a 2 stroke we are talking about here, ANyhow I go on feel, and just guessed what measurment I dump them at. I really suggest though if they can be rocked side by sideby a visible amount they get tossed.

I realise it's a 2 stroke Joe. The difference that makes is that the lubrication isn't really good enough to get enough oil into the big end at high speed for a journal bearing to work well, so the antifriction bearings are a good idea. On the positive side, the crankcase induction keeps the inside of the crankcase much cleaner than on a splash-lubricated 4 stroke, and that makes the antifriction bearings quite durable. Many 4 stroke motorcycles have fairly successfully used antifriction big end bearings, but they have dry sump pressure lubrication and a full flow oil filter, which keeps things fairly clean in there.

If its the only one youve got whats the worst that could happen???


It will blow up and destroy itself and ruin $30 worth of new crank bearings.


Bob.