jueves, octubre 09, 2014

Loose Needles

Many of the Montesa models of the late 1960’s and early 1970’s didn’t use caged needle bearings on the wrist pin in the small end of the connecting rod.  When removing the piston there is a shower of loose needles onto the bench, top of the engine case and into the crankcase.  This article is to explain a way in dealing with the removal and assembly of this unusual bearing solution by the engineers of Montesa.

Early 175 and 250 models used an oillite bronze bushing for connection of the connecting rod to the piston pin.  When this is subjected to severe service it doesn’t carry the load well.  When the engines were produced in the mid ‘60’s they were equipped with a caged needle bearing.  This needle bearing was weak because of the sparse number of needles.  The bearing needed replacement more often.  The 20:1 oil ratio wasn’t adequate with this bearing.  LaCross’s beginning with serial number 23M0231, Scorpion’s at 24M0401, and 250 Impala’s at 25M0201 were fitted with 28 uncaged needle bearings as a replacement for the earlier bushings and caged needle bearings in order to support the increased load.  They are a direct replacement for the earlier models.  The loose needles continued to be used into the early ‘70’s in all Cappra’s up to the VR and the 247 Cota’s to serial number 21M13106.  They were used in all 123 Cota’s (only 26 needles) and all Cappra 360 GP’s and DS (30 needles) models as well.

You will need at least one special tool to remove the piston and help in its reassembly.  Go find an electric paint remover.  It looks much like a hair drier but is typically adjustable for 750 degrees or 1,000 degrees.  Some go higher.  They are cheap.  I found mine for under $15.00 at Sears.  This is a much better heat source than a propane torch.  It doesn’t concentrate the heat in such a small area and is much faster.

Piston Removal

  1. Place the engine over a cleared bench or table.  You might want to place a few sheets of newspaper down under the engine.
  2. Remove the head and cylinder.
  3. Pull the piston up and stuff rags or paper towels around the crankcase opening.
  4. Remove one wrist pin-retaining clip.   Note:  The wrist pin is a very snug fit.  You don’t want to hammer the wrist pin out.  This can damage the connecting rod.
  5. Secure some type of drift that will fit into or against the wrist pin that is smaller than the wrist pin hole.  I use a large common screwdriver that wedges into the hole near the wide end of the blade.
  6. Put on a pair of cotton work gloves.
  7. Heat the piston with setting on high for 2 to 5 minutes.
  8. Grasp the piston with one gloved hand and push the pin out with the drift away from the remaining retaining clip.  It should push out easily.

When the pin and drift is removed the piston will easily lift free.  You will see a shower of needles and two location washers fall and scatter around the workspace.  Pick up the needles and spacer.  Count the needles.  If you don’t have the correct number then one or more might have fallen into the crankcase.  This is why you stuffed the rags into the crankcase.  A pencil magnet will help in the recovery.  I have measured used needles against new ones to the 10,000th of an inch and have never found any difference.  I haven’t reused any because I have access to new ones but in a pinch I would likely reuse.

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Como diría Juan Ramón Jiménez, mi troll es pequeño, peludo, suave; tan blando por fuera, que se diría todo de algodón, que no lleva huesos.

Probablemente no tiene huesos y por eso insulta bajo seudónimo. Pero además de cobarde es tan coñazo que he decidido que sólo me moleste a mi. De tal modo que a partir de ahora me quedo con la exclusiva de leer sus bobadas. Disculpadme el resto que os haga pasar por la "moderación" de vuestros comentarios.