Scott Bonnar 45 Rebuild (27 - Chain Tensioning)

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0:00 - Chain Tensioning
0:45 - Drum and Intermediate Chain Tensioning
7:12 - Reel Chain Tensioning
8:43 - Lubrication

Before finishing up with the chains, they must be tensioned.

🎬 Watch the chain installation video
   • Scott Bonnar 45 Rebuild (26 - Chain I...  

Chains should be neither too tight nor too loose. Too tight and they'll increase wear on the chains and sprockets and increase load on the engine. Too loose and they'll flop around unnecessarily.

The drum and intermediate chains are tensioned first by adjusting the position of the intermediate clutch bracket. The bracket pivots up and down on the top retaining bolt which secures it to the mower side. The bracket also slides backwards and forwards along the two retaining bolts. Loosen both bolts to begin.

We're aiming for between 1/4" and 1/2" (about 6 - 12mm) of slack on each chain. The tension of the short drum chain will be difficult to gauge; it simply needs to be tight enough so the rear rollers don't lock up when the mower is pulled backwards (ensure your intermediate clutch bolt isn't too tight either if you do experience lock up).

The two chains connected to the intermediate clutch are tensioned at the same time. The clutch bracket is simultaneously pulled backwards and pivoted upwards. The weight of the bracket and chains, as well as any applied tension, works against us until the retaining bolts can be tightened again so dial a friend if you can for this job.

Tighten the upper retaining bolt first while holding the clutch bracket in position. Then tighten the lower bolt, adjusting the clutch bracket pivot if required.

In our case intermediate chain tension fluctuates slightly as the sprockets turn. This may be due to any number of reasons including an out-of-round sprocket or a shaft or bearing not sitting square. We account for this by noting the tightest location of each sprocket and applying suitable tension at that point. The chain slackens slightly at it rotates but it's likely acceptable.

The chain tensioner (or idler sprocket, in this case) is used to tension the reel chain. The original style chain tensioner is adjusted in a similar way (by pivoting up and down) whereas twin rail machines use a tensioner that slides up and down.

🌐 Buy the idler sprocket kit shown in the video from SB Fabrication & Parts (available for both solid and twin rail machines)
  / sbfabandparts  

Draw the tensioner upwards to achieve the desired chain tension and lock it in place. The solid deck idler may want to flop down as the chain cover mount is tightened so we used a block of soft wood jammed between the sprockets to hold it in position.

Review the tension of all three chains, adjust again as necessary, and then apply a quality chain lubricant. A motorcycle or anti-fling chain lubricant works well here. Aim to lubricate the chain pins and bushings.

🎬 Watch all the rebuild videos here
   • Scott Bonnar 45 Rebuild  

🎬 Watch all the teardown videos here
   • Scott Bonnar 45 Teardown  

🎬 Check out the SB 45 walkthrough videos if you're new to cylinder mowers and Scott Bonnars
   • Scott Bonnar 45 Walkthrough  

⚠️ Your safety is your responsibility. Don't forget to disconnect your spark plug before starting work on your machine and always read and follow the instructions/labels on your equipment, tools and chemicals. ⚠️

#ScottBonnar #Chains #Tension
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Additional Resources
🌐 Scott Bonnar Restorations FB Group
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🌐 Scott Bonnar - Buy, Sell, Swap Australia FB marketplace
  / 1207116562722291  
🌐 Scott Bonnar Addicts FB Group
  / 791569104336316  
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About me and this channel: I'm Michael, a homeowner and aspiring lawn addict. I make these videos to share as much as I can, as best I can, from my recent experience rebuilding two Scott Bonnar 45 mowers. Although I'm a passionate wood worker and ex-IT guy, I have zero mechanical training. I'm no expert on these subjects but I learn by sharing and it encourages me to grow a better lawn and build a better mower if I know you're watching!

© 2020 Michael Hanes. All rights reserved.