Author Topic: Motor tight  (Read 2652 times)

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Offline ste

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Motor tight
« on: 29 Oct 2016 at 10:02 »
The TS fired for the first time today after many years of restoration. It was wonderful!!! However, after spending some time fettling and putting the belt back on, the motor was very tight as you turned the flywheel, a lot tighter than at the start. No oil is leaking on the floor, but it looks like the drip feed is working. We changed from Shelsey medium to light as it didn't seem to want to go through the pump. Should I change the oil to a lighter grade again or is something more sinister happening. I refrained from taking it for a ride sadly, in case the I would seize the motor, as I'm worried it's not being lubricated enough.


Offline cardan

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Re: Motor tight
« Reply #1 on: 29 Oct 2016 at 10:50 »

Well done Stephen - almost there.

The motor should not be tight. Don't run it.

As a starting point, take the spark plugs out and remove the primary chain. Squirt some of your light oil down the bores and turn the motor over by hand using the flywheel. If all is OK the motor should turn over smoothly with no particular tight spots.

If it's noticeably tight anywhere, you'll need to investigate step by step to identify the problem.

Did you use plenty of oil when you assembled it? Usually starting a freshly-assembled motor for the first time involves clouds of smoke.


Offline eddie

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Re: Motor tight
« Reply #2 on: 30 Oct 2016 at 08:40 »
             It sounds as if the motor may have been assembled with too little clearance on the big ends. you have to bear in mind these motors with splash feed oiling need enough clearance for the oil to be able to get into the bearings. Poor oiling on phosphor bronze shells often causes the shells to curl tighter around the pin as they cool down again - resulting in the bearing assembly remaining tight after it has fully cooled. On stripping down, you will probably find that the bronze shells are tighter on the pins than they are in the con rods, in which case, you will probably need to scrape the shells out to get the correct clearance at the ends. When finally reassembled, the rods should be loose enough to fall freely under their own weight.


Offline steveale

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Re: Motor tight
« Reply #3 on: 31 Oct 2016 at 20:37 »
You may first want to insure the sprocket on the back of the flywheel is properly spaced and not rubbing on the engine case.  This didn't happen on assemble for me but after running/decompressing under power the sprocket worked loose on the threads and backed off and tightened up against to the crankcase to the point where it appeared the motor was seized.

You can imagine the sigh of relief when I found this was the issue.  Some red loctite and a good tightening eliminated this problem from re-occurring and the engine was fine.

fingers crossed...

Offline Dewey

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Re: Motor tight
« Reply #4 on: 01 Nov 2016 at 23:25 »
A little more info can narrow this down. Was the engine tight right from the beginning or after it warmed up? If it tightens up only after being run a bit, and then frees when it cools down I'd say you don't have enough clearance in the top end. Could be pistons, ring end gap or both. Are the pistons new original? I've had aftermarket pistons stick when warm because the wrong alloy was used so they expanded more than the factory ones. Air cooled engines run hotter than water cooled so extra clearance is needed for both pistons and rings.

If it's tight even when cold - find out why - now. It's probably in the bottom end like eddie is saying.