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Before installing a (used) replacement motor who’s mechanical history is unknown, what components or assemblies would be considered wise to tear down and/or inspect? Also, what are the most common failures that could be checked, replaced or repaired before installation/operation? Thanks!
 

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Check the oil for particles and maybe open the oil filter and wash out the screen and look there. pull the spark plugs and place a long screwdriver down the hole. turn the crank whilst holding the screwdriver firm but not hard against the top of the piston. rotate up and down at several positions in the throw. damaged big ends often show up as a lag between the crank rotating and the piston moving and pushing on the screwdriver.

Shake the sprocket up and down and check for wet oil around the seal. (bad seal or bearing.

Check the exhaust ports. wet valve stems and backs of valves = worn seals or guides.
balck sooty deposits are ok wet black deposits mean worn rings

check inlet ports black soot on the backs of the valves means poor valve seating

wet black deposits = worn guides or seals

if you can get alight into the pistons through the spark plug and actually see the piston crown ( you can on some engines) black soot on piston = ok
wet black on the edges of piston crown = worn rings
wet all over = stuffed rings
wet cylinder head too = stuffed rings guides et al

do a compression test

add a small aount of oil onto the top of the piston and do it a gain
it shouldnt rise very much
add a lot of oil and it should nt rise much again .

If it rises its because
1 the engine has been standing and its dry = ok ( a little oil added)
2 the engine is worn and the oil has filled the gaps = bad ( lots of oil added)

both cylinders should have similar pressure , the figure is not critical, ( see adding oil)

do a leak down test ( requires a leak down tester)

check the plugs that come out ( same rules as checking ports)

check the mounting points for cracks or elongation of the holes( loose mounting bolts or crash damage. check around the sprocket for gouges ( previous chain brakage)

Check drain plug worn thread and leaking

rotate crank back and forth and listen for loose cam chains

Check the area around the exhaust ports there is usually a little bit of oil but if this looks to have been burnt it may indicate the engine was run out of coolant at some time.

Where to start.

compression test and visual on the ports

make sure there are no cracks or damage in the places mentioned.

happy hunting
cheers stu
 

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And once you're happy with all that, might as well check / adjust the valve clearance while the engine is out, it's not real hard withthe engine fitted, but it will be easier with it out.
 

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just buy my TLR motor and you can be rest assured it's top-notch! :devious
 

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On a tlr motor on the same case half and rearward of the oil drain plug there is an allen drain plug that is magnetic, pull it out and look for bits of metal if so pull the right side cover off and pull the screen out and see what you have, it will indicate if the whole motor should be torn down.
 
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