Rotax 185 aircraft engine rebuilding, Rotax 185 aircraft engine rebuilding manual, Rebuilding the Rotax 185 aircraft engine used on the Lazair ultralight.

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Rotax 185 aircraft engine tear down and decarbonizing

The Rotax 185 is still a very popular engine primarily used to power a portable high pressure pump used for forest fire fighting ,so all parts readily available. This page is going to deal with replacing the crankshaft bearings and seals ,which is recommended every 300 hrs as well as piston cleaning which can be as little as 30 hrs. They are quite easily done ,the key is using the right tools for the job, plus there is a few very helpful tips that I learned from a long time local Lazair pilot, that make the job that more enjoyable.

This page contains many pictures they are not meant to be in any order, some steps considered incidental are omitted,  all have been edited down to size to speed up the download.

Anyway I got most of the important and helpful procedures. On my next rebuild I will fortify the picture collection.


Disassembly procedure

The right tools for the job an assortment of pullers, from left to right, Rotax copy, fly wheel puller custom made front bearing puller, but available from Rotax (bearing positioned against crank flanges no room for regular puller) typical bearing puller works for rear bearing. 


Inspect and clean cylinder, remove carbon from exhaust port and inside de-compressor ally. This job is made easier by soaking the whole cylinder in carb cleaner, removes varnish and softens carbon
Here I am being a little cheap but it seems to work well with minor build up, I am brushing on poly stripper paint remover, instead of soaking the piston in stinky carb cleaner.

The stuff I used on my first engine rebuild I'll tell you this stuff worked great dissolved even the most stubborn carbon build up but the odour was permeable.

It would last for days on your hands and on your clothes,
the stuff was hardcore ,loads of warning labels. anyway one of its ingredients, methylene chloride is also in poly stripper.

pic 2 once brushed on cover with plastic meth chloride is extremely volatile and won't last if not covered 
Wait several hours then scrape carbon off  piston crown
(Note : try to leave the thin hard carbon film ,this apparently protects the piston crown from heat)

To scrape carbon from the ring grooves ,make a tool from a old piston ring you can see the shape that's been grinded on the end.
 Carefully scrape away all traces of carbon in ring grooves ,this job also easier if piston has been soaked overnight.

Now your wife will love this one.
To dis-assemble crankcase insert in oven heated to no more than 210 degrees F, wait till completely heated then rap the case halves with a rubber mallet or a block of wood they should fall apart with relative ease.
Here I am pushing in new oil seals, with a homemade 1" pvc tee and socket tool works good for removing also.
Now here's the fun part. The bearings are friction fitted to the crank, here is a way you can put the new ones on without a press.

This works slick, toss the crank in the deep freeze, put the bearings in a small pot submerged in motor oil and heat till hot.

When every thing is as hot and cold as its going to be whip the crank out of the freezer, grab the bearings with pliers and slip them onto the cold shrunken ends IMPORTANT do not forget to replace all the shims and spacers in there correct position

To re-assemble crankcase. Into the oven at 210'F goes the case halves.
Back into the freezer goes the crankshaft with new bearings wait (be patient) till they are up/down to temperatures
When ready whip the back crankcase half out of the oven and place on a block of wood, whip crank out of freezer and carefully and firmly press crank assy into rear case.

Make sure to put magneto end of crank into this side!
POSITION case gasket then quickly remove front case from oven and place over crank and press down to bring halves together.
Notes  you must me gentle at first to make sure bearings align with there sockets in the cases to prevent jamming also check gasket placement.
Replace the 4 crankcase bolts and torque down to correct value, remember to use cross sequential incremental torquing .
At this time you will usually find a little resistance when the crank is turned over, This is normal before the anealing step. Put the unit back into the oven to again  bring all parts up to uniform temperature, remove crankcase from oven then rap both ends of the case halves and bearing housings with a 2X4 block or wooden mallet, this will settle the bearings into there proper place. Recheck bolt torques when cooled .
Use ring expander to re-install piston rings, some say that you must use new piston rings every time , I don't think its necessary myself especially if I am de-carbonning every 50 hrs I'd probably replace them after 200-300 hrs
Install gaskets, cylinder and head Torque to proper values
Special Notes
Cylinder head placement is important, you will notice that the top of the combustion chamber has a defined shape, make sure that the shallow lobe that assists the removal of exhaust gasses is placed over the exhaust port side.
Also regarding the cylinder head gasket, myself and others I know re-use it several times, as long as you replace it in the original position, I have never had a failure and I put lots of hours on my engines. This is especially handy for just routine combustion chamber cleaning and inspection.

Reinstall magneto. If you did not put a reference scribe mark on mag and case you will have to reset timing using a dial gauge inserted into the spark plug hole. Its a good Idea to fill all electrical connections with silicone to exclude any moisture.
This is also a good opportunity to check or replace the points.

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