Dismantling a Thunder Tiger 91FS.

[David Davis]

My Thunder Tiger 91 needs new bearings. My fault, I should have squirted some oil into the engine after the last flights. Lesson learned.

I managed to dismantle the engine in about fifteen minutes last night and I'm now at the stage where the only parts still inside the crankcase are the crankshaft, the camshaft and the bearings. I have removed both cam followers but cannot get the camshaft out.

My guess is that the lobe of the inlet cam is fouling the crankshaft, either that or there is some knack to disengaging the drive to the crankshaft.

Help!

All advice gratefully received.

[cymaz]

The skew gearing is preventing it from dropping out. Try moving the crank back or forward and turning the cam shaft with some pliers. Give it a gentle tug and twist at the same time.

[David Davis]

Thanks Cymaz. There is next to no fore and aft movement on the crankshaft. I'll try a better pair of pliers after breakfast!

[Jon H]

Some plastic tweezers can be good too but as the cams are hardened you shouldnt do any damage with pliers. One possibility is that rust/gunk is holding the cams into the bearing on the far side which is not an easy problem to fix. I had this on an sc52 i was fixing for someone and had no choice but to drill a hole in the crankcase so i could tap the shaft out. A far from ideal situation, but the engine works fine now and the hole was blanked off so no harm done

[ken anderson.]

if is gunge...try a hair dryer around the area...soften it all up and then you may get it oot..

ken Anderson...ne...1....... oot dept..

[Jon H]

if its rust a sharp tap with a small hammer on the other end of the camshaft might crack it loose. I tried this on the SC however and it was unsuccessful

[David Davis]

Thank you for the advice gentlemen.

I was able to move the camshaft about 1/8" side to side. Figuring that the front of the crankshaft is likely to be smaller than the rear part and that if I were to tap the crankshaft out, all that would happen is that the camshaft would simply rotate, I resorted to gently tapping the end of the crankshaft with a plastic hammer until I was able to remove it. Still the camshaft would not come out!

I resorted to putting the crankcase into a hot oven for about twenty minutes, then holding the crankcase in a welder's glove I was able to pull out the camshaft very easily. I then turned the engine onto its back and whacked it fairly vigorously down onto the bread board! Actually it was a piece of wood, once part of a wine box, which was placed on top of the bread board! I repeated this a few times and the rear bearing fell out. I then poked the front bearing out with the handle of a wooden spoon. Next time I'll heat the crankcase before removing the crankshaft.

If I may beg your indulgence for one more question, the front bearing is sealed on both sides as is its replacement, however, the replacement rear bearing, bought in an automotive spares shop, has rubber seals on both sides. Do I remove these seals prior to reassembly?

[Nigel R]

Posted by David Davis on 30/11/2016 10:09:00:

the front bearing is sealed on both sides as is its replacement, however, the replacement rear bearing, bought in an automotive spares shop, has rubber seals on both sides. Do I remove these seals prior to reassembly?

The only route the oil mist has to get from the crankcase to the cams is via the rear bearing, so yes, pull the seals.

BTW a bit of heat on the crankcase again, before you pop the bearings back in will make that job much smoother.

[Jon H]

I would do as Supernash says with the rear bearing and i would also remove the rear seal from the front bearing. Often the grease will escape and the seals will prevent crankcase oil getting in to replace it leading to wear/rust.

[Mike T]

You would know far better than me, Jon, but if the grease can get out, can't the oil get in?

I ask as the last time I changed bearings (RCV91CD) I'm sure I left both seals in place at the front, but removed them on the main.

[Jon H]

Posted by Mike T on 30/11/2016 12:36:36:

You would know far better than me, Jon, but if the grease can get out, can't the oil get in?

I ask as the last time I changed bearings (RCV91CD) I'm sure I left both seals in place at the front, but removed them on the main.

You would think that but some reason it dosent work that way! I suspect its due to the thicker grease on the inside being able to put a hydraulic force on the seals and forcing its way out. As their is no force of any kind pushing the oil in the seals stay put.

Thats my theory based upon what i see here with our bearings, but in truth i dont know. All i know is that very often sealed front bearings end up running dry and going rusty

[Engine Doctor]

Hi heat should help loosen it together with some penetrating oil. Heat the whole assembly until a drop of spit jumps off . you will need some decent gloves .  I use a hot air gun or you could put it in the oven for half an hour .Now dose the cam box with a drop of penetrating oil , not enough to cool it quickly or distortion can occur, but just enough so that it can soften any gunge. It should now pull out with any luck the cam bearing will come out with the shaft .While its hot the crank and mains can also be removed.Clean case and re-heat to fit new bearings. Definitely remove any seals from the rear bearing and clean out grease before fitting. The front bearing usually only has seal left on the front outer side . Again washout grease and oil prior to fitting. Make sure they are well oiled before running engine. I have in the past tried fitting a fully sealed front bearing with no problems and they last very well. If you compare the front bearings with those of similar  size fitted to  car alternators that run for years without fault with good belt tension then it seams odd to remove the seal so that methanol can do its thing .

Edited By Engine Doctor on 30/11/2016 15:30:02

[Fatscoleymo]

When you re-assemble pop the bearings in the freezer for 20 mins beforehand....it does work...or it did for me...as well as cooking the engine in the oven..

Fats

[Shaunie]

I often rebuild alternators, it's remarkable how often the bearings come in useful on engines! Nevertheless I would always remove the rear seal for an engine application. The temperature on an engine will be higher and the acidic byproducts of combustion will contaminate the bearing, with seal removed the contaminants can be washed out, either by engine operation or by after run oil.

Shaunie.

[Alistair Taylor]

Top tip for driving new bearings in (if they don't drop in);

Use a socket from a socket/wrench set that is the same diameter as the outer ring of the bearing you're driving in. This should ensure you drive it in square.

If you've heated the crankcase, and frozen the bearing, nine times out of ten they just drop in, but having correct socket handy means you can ensure bearing fully seated.

I've found it's sometimes difficult to manoeuvre the assembled piston and conrod onto the crankshaft, even with the liner out.

My solution (which might be how they assemble them in the factory, dunno), separate conrod from piston. Carefully insert piston part way into liner, so that hole for gudgeon pin is still accessible.

Insert crankshaft, and then pop conrod on on the end, the right way round. I believe this usually means radiused side of bushing faces forwards, to collect oil from intake charge (I'm open to correction!?). Then slide liner/piston in and rotate until gudgeon pin hole lines up with hole midway up the rear of the crankcase, about level with top of conrod at BDC. It's usually just under cooling fins.

Ensure the circlip you'll probably find at one end of the hole for the gudgeon pin in the piston is facing forwards, i,e, is on opposite side of piston to the side you can see thru hole in crankcase.

Rotate crank and wiggle conrod to get it to line up with this hole as well. Then slide gudgeon pin thru crankcase hole, thru piston, and thru conrod. Usually takes wiggling. Then push gudgeon pin home gently against circlip. Some engines also have teflon pad that sits on the end of the gudgeon pin not pressed against the circlip. Now is the time to make sure it's in there!

Liner should now slide down past piston and home, and hey presto, you should have assembled crank/piston/conrod/gudgeon pin/liner.

Sorry if this is teaching you gents to suck eggs. It was this or some knotty homework problem, and this is more fun.

AlistairT

[David Davis]

Thank you gentlemen, the engine is now completely dismantled.

There is a certain amount of baked on oil inside the bearing housings, though I don't recall ever running it on fuels containing much castor oil. I intend to to re-commission an old Merco 35 which has seized due to congealed castor oil. My technique is to boil the engine in antifreeze for half an hour. This gets rid of the baked on oil and warms the crankcase simultaneously! I'll put the Thunder Tiger crankcase into the pot at the same time!

Thanks for the tips about removing seals, using sockets and putting the bearings in the freezer.

[Denis Watkins]

Having followed this thread with interest for four strokes, the theory being to remove seals for oil flow over the bearings; with 2 strokes, would we remove seals on the rear bearing? and just keep the front bearing sealed to maintain pressure? Or remove the inner front seal too?

Thanks in advance

[Jon H]

If you have shields on your rear bearing you can probably leave them, but if you have rubber seals then i would remove them and still remove the rear seal from the front bearing.

[Cuban8]

Re cooling parts to fit..............We used to use freezer spray to cool bearings and make them easier to fit when out on a repair job. Seal the bearing in a plastic bag for protection, (often they'd already be in a sealed bag) and then pop that in another small plastic bag and squirt freezing liquid (pipe freezer) between the two. Wrap the bags up in a rag for insulation and in a minute or two it'll have cooled sufficiently.

[GONZO]

I've always used differential expansion of engine parts for assembly and dismantling, makes life soooo much easier. In fact some engines will not come apart without it. The Super Tigre G23(the big one 23cc) has a liner that is an interference fit in the crankcase; bag engine(less bits that will come off) and place in freezer over night, remove and locally heat barrel with hot air gun. Easy peasy to remove liner, a no go otherwise. Just an example.