Evo 20GX Crankshaft repair

[Applewood]

After a bad crash, I've noticed the crankshaft has bent in my Evolution 20GX petrol engine. It wasn't noticeable initially, but on closer inspection its about 3-4mm difference on the prop tips. No other damage noticeable to the engine, bearing sound quiet and smooth etc.

Having trawled the internet, I've not found one shop that holds stock, and doesn't look like distributors have them readily available.

Instead, does anyone know anyone who does engine repairs who can straighten a bent crankshafts?

Thanks in advance.

[Jon H]

Bin it and buy a new one. You will never get it perfectly straight and even if it runs ok you will get through bearings at an alarming rate. Depending on the price of the crank it may or may not be economical to repair the engine

[Applewood]

Thanks Jon. It doesn't look very promising on the repair then.

Also no replacement shelf stock from uk retailers, distributors Horizon Hobby UK & Germany have no stock. Awaiting to see if Horizon Hobby have any in the US. The replacement crankshaft retail at £53, new engine £250 so it's still worth repairing (just), as long as nothing else is wrong.

[Jon H]

if its had a smack that hard its worth replacing the bearings while its in bits, but even if its a 70 quid repair its cheaper than a new engine so assuming the rest of the engine is ok then it is worth while

[Denis Watkins]

TJDModels are worth a call AW, as they deal with replacement crankshafts, and they advertise on this homepage

[Applewood]

Hi Denis, thanks. I've been in touch with John from TJDModels, he's spoken to HH on my behalf, and gave me the above info. He's suggested ordering it, then that puts pressure on distributor to get stock.

[Levanter]

Applewood - Just a thought - Are you certain the crankshaft is actually bent. If you are checking the tips of the prop that was involved in the crash they could be out of place themselves. I suspect many props do not track with complete accuracy. Do you have a local engineering company or anyone who could put it between centres or on V blocks and check with a dial gauge?
​In any event I would still take Jon's advice and change the bearings.

[Engine Doctor]

Posted by Jon Harper - Laser Engines on 04/01/2017 15:04:56:

Bin it and buy a new one. You will never get it perfectly straight and even if it runs ok you will get through bearings at an alarming rate. Depending on the price of the crank it may or may not be economical to repair the engine

Sorry John . I disagree . As a replacement isn't readily available It's worth having a go at straightening if it's not too badly bent as you have nothing to lose. I have straightened many bent crankshafts with very good results and no more vibration than before they were damaged. The evo  engines are probably made in China and the cranks on most Chinese engines are very soft so bend and straighten easily. 

[Jon H]

I guess I look at it as an engineer and know you wont ever get it straight straight. But, if no replacement is available then you can try and you are right, you couldn't probably improve it, just be aware that you might make it worse and be more in trouble than you are already

Edited By Jon Harper - Laser Engines on 04/01/2017 22:03:34

[Shaunie]

If the engine is, as I presume, of a conventional design then the bend would be confined to the threaded end of the crank where the prop bolts on. The area between the two bearings is of a much larger diameter and is much less likely to get bent. What I am saying is that straightening the nose of the crank should be fairly straightforward and there is unlikely to be any runout implications on the bearings in my opinion.

Shaunie.

[onetenor]

My thoughts exactly Shaunie but with a caveat that the bending and straightening could leave a weakness that could cause a fracture down the line I wouldn't want to be around if that happened with the motor running.Lethal.

[cymaz]

The Evo 20gx has been superceeded by the 20gx 2. Same engine I believe. You might have better luck with finding the gx2 crank.

Found one

Horizon have them in stock. £44 but they sting you £22 postage !

Edited By cymaz on 05/01/2017 06:36:24

 

Edited By cymaz on 05/01/2017 06:44:59

[Jon H]

Posted by Shaunie on 04/01/2017 23:13:31:

If the engine is, as I presume, of a conventional design then the bend would be confined to the threaded end of the crank where the prop bolts on. The area between the two bearings is of a much larger diameter and is much less likely to get bent. What I am saying is that straightening the nose of the crank should be fairly straightforward and there is unlikely to be any runout implications on the bearings in my opinion.

Shaunie.

Our engines have conventional design cranks too and if you really bury one it will bend. Normally it will bend behind the front bearing where the rear face of the bearing touches the forward face of the main journal.

I am not quite sure why this is being debated. While not the same brand, i manufacture these engines every day and i am saying that if a spare is available the crank should be replaced along with the bearings and prop driver. This is what i recommend to my customers if we find a crank bent beyond a certain limit after a crash.

If a spare is not available then ED's comments are perfectly valid and there is potential to save the engine by attempting to straighten the crank but it will never be perfectly straight and you might make it worse as it is going to be hard to make it bend exactly where it bent before.

If the crank cymaz found is the right one, which it looks like it is, then it will be far better to repair it properly and get the maximum life from the engine instead of potentially making it worse and causing more damage in the long term.

All that said, Levanter is quite right and it is worth getting the crank clocked up so you know where you are to begin with. If you were local and could pop in i would quickly check it for you. If its not bent, the prop driver is the likely candidate as they can distort in a crash as they are soft ali.

[Applewood]

Thanks for your help guys. I'm going to take the engine to our club's resident expert tonight, hopefully he will take a look at it. I know some club members have access to machinery mentioned above and can put in a lathe and clock it up etc. This way they can check the extent of the damage. I'll come back with an update.

I didn't consider the damage to the prop driver. I did think the face of that seemed off, so that may need to go on the shopping list too. I'm at work so cannot check.

I would prefer to have trouble free running, but having forked out for a replacement fuselage I'm running out of £ quickly. Also with parts shortage, the shipping is look like what I've been quoted from uK shops 6-8 weeks from the US.

With lack of parts support, in the future I'm going to stay with brands with more readily available parts. (Jon - I would love to own a Laser at some point, our club chairman swears by them).

[Jon H]

Sounds like a good plan, and if you ever decide you do fancy a Laser i will be happy go arrange that for you. Our 155 would put that gx20 well in the shade in terms of performance

[Shaunie]

Replacement is the first option but if the crank is not available, as the OP first thought, then straightening is an option providing the deviation is minor. If the tip runout is only a couple of mm then surely if the bend is in the crank and not the prop or driver then the bend must be quite small. My engineers mind says that nothing is ever perfectly straight, round or true! How much runout you tolerate depends on the application. The largest machinery I have worked on is a Vertical Turning Lathe with a 12ft dia table, the smallest, a gold wire bonding machine working with 1.2thou wire, (two different careers).

Shaunie.

[Jon H]

Posted by Shaunie on 06/01/2017 00:35:49:

My engineers mind says that nothing is ever perfectly straight, round or true!

Shaunie.

pretty much, its all a matter of tolerance at the end of the day but if you can see the run out its probably a bit excessive