My thundercat

[Roflhat]

Going to run a fair bit more oil from now on. Got the bores honed out today and I cleaned up the ports just now. Ordered 2 full sets of pistons and rings, will weight match them when they arrive.

It's not rust on the conrods, that's just the colour they are. Crank assembly is in pristine condition fortunately, bearings etc. and conrods all good

Also retested the trigger coils, the one feeding number 1 cylinder is absolutely dead, infinite resistance. Got a new (used) ignition plate and all 3 trigger coils are good so hopefully that's the 2 cylinder issue sorted.

[Lexagonal]

Your cat is awesome. Can I ask how big a fuel tank you use? I’m considering getting one to explore waterways and was wondering how big a tank I would need and how long it might last.

[jwalker]

Just a wee note which you likely are aware of but you tried weaker mixtures so I'll mention it. As you weaken the mixture, you also supply less oil for lubrication. Rich mixtures help cool the pistons because as the extra petrol evaporates it draws heat from the surrounding metal, ie. the pistons.

2 stroke engines will 4 stroke on excessively rich mixtures so, as a starting point, go very rich for safety then work towards weak, when you stop gaining power, go back one size of jet to be sure you're on the safe side.

[Roflhat]

Originally Posted by Lexagonal

Your cat is awesome. Can I ask how big a fuel tank you use? I’m considering getting one to explore waterways and was wondering how big a tank I would need and how long it might last.

Thanks very much! At the moment I use the flexi tank which is only 22l but really I need a bigger one as it guzzles through that. With a standard 50hp a 25l plastic tank is perfect, will do around 40nm on that with normal driving.

Originally Posted by jwalker

Just a wee note which you likely are aware of but you tried weaker mixtures so I'll mention it. As you weaken the mixture, you also supply less oil for lubrication. Rich mixtures help cool the pistons because as the extra petrol evaporates it draws heat from the surrounding metal, ie. the pistons.

2 stroke engines will 4 stroke on excessively rich mixtures so, as a starting point, go very rich for safety then work towards weak, when you stop gaining power, go back one size of jet to be sure you're on the safe side.

Thanks, I've put bigger jets in just now to make sure there's enough fuel getting through as you say. The theory with originally going for smaller jets after testing was that it was 4 stroking, but the results didn't point that way after I tried it.

[Lexagonal]

Thanks for the info re the tank. I have another noob question.

Is it possible to recover a capsized Thundercat with one person. Have watched a few videos online and people seem to struggle with two!

[Roflhat]

Originally Posted by Lexagonal

Thanks for the info re the tank. I have another noob question.

Is it possible to recover a capsized Thundercat with one person. Have watched a few videos online and people seem to struggle with two!

Yes it is, I've never flipped but a friend of mine has and managed to right it on his own, he's not particularly heavy either

[Roflhat]

Photos don't seem to be displaying at the moment, hopefully they come back up

Got the engine back together, all new gaskets and running really well. Compression is up to 175psi across all 3 cylinders. Got some testing done on Saturday, hopefully more this weekend. My mate Andy White is very kindly making me a carbon fiber cowling. A lot of work involved but if it works should save around 3kg.




Also had the flywheel painted while the engine was apart, looks really smart. Did the pull start cover and a few other bits too

[Roflhat]

Had the engine down on a dyno last month, Pete at Swordfish marine kindly sorted it out.
Didn't go to plan but still some useful progress, the oil pump block off was leaking, also had a pin hole in the cylinder head and a gas leak between the block halves, all now sorted. Performance started going down hill so took it apart, bottom piston had melted, most likely due to the oil pump plate leaking so not enough oil getting to it.



All back together now ready to test, changed to Castrol oil and different gear oil

[Roflhat]

And here we are again,



Was hoping to get some decent testing done today, first calm day this year but had another breakdown. Only managed 2 hours running since the last rebuild.
Still a few things to test, best speed today was 62.51 knots which is the fastest I've had since Coniston. So things going in the right direction but some problems, thinking is the fuel is not upto the job. So going to try and get hold of Avgas 100LL.
Getting pretty tired of this now, can't afford to keep rebuilding it all the time. Don't usually lack motivation but struggling a bit at the moment.

[Roflhat]

Had a thrust washer machined down, for the top speed runs I'll run this

[A1an]

That's a cracking speed Seamus.


When you are pushing the limits of an engine, failures are going to happen. To be honest, Im impressed none of your problems have been catastrophic.


What oil and ratio are you using?

[jwalker]

Originally Posted by Roflhat

...Getting pretty tired of this now, can't afford to keep rebuilding it all the time. Don't usually lack motivation but struggling a bit at the moment.

You've been there, done it, Seamus, possibly time for a new project.

With regard to your pictures, is the piston simply picking up due to lack of lubrication or is it seizing in the bore? If it's seizing, you need more piston to bore clearance to allow for the piston expansion, if it's picking up and your mix is right you might just have reached the limit of your motor the way it is.

[Squid Boat]

Smokin top end mate, have really enjoyed watching the progress.


Cheers

[Roflhat]

Cheers,
weather is crap for the next while anyway so time to plan things out. Putting the engine back together, brand new Yamaha pistons and rings, skim the head to make sure it's flat, hone the block, new head and base gaskets and a few other bits. The pistons that were in it were remakes of Yamaha ones i believe, they are very slightly different to the genuine ones I have.

Running plenty oil, Castrol 2 stroke at 3% (33:1). Only the top piston had seized, but changing them all out anyway. Bore isn't too bad just needs a clean up.

Already booked accomodation for Coniston this year so will see that through anyway I hope. Got another project already lined up, got a wee Suzuki Gp100 to go for a land speed record
But still feel like I've got more to go with the boat first

[jwalker]

What did occur to me is that the piston melting is not on the piston thrust side where there is greatest load on the piston. Pistons don't expand evenly as they heat up, partly because the temperatures are different around the piston and partly because there is more metal in some parts of the piston. It is fairly common for pistons not to be cylindrical so that as they heat and expand their shape actually improves.

We need more pictures, where both sides of the piston damaged, was it equal damage in the same areas on each side? Could there have been contamination inside the gudeon pin which escaped? Could you have a slightly bent con rod so tilting the piston? We need more info!!

[GuyC]

Originally Posted by Roflhat

Cheers,
...got a wee Suzuki Gp100 to go for a land speed record

Down Clisham?

[Roflhat]

Interesting what you're saying about the pistons there, the ones I changed out just now were replicas of the Yamaha ones, not genuine. Bought from a reputable company but side by side with the genuine ones there are differences. So you might be onto something there with the way the pistons expand, especially when pushed as hard as these ones are.

Everything this time is genuine Yamaha, and going bigger still on the jets just in case. That along with Avgas if I can buy it should do the trick. Then it'll be smaller cc heads to extract more power from the Avgas

[Roflhat]

Originally Posted by GuyC

Down Clisham?

With a strong tail wind!
Plan is to take it to Bonneville, it's on the back burner for the moment maybe 2021

[jwalker]

Originally Posted by Roflhat

Interesting what you're saying about the pistons there, the ones I changed out just now were replicas of the Yamaha ones, not genuine. Bought from a reputable company but side by side with the genuine ones there are differences. So you might be onto something there with the way the pistons expand, especially when pushed as hard as these ones are.

Everything this time is genuine Yamaha, and going bigger still on the jets just in case. That along with Avgas if I can buy it should do the trick. Then it'll be smaller cc heads to extract more power from the Avgas

I don't know what piston/bore clearances you are using but you need to consider them very carefully. Compared to a standard engine your block will stay cooler unless you are running for an extended period sufficient for the block temperature to stabilise. I guess you might also be running it at a cooler thermostat setting plus the water you run in will be comparatively cool. All this will tend to a cylinder diameter not expanding as much as it might with prolonged, stabilised running. Conversely, your pistons are going to be subjected to severe heating in a very short time and therefore have consequently greater expansion than a piston in a standard engine. The expansion of the piston will also be greater than the bores because the aluminium alloy will have a larger expansion per degree of heating. Different piston alloys are available and they expand different amounts per degree of temperature change. You need to know these specifications to calculate, or, at least have an educated guess at the required piston clearance. Pistons with a slightly large clearance will tap, especially in a cold engine but they also reduce friction on the non-thrust running surface so can give a little more power. You can view a piston as a devise which provides the piston rings with a stable platform to operate and seal the cylinder. In this respect it's little more than a plug for carrying the rings.

However, it must at all times have sufficient clearance for a suitably thick oil film to be present over the whole of its surface when heated to its maximum operating temperature. The oil film provides friction lowering lubrication (obviously) but it is essential also for effective transfer of heat from the piston to the bore.

The piston skirt will have the lowest operating temperature and the piston crown the highest temperature so there will be a differential expansion of the aluminium alloy from top to bottom. From your pictures, the tightest part of your pistons appears to be towards the top where they have the most picking-up of the aluminium. It's possible and common to have the upper portion of pistons reduced so that they never touch the bore and their alignment is left to the skirt portion. If you get the skirt clearance correct and keep a good oil film there, reducing the diameter of the upper part of the piston to have no bore wall contact could be a good thing. Upper piston cooling will then be down to the piston rings and conduction to the skirt area plus the cooling from the intake fuel. Having some excess fuel will require heat to evaporate and so provide a fair measure of cooling to the piston. Going a little rich in mixture ratio is the safer option.

[jeffstevens763@g]

is it only picking up in the gudgeon pin area or in line with as in the photo