Saito 300 Twin

[Tony Martin]

I have a Saito 300 Twin, single carb version, which was originally run on glow for around 3 years before being converted to gas using the Morris Mini Motors full gas kit. The engine has run well on gas for almost 2 years, but has now developed a serious problem.
The exhaust valve seats are worn out in both cylinders rendering the engine pretty much useless. There is no metal left for the valves to seal and they both drop inside the exhaust chamber in the head.
My question is does anyone have a similar experience or suggestions as to what may have caused this. Is it likely the increased head temperature of a gas engine could be to blame?
One thing I do notice is that Saito fit bronze inserts for the exhaust guide and seat on their dedicated gas engines, whereas only the guides have bronze inserts on glow engines.

Any help here will be most welcome.

[paul d]

Soft valve seats, it was never designed to run on petrol.

You may be lucky and find someone to fit hardened seats but realistically your looking at new heads I'm afraid.

[Jon H]

I agree with Paul. The engine was never designed to run on petrol and so its very likely the heat in the exhaust gas is what has done the damage. I read a review somewhere and they found the EGT of the saito fg30 was over 300'c as it left the exhaust system so i dread to think how hot it was at the port. As the valve recessed its likely the clearance went tight and the valve started to leak which would not have helped matters at all. Its likely saito discovered the problem with burning valve seats during development of the FG series and so it got bronze seats.

Its time for a pair of new cylinders i suspect as i cant think of a way you could press new seats in given the design of the head.

As a side note (and this is not a jab at you personally Tony) its situations like this that show why i am so opposed to these conversions. Many never run right so are not worth the trouble, but even when they do work well enough you risk problems like this. Given the cost of repairs now needed were the savings on fuel over 2 years enough to balance it out? i really doubt it.

[Andrew Calcutt]

I have to agree,can't see the point of converting to petrol,with the addition of the ignition especially on small engines,they don't run as well and the bearings are probably not suitable.Saito four strokes are lovely engines leave them as god intended.

[Paul Marsh]

Ive got the 300 twin, twin carb version was thinking of converting it to petrol, but seems that glow engines never run well and in this case was detrimental to the engine - destroying it. Might as well keep on using the fuel it was meant for, I reckon.

I suppose the 57 T has hardened valves as this is a bespoke petrol unit from the start, though it won't go amiss to up the oil somewhat.

[Denis Watkins]

Putting aside fuel costs

Glow is more powerful, in a glow motor

And is proven that the chambers cannot handle the already high temperatures of a 4 stroke being even higher with petrol.

Looking at the picture of the Saito, you cannot knowing break such a work of art

[Jon H]

Paul, if you run that on the model technics laser 5 or techpower 5 it will be a monster. I run my OS FF240 and FT300 using the laser 5 and they both run really well on it. Admittedly the 300 only flew 3 or 4 times for testing after rebuild but it went like a train

[paul d]

Paul, leave it on methonal, it's not the valves it's the seats that need to be hardened, methonal burns a lot cooler than petrol at the expense of greater consumption.

Just my thoughts ( hope Jon jumps in and backs me up)but I would carry on using the oil ratio recommended, I think a lot of trouble comes when modellers try to extract the absolute maximum out of there fitted engines when a larger engine ran below of its capabilities would give far less trouble and last longer.

I only fly vintage jobs now but I do like to use a slightly larger engine than recommended and run them rich and undercompressed ( diesels obviously!) partly due to my mechanical sympathy but mainly because spares are unobtainable...just my thoughts.

[Jon H]

The biggest difference between methanol and petrol combustion is not the flame temperature as such but the fact that methanol itself is an awesome coolant. Get some on your hands and you soon know about how readily it evaporates and how much heat it takes with it. This is also why WWII era engines had water/methanol injection to cool the fuel charge before it entered the cylinders. This helped prevent knocking at high supercharger boost and helped keep the engine cool as the fuel charge itself cooled the combustion chamber before the engine fired.

Its this effect that allows glow engines to run cooler than petrol.

When it comes to oil we use too much in glow fuel and 10-15% is plenty. To be honest, even less than that is likely to be just fine so i would not worry too much.

The only limiting factor on modern saito's is that for some strange reason they dont use bronze big end bushes. I know this has caused issues on their dedicated petrol engines and i think all their current petrol's have bushed rods. The 300T does not but it really should not be a problem if you are running 15% and certainly it is where i would start if i had one of these myself.

As i mentioned before, i run my FF240 OS on 15% synthetic oil when the instructions clearly call for 20% castor. As the thing is like an oil fountain even at 15% i am going to consider myself well covered!

[Jon H]

Posted by paul d on 03/03/2020 19:19:48:

Just my thoughts ( hope Jon jumps in and backs me up)but I would carry on using the oil ratio recommended, I think a lot of trouble comes when modellers try to extract the absolute maximum out of there fitted engines when a larger engine ran below of its capabilities would give far less trouble and last longer.

i forgot about this part.

I see the logic here but with glow engines being gentle is often the end of them. Running them 'a bit rich for safety' and running them cool at low power is a really good way to kill them. Unburnt fuel can accumulate in the crankcase and cause rusting of the bearings and other parts. Its very important to always tune a glow engine for maximum performance, and if you do then just float around its a good idea to give it a good blast before you go home to try and clear all the rubbish out of it.

I often hear people say they are worried their engine is too hot because they cant touch it after a flight. My first question is 'why would you want to touch it?', but ignoring that i would expect a glow engine, a large 4 stroke especially, to run at 80-120'c which is not too far off what you would expect from the engine in a car. You wouldnt give that a cuddle after a blast up the motorway so why would you poke your model engine?

So, my top tips for glow engine long life:

Choose the right size engine. Too small and you have to hammer it, too big and it sits at idle
Choose the right fuel. 5% nitro, 10-15% synthetic oil. No castor!
Tune for maximum performance. Note that this does not mean needle twiddling for hours and hours. Tune it, check it, fly it. Its a 30 second job.
Provide proper cooling. Cutting a gaping hole is not cooling. You need baffles etc to do a proper job.
Dont spare the horses. Give the engine a bit of a spanking every now and then. As a friend of mine likes to say, thrash it to within an inch of its life...and then another 5/8
Use the engine all the time. Storage kills mechanical things. Try to keep them in use as much as possible.

[Engine Doctor]

As your Saito was designed for use with methanol fuel then changing it to run on petrol has caused the valves and seats to erode due to the hotter exhaust gas eroding the valves and seats . Classic cars and motorcycles suffer from a similar problem of valve seat regression due to the use of modern fuel/petrol not having any lead in it. The lead would act as a lubricant and stop the valves and the standard softer seats sticking or being damaged by the excessive heat . The cure is to have new hardened valve seats fitted that can withstand the temp of the new fuel without the lead . The bronze seating fitted in the Petrol Saito' is there for that reason . If the valve seats are not too damaged/regressed (any pictures ) then a new set of valves may fix the engine but the cure then will be to run it on methanol in future . You could still use the spark ignition with methanol for added reliability and get more power than petrol .

New cylinders for Saito' are expensive as head and cylinder is a one piece casting  so fixing your engine will probably cost £250 plus Unless you can find a cheap breaker with the parts you need .

OS engines have bronze valve seats as do other FS engine makers . Not sure though how they would stand up to use with Petrol as the composition of the seat material may not be up to use with petrol unless its purpose deigned for petrol.

Best of luck with re-build

Edited By Engine Doctor on 07/03/2020 14:20:05