I recently purchased an MVVS26IFS NIB never run but second hand if you know what i mean. I have run the engine once on a test stand for around 15 minutes, after that i took it off the mount and packed it away until the plane is ready. A couple of days ago i took the engine out of the box and had a look through the exhaust port and noticed there were some fine scratches on the liner wall which i thought was a bit strange. This enticed me to take the back plate off to see if i could find anything.
Well it looks like the crank pin and big end have been scuffing up the back plate quite badly and subsequently depostited some fine aluminium through the engine. I can clean the engine up and get the garbage out of there, but what would make this happen?
By the way this is the current red head version.
Cheers

Can you also supply information on the oil mix and oil brand? Propellor dimension?

Hi Pe
The prop used was a 16x10 zinger pro, mix was 25:1 using shell advance sx2 synth 2 stroke oil.

Cheers

Preamble:
I will answer this in a bit more depth than usual, because the problem is not typical for MVVS, but for the cantilever crankshaft construction used. This construction initially was used for low performance engines only. The advent of extreme high pressure synthetic oils allowed to use this construction for our high performance gas engines with their high running temperatures. This is not the place to write a book on oil; others have done that. Let it suffice to stress the use of the best oil you can get for the engine, without asking:"will this oil do as well??"
These engines need a fully synthetic two stroke oil exceeding JASO FC class. Period.
end of preamble.

The prop you used seems OK for running in (see ad2). So was the mix.
I suspect two things:
1) Oil quality. and
2) Initial high loads.

ad 1)
MVVS demands JASO FC exceeding fully synthetic racing oil, and my experience is in line with their requirements.
I have seen engines go to pot using inferior oil running just a few quarts of fuel. For your information, I include the Shell specifications below. Only the shell racing X2 fulfills MVVS demands. The SX2 type is just JASO FB class, and semisynthetic (synthetic fortified), and lacks the high pressure additives and scuffing protection that the X2 racing oil offers. Saving on oil is a dead-end philosophy.

Racing X2
A unique, Ester/PAO fully synthetic, 2-stroke extreme performance engine oil, specifically formulated for the most extreme race conditions. The highest levels of performance and protection is demanded by world GP race teams that use and recommend Shell Advance Racing X2. This ensures that whether you're involved in motorcross, supercross, road racing, trials, go-kart or off-road endurance, you can be assured of the quality and extra edge Shell Advance Racing X2 can provide. Pre-mixing ratios up to 50:1 are achievable after consultation with your manufacturer's handbook. Available in 1 litre.
VSX2
A fully synthetic super high performance, JASO FC rated 2-stroke motorcycle engine oil that gives superior protection for all modern 2-stroke motorcycle engines. Shell Advance VSX2 is formulated with polyisobutylene synthetic fluids that ensure a low smoke, low ash performance, superior lubrication and engine cleanliness under high pressure.It also provides excellent resistance to exhaust system blockages. It is suitable for both air and water cooled 2-stroke engines with oil-injection or pre-mix systems.Pre-mixing ratios up to 50:1 are achievable after consultation with your manufacturer's handbook. Available in 1, 4, and 20 litre.
SX2
A synthetic fortified, JASO FB 2-stroke motorcycle engine oil that provides extra performance and protection for hard working 2-strokes that require smoother, cleaner performance. Suitable for both air and water cooled production motorcycles with either pre-mix or oil injection systems, especially older 2-strokes that experience ring sticking and piston scuffing. Pre-mixing ratios up to 50:1 are achievable after consultation with your manufacturer's handbook. Available in 1, 4, 20 and 205 litre

ad 2)
The conrod in this construction type sometimes rubs against the backplate, mostly in the top of the stroke part. In addition, Aluminum against aluminum is not the best material combination for wear prevention. Part of the purpose of running in, is to "harden" the aluminum surfaces and prevent fretting by lightly rubbing them together. This process is supported by lighter loads, and the right oil, so some kind of plating is achieved. Increasing the load on the engine step by step allows this to take place, and the backplate will then show shiny rub marks without any fretting or metal flaking.
This way you teach the engine to work, and the engine will break in without being broken.

HI Pe
Once again thanks for the in depth explanation, I will loose the SX2 and source some of the Racing X2, beleive me i am not trying to save on oil, generally i always use the best i can get. I will rinse the engine out using what was left of the fuel and run it up with a fresh mix.
I have included a picture of the backplate.

Rinsing is a good idea. It will also get rid of the old oil.
Smooth the marred backplate face with a file. Just smooth it. there is no need to remove all scoring marks, because they fit the conrod wear pattern.
Do not use sandpaper. Minute corund particles may embed in the aluminum and play havoc with your work.
A slight burnishing operation using the new oil and a smooth hard steel surface will harden the aluminum surface, and prepare it for the next running in. Wipe clean, and apply a fresh coat of oil.
Soak the conrod bigend bearing and ball bearings with oil before you re-assemble.