OSFS 1.20 Surpass Mklll

The OS 1.20 has just been removed from Simon's WOT 4 XL. A while ago, Simon had problems with the engine, it would start but wouldn't run cleanly. At the time it was thought that either the pump or the carburettor diaphragm was the cause. Both items apparently difficult to find. All I have done so far is set up the throttle servo and linkage, this had the engine starting easily and running well. However, at tick-over, the sound of the exhaust was drowned out by the sound of noisy bearings. Today being warm and sunny, it seemed like a good time for an engine strip down. Everything went well until I tried to remove the screws holding the cylinder head. After heating the head and tapping the screws with a small drift, most of the screws came out easily, only two are left, a 3mm buried in the area of the rockers and a 4mm just behind the glowplug. I can't move the 3mm as the only key I have is a ball ended one, and its a slack fit so can't be used. The 4mm screw is rounded over and the key will never grip it. I've ordered some new keys from Screwfix, all without ball ends so can be used in tight areas. The method I'm going to use to get the 4mm screw out was shown to me when I was an apprentice, its a bit brutal to the Allen key, but it seems to work quite well. The method is to use a grinding wheel, this is the brutal bit, the Allen key is rammed hard into the wheel (which should be spinning at full speed) Just a short sharp jab is all that's needed. The result is a distinct burr on the end of the key. Making sure that the damaged socket is completely free from debris, align the key with the socket and push it in firmly. If it doesn't work the first time, it usually does on the second attempt..
No wonder the bearings are noisy, the main bearing is a mass of rust, I assume the front bearing's the same as its been leaking nasty brown oil behind the prop driver.

The "4mm" screw was in fact a 3mm screw and matched the rest of the screws in the cylinder head. Don't know why I thought the screw was bigger. Perhaps today wasn't a good choice for engine stripping, I was lulled by the sunshine and needed something to concentrate on. Yesterday morning we had our third Covid jabs, I was OK until lunchtime, then things went a bit wobbly for the rest of the day. Only began to shake off the after effects late this afternoon.
I've just been to Screwfix for some cheap Allen keys, the burr on the key did its job first time, and now all the screws are out. As a precaution I'll replace all of them and re-tap all the holes before assembling the head.

They are NOT 3mm screws Brian - they are 3.5mm and as rare as hens teeth and brand new too! The threads in the cylinder were perfect when I replaced the head, and they were lightly oiled (no loctite or similar used)!!

No eBay APP ID and/or Cert ID defined in Kunena configurationsorry if i'm telling you how to "boil an egg" Brian but i would be be very reluctant to re tap the threads as you could very easily remove metal. I would prefer to just run a good bolt in & out. Also double check the threads which may not be M3 x 0.5 pitch

These hex drive bits would provide better performance on stubborn screws

THEY ARE M3.5 !!!!!!!!!!!!
THAT IS WHY THE M3 HEX KEY DOES NOT FIT !!!
DO NOT RE-TAP THE THREADS M3 or M4 OR YOU WILL RUIN THEM !!!!!
www.modelfixings.co.uk/high_tensile_steel_caps.htm do the screws if the heads are now trashed (MF-SC31 M3.5 x 20 Cap Screws (pack of 5) )

Thanks for your input Kevin and John, the screws are of course M3.5 X 20mm, I've found the relevant OS manual and printed it, Model Fixings have the screws, unfortunately they sell them in 5s, nuisance when you need 6. I don't know much of the history of this engine, or who did what to it. All I'm concerned with is getting it right and getting it running properly. It has been encouraging to find that setting up the throttle throw had the engine starting very easily, running and throttling quite well. There is an issue with the bearings, the main is very rusty and I assume the front bearing is the same due to the dark oil leaking out. The tappets are a bit noisy and are worn, perhaps caused by lack of oil. Repolishing the contacting surfaces should cure that.
I noted your second reminder John, I could read it without my glasses!
The reason I'm considering retapping at least one hole is that the screw which I mistakenly described as "M4", was rounded over and difficult to get out. Funny thing though, when it was the only screw holding the head on, the head was free to move although the screw was seized solid, the screw hadn't gone home fully as there was at least 2mm of head movement, hence I feel that something is preventing the screw from being fully driven home.