OS head gaskets - 40 Surpass, 46 SF, 61 SF

[Nigel R]

Afternoon chaps

 

I've recently been refreshing my box of older OS lumps (FS40 Surpass, 46SF & 61SF). Not one of them seemed to have a cylinder head gasket. Including several duplicates of the same motors I bought as parts donors. Yet I note the parts list for all these engines includes a head gasket.

 

Question is, where they usually supplied with a gasket? Was it something added to allow high nitro? Do I need the gaskets? Are the surfaces good enough to mate without them?

 

Before discovering this I had already swapped bearings, O rings and whatnot, and run all of them, so they seem to go OK without one. But if they are meant to have one I had better get some.

 

Any information gratefully received!

Edited March 23, 2021 by Nigel R

[Martin Harris - Moderator]

Check inside the recess in the head very carefully - they often stick in there and being aluminium, blend in particularly well!

[Richard Acland]

As Martin said it can appear that there is no gasket, but in fact it can appear to be part of the cylinder head itself.

[Don Fry]

Not broke, don’t fix. If it goes well, bow to the God of good motors.

[Engine Doctor]

As already said the gasket/shim quite often sits in the recess and looks like the machined  surface of the head. Leaving this shim out on some of the four-strokes is a contributor to pre-ignition and prop throwing .

[Nigel R]

Thanks chaps

 

I will check again... 99% certain there is nothing there. But will check.

 

I'm getting the feeling the thought is, if the gasket is listed it should be fitted. 

[Jon H]

a quick and dirty method of gasket replacement is to use some of that aluminium foil tape. Stick it to the top of the liner and trim with a sharp knife without scratching the bore. I did this on that busted old os 40 i repaired last year and its quite happy. 

[Nigel R]

Thanks Jon I will bear that in mind as an option to keep in the back pocket.

 

I've checked the two strokes again - definitely nothing there. I stumped up for some laser cut gaskets from icbim products, as they're only a few quid. The thickness (0.3mm) closely matches what I found reported in the RF Hanno review on sceptreflight (0.016" alu, which is 0.4mm ).

 

Will report back on fit & finish when they are here.

 

Haven't double checked the four strokes yet.

[Don Fry]

If I’m hijacking the thread, apologies. I’m an engine user, rather than a fixer of engines. But a user with curiosity.

But why would an engine run properly with a part missing? 

[Nigel R]

No hijack Don,

 

Now you got me wondering again.

 

So I took a fine pick and on one of the 61s I found I could lift the gasket that I didn't think I had (!)

 

I'm off to triple check the 46s and the four strokes now...

 

I feel like I have 'just enough knowledge to be dangerous'.

 

I don't think I qualify as 'fixer' yet either!

Edited March 24, 2021 by Nigel R

[john stones 1 - Moderator]

Bit of gunk holding it in Nigel, even Poirot would take an episode discovering the truth. ?

[Nigel R]

?

 

OK, I'm officially dangerous and shouldn't be let loose near engines. Let alone near the internet to admit it - mods please delete this embarrassing thread!

 

Both 46s had a gasket. More time carefully picking at the other 61 revealed one too. I'm betting I'll also be finding one in the four strokes and now questioning whether its worth me even bothering to disassemble them to check!

 

But my goodness they don't half get stuck up there in the head though!

[J D 8]

Don, in a model engine the gasket/shim not only helps seal the head but when in place lowers the compression at tad so it runs at its designed ratio. Even with no shim the seal may be good enough and the compression ratio close enough for it to run, just not 100% performance.

  In days gone by when car engines were often rebuilt a warped head would be skimmed to get it flat again for a proper fit, sometimes done more than once, this would up the compression ratio so a thicker head gasket would be fitted and if all else failed one could try fitting two gaskets!

Cheers, John D.

Edited March 24, 2021 by J D 8
more info.

[Don Fry]

Cheers JD 8. That’s clear. That resurrected memories of watching my dad making over thick head gaskets for the hill climber engines he used to prepare. He reckoned the lower compression ratio made the unit kinder in power output, valuable in attempting a climb in a mud bath. I just didn’t realize our motors are so tolerant. 
 

[extra slim]

If you didnt spot them Nigel, alot us wouldnt have either.. we have all experienced much worse, but most never admit it!.. Good on ya for coming clean so we can all learn.