Returning My G3C to 350cc

[RichardS]

In my other thread I am detailing a project to put together a late scrambles age-wise bitsa, but in parallel I am reverting my old '49 C trials bike to 350cc as it should be from it's current 500cc top end.

I got the bike frame as a "you may as well take that too" back in 1977 (together with a WD G3L) and restored it a few years later after scoring a couple of alloy engines in a barn-find while asking about Bantam bits - sometimes you just get lucky! From the disparate heap of parts I could only create a complete 500cc engine. Although there was a 350 barrel, it was badly damaged due to an errant gudgeon pin, which is a shame as it is otherwise virtually unworn. Also I had no 350 head at the time either so it was a done deal to make it a 500.

Fast-forward, good grief what the hell was that? Gulp, 4 decades, oooer where did it go?! And it's time to put things right.
Along with the scored barrel I have a brand new Std piston and this is where the first quirks show up. I need to rebore the cylinder and, as well as a brand new, un-run Standard piston, I also have a used +060 one.
The bore damage is about 007" deep so a +020 should clean them out adequately. Then turn the +060 piston down to match. There was some agonising over the appropriate clearances as published data varies widely.
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The unworn top edge of the bore measures 2.7195" and the brand new piston is 2.717" (fore-aft at the base of the skirt) which seems pretty snug. Even in an alloy cylinder 0.0025" strikes me as a bit meagre for clearance, but some reading on the WWW seems to broadly confirm that it is actually reasonable so I'll shoot for that.

I can't find any actual stated figures from Hepolite or AE and although Phil Irving's excellent 'Tuning for Speed' gives some numbers I am sure they assume an iron cylinder as the clearances are vast by comparison. So, if I aim for 0025" clearance (on dia) I can always increase it later if needed, I can't hone it smaller!

I don't have a cylinder boring machine, but I do have an entirely suitable lathe so I set up the cylinder on a face plate, clocked up the top unworn lip as the reference and bored it to +020 relative to that original unworn tip edge of the bore. I can now use the cylinder and a bore gauge as the reference for finishing the piston to 0025 below the barrel.
Set the bore gauge at -020 on the unworn Standard lip and bore it out to 000.
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All sorts of ways to do it, but that's mine
I know it's more conventional to finish a bore to match a piston, but in this instance the reverse is more convenient.
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As always, setting up took way longer than the job, but the result looks servicable.
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Next task was to reduce the piston, pretty straight forward job. I checked the diameter differences of each land and simply duplicated that based on the reduced diameter. I also deepened the ring grooves accordingly as well. After cylindrical turning, there is a neat 'trick' to approximate to a 'cam-turned' profile. Offset the piston by 010" in line with the gudgeon pin and then take a 005" cut. Do the same thing the other side and the result is a close approximation to a 'bought one' with aproximately a 60deg contact patch front and rear and the sides relieved by up to 005".

The marker hatch marks are just to check where is original and where has been relieved
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[RichardS]

So far so good. Can fit a 002 feeler and the piston doesn't drop, but I can't fit a 003" feeler. Good enough for Government work.
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I need to get some new rings, I can't use +060's in a +020 bore, even if I set the gaps correctly I can't imagine they last longer than a few seconds.

[Joker_Bones]

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[Joker_Bones]

I have a 350 ally barrel that has been sleeved back to standard... I don't know if the sleeve has moved a bit or wasn't fitted flush.

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[Expat]

Lovely work Richard. Not sure of the make of your lathe but I know and had used for years the multifix tool holder, a wonderful thing especially for multi tool changes when batch manufacturing.
Very interesting project you have there, hope we can follow your progress to see how it works out. Thanks.
Steve

[RichardS]

I have a 350 ally barrel that has been sleeved back to standard... I don't know if the sleeve has moved a bit or wasn't fitted flush.
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Ooer, I'd be a bit nervous about that. There certainly shouldn't be any alloy wall exposed to combustion gases and I think it may be tricky to get the head gasket to seal. What does the bottom of the barrel look like? If you have 1/16" exposed there you can be pretty sure it has slipped. My instinct is that the liner would be pressed in from the top so again I would strongly suspect it has slipped.

[RichardS]

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Great infor JB, thanks. Looks like I may be a bit on the tight side. 69mm (~ 2-3/4") x 0015 is 004" clearance, I have 0025 or a tad more.

Food for thought, but it doesn't remark on cylinder material (alloy vs iron).

[RichardS]

Lovely work Richard. Not sure of the make of your lathe but I know and had used for years the multifix tool holder, a wonderful thing especially for multi tool changes when batch manufacturing.
Very interesting project you have there, hope we can follow your progress to see how it works out. Thanks.
Steve

Thanks for the kind words Steve, I will post more as things progress.

For boring the barrel I used my Smart & Brown 1024. Similar in size and class to a Monarch 10EE or maybe a Rivett 1030 or a Holbrook C10.
I used a little Smart & Brown Sabel (reworked and upgraded Southbend 9" clone) to do the piston as it's in my shed at home, nearer than the workshop.

Tool room grade machine tools is another interest of mine - you might have guessed.

[Expat]

[Joker_Bones]

I'd never thought of there being different piston clearances for ally and iron barrels... the ally barrel has a pretty substantial core of what I assume to be iron, will it not behave the same with respect differential expansion between the bore and the piston as an iron barrel?
The chart also gives some typical values for piston ovality... it appears to me more at the at the crown end.

I agree, I in all likelihood the liner in mine has moved a tad, there is no ally to combustion gas interface, the liner is let within the width of the iron core. I understand a head gasket was not used originally on the ally barrel, the head and barrel were lapped together. I have tried that a couple of times but with little long term success and have fitted gaskets.