Returning My G3C to 350cc
Posted: Sat Aug 26, 2023 4:13 pm
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.
- -
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.
- -
As always, setting up took way longer than the job, but the result looks servicable.
- -
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
-
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.
- -
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.
- -
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.
- -
As always, setting up took way longer than the job, but the result looks servicable.
- -
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
-