125 cylinder heads

[hullygully]

I'm building a stock GP125 engine that came with its original frame. My problem is, the engine is complete except for the cylinder head...............
I've 2 x 125 heads in my stash, 1 x central squish & 1 x side squish.
Deep in my Lambretta knowledge ( ) I seam to recall that GP125's had a central squish.....................but, I cant confirm this anywhere in any
of my info or books, or even on Google
Are there any adults out there that have this info pleeeeeeeeeeeeeeeeeeez

[Warkton Tornado No.1]

Please await confirmation from A N Other, but I think you are correct in thinking that the head utilised was a concentric squish.

It’s decades ago that I tore down a GP 125 engine to rob various parts - is there a less stressed engine? - but besides the gearbox, clutch etc the head could be converted to central plug with plugged & repositioned stud positioning to suit the large block.

I have to ask, though, how far are you prepared to take the intent of as original OEM? If the whole machine including the engine is intended as a museum piece, then I fully understand the intent. In fact, I might be able to help you out with a John Bull tyre

On the other hand, if the intent is a useable ride, then I would imagine compromises will be taken. Besides the tyres, personally, I reason it’s best to optimise the set up with the better head, discard head gaskets to help in getting the best squish & even be prepared to machine the gasket face & combustion area to achieve a good compression ratio.

Presuming that OEM points ignition will also be fitted - for reliability* - also consider fitting the J range flywheel to the flywheel boss. Not only does the lighter flywheel reduce stress, the decrease in inertia makes for a much nicer ride……


* For those readers that doubt the reliability of a properly set OEM contact breaker ignition set-up, just check this here Forum for proof of the reliability of the aftermarket electronic systems. It’s littered with the issues that such systems present & the ‘insurance’ required of riders taking spare pick-ups, resorting to external pick-ups etc etc.

Ba Humbug

[Adam_Winstone]

Yes, GP/DL125 uses a concentric squish head.

[hullygully]

Cheers Adam & WTN1, it'll be the concentric squish then, plus using an electronic setup, but with an AF's light flywheel, but I want to source a thin ringed 125 piston

[Adam_Winstone]

I'm pretty sure that Knowledge (Martin Leech), perhaps a.n.other, undertook a project to see what he could get out of a 125 so may be able to give you info on what pistons he looked into. I have a feeling that the project went down the reed valve route so you might need to factor that in.

TBH I imagine that many have looked into such a project, especially back in the day of 126cc learner limit so would expect many to be able to chip in with info.

WTN1 may have detail of racer's configurations for smaller cc class motors (?).

Good luck with the quest and please keep us informed of progress.

[Warkton Tornado No.1]

As I touched upon the extent of the ‘as OEM built’ to which we all have differing views, I would not recommend tuning of the 125 barrel if other components are not considered. In any case, we’re probably second guessing what this particular remit amounts to.

That said, tuning a 125 barrel for the fun of it was some subject for debate. As for tuning them for racing, I can only recall one circuit racer ever attempting to take on the vesp@s & that was Keith Terry with the Autisa kit designed for the large block. I have one of the kits & in the (very) unlikely event that I were to tune one retaining piston porting - for fun - I’d look to change the configuration from 54 x 54 to under-square as Innocenti OEM