Engine rebuild problem "help required"

Piston problem-. I managed to source a piston I believed would be ok, to rebuild the engine I started my racing career off in the late 70's the motor is a piston ported reed valve motor, this always required us to re peg the piston ring further round the bore to miss the top of the inlet port, that has a small boost pot, this had issues and we suffered a number of pegs comming out.

I believed I managed to source a piston that does not need this mod - its a go-kart TKM piston on a 54.mm bore with .5mm oversize, a perfect fit or so I thought!, This piston does not need re-pegging as the piston ring is pegged on the c/l and located on the very top edge(dykes), so does not drop into the to top of the inlet port window, at btmdc.

BUT !!!!!!!
now that I finshed finally assembling the engine/ pressure tested/ over come all the usual little issues and problems do I find this gem.

problem the piston is sitting too low in the barrel, like its 2mm lower at BTMDC below transfer btm edge, giving wild port timings ("O"no ) its 2mm shorter on the crown height than a std cr/rm125 piston. !!. ?

so Im facing a dilema.

If I can not identify another piston that does fit the criteria the only option I can see is a lengthly one, to strip the lot and re machine the top spigot face of the crancase 2mm off to lower the barrel. ?

I can get this done but im struggling with the time scales to do this in time,

can any one help point me towards an altenative option or piston with the similar deck height as a 125 RM or CR ka3 honda, it needs to have the peg on the rear c/l with a dykes ring on very top edge.

Not sure one exsists but if anyone can help please let me know.

Regards Derek.

Hi Derek,
Alternatives you have...
1. Machine the base of the barrel instead of the crankcases, but you may need to remove the liner, or spin the lot in a lathe.
2. As you say an alternative piston with C/Line peg.
3. An alternative peg position to miss the ports.

Piston suggestions:
YZ125 is 20 degree off set (and some versions are centre line). would that miss the ports?
Husqvarna 125 is centre pegged and very similar in crown height to the YZ125.
None of these pistons are Dykes, but why bother with Dykes anyway? I don't know that anyone uses them these days.
Barrel suggestion:
Why not braze/weld a tonsil into postion to support the ring peg area?

Is this any help Sir?
Cheers,
Alan

I have a boost port from the back, so can not braze a tonsil in central position, but may be able to put one in where pegged to side. klook at this

The barrel can not be machined the thickness between btm inlet port and base already very (too) thin.

The yz 125 may do it at 20 degrees how far round from the c/l is 20 deg 15/18mm ??. is this a 54mm or 56mm piston

The only reason I picked the TKM piston is because the dykes ring is located at the top edge and still in the boost port.

Regarding the details