Lapping

[Bluestoesonnose]

I'm just going through some old RD tuning stuff I followed when I owned a 350LC back in the late 80s, and it suggests deleting the head gasket and lapping the head to the barrels, has anyone tried this with a DT125R motor?

[Calum]

I doubt you'll get it to seal.

Back in the Air Cooled days, the engines were cast iron. As such they were less prone to warping. Aluminium will literally just warp.

The best you can get is a machine O-Ring into the head to eliminate the head gasket. The Rotax engines use these.

You have to be careful with how much compression you run, pre-ignition and detonation is going to creep in at these sorts of levels and kill your engine.

[Bluestoesonnose]

LC motors are alloy and it seems to be a wide spread practice with the aircooled and watercool boys back in the heydays.

[Calum]

@Bluestoesonnose I know LC motors are alloy, I was specifically referring to the AC motors since I'd expect this to be where it would work reliably.

But I've not heard anyone do it to DT's. I guess there must be a reason why it's not so common these days.

[Bluestoesonnose]

Not totally clear in your reply

[Calum]

@Bluestoesonnose I'm not saying it's not possible, I do question it's reliability. O-Ring modification would probably be your best bet. But the discussed factors should be considered.

[2stroketerry]

The O-ring conversion on a DT 125 using a 90,s YZ 125 O-ring , the tighter squish and less combustion volume really helped. On average I would say this mod gives around 1,0 a 1,5 HP. This will lower the squish from +/-1.35 to a +/-0.95mm

[Calum]

@2stroketerry That looks the tits. Who did that for you?

[2stroketerry]

Cost £30 to cut the groove from a machine shop in Southbank

[declan]

@2stroketerry I’m surprised you lot like the o rings I hate them with a passion I love the simple metal one like dt has I’m always having leaks and such from poorly fitted o rings even though I try my best to fit them perfectly

[Calum]

@declan That's your problem then isn't it. Poorly fitted.

I've not had issues with O-Ring gaskets. Usually a dab of grease to hold it in situ is enough to sort any problems out if you're having them.

[declan]

@Calum none of that worked for me they wouldn’t sit still the last one I did I used the lightest amount of super glue just enough to keep it in the groove and that worked any other time and it would always slip out an pinch the ring I should add this is on the cheap replica rs cylinders so the machining wasn’t exactly flawless

[2stroketerry]

@declan using glue is the last thing you want on an O-ring just a bit of silicone grease, but having a poorly machined groove the O-ring sits in probably doesn't help.
Yamaha use an O-ring on there best 125 cylinders the SP although don't actualy make that cylinder themselves.

[NINJA]

I seem to remember someone once said that applying a bit of petroleum jelly, aka the good ole Vaseline works wonders for keeping your 'O' ring in good order.

:grinning_face_with_smiling_eyes: :winking_face: :grinning_face_with_smiling_eyes::winking_face: :grinning_face_with_smiling_eyes:

[declan]

@2stroketerry the only bike I didn’t have a problem with was a yz 125 that was no issues what so ever

[declan]

@NINJA you’d know

[NINJA]

@declan Do not tell me that you did not laugh at that one???

[declan]

@NINJA sorry to say I didn’t i take my o rings very seriously I’m quite anal about o rings

[Calum]

Like with most things, the work is in the preparation.