Blown head gaskets

[vicvega]

I'm running a GT 206, and have blown 2 head gaskets in 2 months, same place. Does anyone know the most probable reason for this or miracle cure!

[Diablo]

Where are you getting the gaskets from. MBs ones are better than most.

[vicvega]

Just ordered a couple from MB, best safe than sorry, but to go in the same place seems like somethings wrong, One was an old gasket that I had knocking about which came in handy when it blew, so could be coincidence bloody unlucky if so. Gonna check carb settings, mikuni 30, but wondered if anyone knew of anything obvious has run perfect apart from this. (kiss of death)

[TS1-200]

Head slightly warped ?, try getting a sheet of wet & dry 240 grit put it on something flat like a piece of glass and rub the head on it in a circular/ twisting motion, keeping it flat on the paper, you will see if the head is warped, if it looks like there are areas on the sealing surface which aren`t flat just carefully keep doing it until the surface is all the same colour i:e flat, I can`t think of any other reason why gaskets will blow,and use a torque wrench to re-tighten the cylinder head nuts to around 15lb.ft or whatever is recommended.

[Rude De]

And Surface the top of the cylinder too, the same way as the head

[Fruitjuice]

And check the head nuts again after 200/300miles after you've built the top end and torque them again...

[jonny snatchsniffer]

lap the head onto the barrel and do away with the head gasket, id rather use .5mm on the base to get the squish right than use a head gasket, unless your port timings are mad

[cezeta]

lap the head onto the barrel and do away with the head gasket, id rather use .5mm on the base to get the squish right than use a head gasket, unless your port timings are mad

do you use a sealant or not?

[vicvega]

It always takes someone else to point out the bleedin obvious. Will check the head I think I've got an MB one somewhere so will see which is best. I don't usually use a sealant on the head gasket, this is the only scoot that has done this though, which makes me think it probably is the head. Hopefully then shouldn't be to hard to remedy.

[Rich_T]

Do not be tempted to run a 206 without a head gasket, it will be over compressed (unless you a custom head and someone really does know what they are doing with squish volumes). Check the head as explained on a previous post.
Also check the head volume compared to the manual, 206cc get a bit critical on the compression so, if anything go on the safer side, and by that I mean add 2.5cc to the head volume (about 28cc or so, the profile for 125cc side squish heads will give you a safe volume).

Do not be tempted to flat the top of the cylinder 1. it is iron and will take forever 2. the top of the cylinder is machined directly off the datum point (base gasket) with a CNC mill, it is as exact, flat and true as you can get, it also leaves nice semi circular cut marks which are great for sealing against a head gasket (flatting will remove these).

Personally, my hunch is a slightly off cylinder head or head not torqued down tightly enough. On a note about tightening down, a lot of lambretta engines do not have the studs tightened all the way in. When I build GT kits (and I've done more than a few) I remove the studs, clean out the M8 threads with a good tap and then sink the barrel studs back in the full length. If you have not already done so, down load the GT build manual and have a good read through.