1980 CM200T MLS head gasket after overbore

[Kywatt]

Hey all,

I have a 1980 CM200T that I am rebuilding for my special lady friend as her first bike. I just received the head and cylinders from the machine shop after having the cylinders overbored 1mm. Everything looks clean enough to eat off of after having the engine parts vapor honed. I am very excited to get this motor together!!!

It is damn near impossible to find OEM head gaskets for this motor but I bit the shipping cost bullet and ordered a MLS OEM head gasket from CMSNL. The stock bore on the CM200T is 53mm and I bored it out to 54mm. There is a slight over hang of the new head gasket into the cylinders. I am thinking of using a Dremel tool to ream out the bore of the head gasket to about 54.5mm. My thinking is that when the head is torqued down, the gasket will squish into the cylinders some and the last thing I want to do is cause any hotspots.

Do you guys see anything wrong with using the Dremel tool to increase the "bore" of the MLS head gasket? Will this compromise the Viton coating at all? I am nervous to proceed until I have consulted with the professionals...

 

[LongDistanceRider]

The head gasket only compresses, it doesn't swell to the sides. The issue here is that you're talking about cutting into the fire rings which are there to keep the gasket from burning away as well as giving a good seal. This isn't something that is ever done.
Honda only offered +.5mm max oversize and the head gasket was designed/produced with that information. So now you're looking at a custom headgasket from someone like Cometic https://www.cometic.com/custom
I'm not familiar with anyone producing a MLS, Multi Layer Steel, gasket for the vintage Hondas although some of the racers are using copper head gaskets.

 

[Kywatt]

Yeah... I looked into copper gaskets but I have heard they are really meant for race engines that are expected to be dismantled regularly. I am looking for reliability and longevity which is why I am trying to use OEM gaskets as much as possible.

I understand about the fire ring on non MLS gaskets. Do MLS gaskets have a fire ring though?

Here is the one I purchased:
https://www.cmsnl.com/honda-cm200t-twinstar-1980-ausa_model1044/gasket-set-cylind-nas_12251kb4315/

 

[LongDistanceRider]

Difficult to say if there's an actual fire ring on that gasket, I do see that the edge is rolled like a fire ring and there appears to be some sort of sealer band. I would contact Cometic and see what they say about making a more standard fiber gasket.
For a daily or regular use engine I would be hesitant about the copper gaskets or any all metal gasket.