damaged gasket mating surface- expierence people plese help

[themadatter]

Hi there,

I have a cb750.
to cut a long story short, i have damaged the mating surfaces on the crankcase and the bottom on the block.
at the worst spots; the damage is perhaps 2mm thick, by 1.5-2mm deep, all the way across the surface.

Will a high quality, hig temp, instant gasket save me ?

The worst area is around the cam chain tunnel.
Im guessing there should be little or no pessurisation in this area ?


When I had the old cg engine, somebody had put alot of the red instant gasket stuff around the head to try and solve a head gasket leak.
Fair enough, thats daft- especially the fact that they just squezed it round the outside of the barrel.
I will NEVER forget how utterly resistant that bloody stuff was though.

anyway- If anybody can offer advice on the correct thing to use for this application then you help is NEEDED !


Jamie

[the grim reaper]

The best way is to get a proper ally welder to add some material and then file it back to the original level material. The problem with the instant gasket type stuff is that bits of it can break off inside the engine and end up blocking oilways. If the area you need to repair is 2mm thick, this is a real possibility.

Cheers

Grim
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[stinkwheel]

That is a pretty damned big bit of damage. You might be better off getting a scrap barrel for one thing. Skimming that much off the base of the barrel is probably going to bugger up the stroke, you'd land up bumping the compression through the roof.

What springs to mind would be to get the crankcase surface skimmed and a replacement barrel. Then fit it with two base gaskets or one extra thick one (copper sheet?) to decrease the squish back to something more realistic (although it would be pertanent to measure the squish when you get it back together again).

If all that sounds like too much time and effort. People have done amazing things in the past using low temperature/chemical metal repair products such as lumiweld or JB weld. I have seen dirty great holes in peoples crankcases repaired using this stuff. If you thoroughly degrease the area, build it up with chemical metal then sand it down level again, you might get away with a thin layer of gasket goo and a good thick paper gasket.

JB Weld works like araldite, mix the two tubes to form a glue then machine/sand/drill/tap when it is dry. Lumiweld is like a brazing stick that you can do with a normal blowlamp.

I have heard of both being used to fix major engine componants (even cracked heads and stripped sparkplugs). It will either fail immediately or work forever.
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[ZRX61]

Get some runny eopxy & use that to fill the voids. I've used it to fill a hole the size of ya thumb in a crankcase before...

[Guest]

What they ^ said - try some metal filler type stuff.




And next time, don't take the barrels off with a hammer and screwdriver

[themadatter]

thanks for the advice guys.

If this bike was a mint classic bike then i would not have used a hammer and chissel-
So I will go for the cheap option.

That Jb welding stuff sounds good.

~Thanks Guys- as always !!!

Jamie