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PostPosted: Mon Nov 04, 2019 10:09 pm 
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On the street
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Joined: Sun Dec 18, 2016 6:15 pm
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Location: Northern British Columbia
Country: Canada
Bikes owned: 72 GT750, 74 GT750, 75 T500, 76T500, 09 DR650
I will shortly begin re-assembling the engine for my '72 J. The cylinder block has been vapor blasted and bored to 0.5mm oversize. The cylinder head has been given the lightest possible skim to address a 3 mil warp, and crankshaft rebuilt with new seals.

My questions:
What is recommended practice for when refitting a skimmed head. The 'skim' on my cylinder head was to fix a 3 mil warp and the skimming process probably removed about 5-8 mils of material. I'm not interested in improving performance and would like longevity and reliable operation. So, i don't need any extra compression. To make up for the lost head material, what are fellows thoughts on using two head gaskets?

The skimmed head has a smooth mirror finished surface that should not be a problem for gasket sealing. But the cylinder barrel isn't so pristine (see the photo below). Is it recommended to use a jointing compound on this less-than-pristine cylinder barrel such as Wellseal? Or, one of the Permatex products?

Olaf
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PostPosted: Tue Nov 05, 2019 6:18 pm 
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Joined: Sun Jul 13, 2014 7:55 pm
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Location: Greendale Vic, Australia
Country: Australia
Bikes owned: AS50, GS500, GT500, ex GSXR750 slabbie
5-8mm removed? Mils? Well I'd say you're going to need one mother of a cylinder packer! (Perhaps you mean 'thous')
?


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PostPosted: Tue Nov 05, 2019 6:53 pm 
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Joined: Mon Dec 13, 2010 4:32 pm
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Location: southwestern Pennsylvania
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Bikes owned: 69 T350 thru 75 GT750
joolstacho wrote:
5-8mm removed? Mils? Well I'd say you're going to need one mother of a cylinder packer! (Perhaps you mean 'thous')
?

Here in the US, a mil is either 0.001 inches (0.0254 mm) or a mother-in-law.

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PostPosted: Tue Nov 05, 2019 10:06 pm 
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Location: Greendale Vic, Australia
Country: Australia
Bikes owned: AS50, GS500, GT500, ex GSXR750 slabbie
Oh well that's why you never made it to the moon! :wink:


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PostPosted: Wed Nov 06, 2019 9:07 pm 
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Location: Illinois
Do NOT use 2 gaskets on the head. That is pretty much guaranteed to leak.

First question is how much metal was actually removed from the head. Those motors are really low compression stock - even early ones with low exhaust ports, so up to 20 thou or .5mm is fine. No changes needed and no gasket goo.

If they really took 3mm off, the the pistons would hit the head, so that ain't goin' to fly without a 3mm spacer under the cylinder which will make more power.

So that takes us back to how much did they take off?


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PostPosted: Thu Nov 07, 2019 11:50 pm 
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Joined: Sun Dec 18, 2016 6:15 pm
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Location: Northern British Columbia
Country: Canada
Bikes owned: 72 GT750, 74 GT750, 75 T500, 76T500, 09 DR650
Thanks for the replies.

In the English speaking parts of Canada the machining terminology is the same as in USA. The 5-8 mils (maybe a little less) that my local engine shop removed is 5-8 'thou' or .005"-.008" . In metric terms that's .13-.20mm or 130-200 microns. Visually, it's impossible to tell a difference between the 'height' of the skimmed head and a spare that I've got from a parts bike. (One interesting observation is that the casting quality on the parts engine (which is a '74) is considerably better than it is on the '72). My take-away is that engine runnability probably won't be affected by by the small amount of material that was removed from the head.

What are fellows' recommendations around using a jointing compound like Wellseal?

Olaf


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PostPosted: Fri Nov 08, 2019 12:26 am 
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Joined: Sun Jul 13, 2014 7:55 pm
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Location: Greendale Vic, Australia
Country: Australia
Bikes owned: AS50, GS500, GT500, ex GSXR750 slabbie
In over 50 years dealing with this stuff in the UK, Europe and America, I've rarely heard of the term Mils used in engineering parlance.

Thous as in thousandths of an inch yes, and decimals of millimeters yes.
Mils is commonly used in Fluid measurement, millilitres, e.g. 5mils is 5 millilitres.
The term MILS is too easily confused with mm or Millilitres IMHO.

My old American lathe displays nothing about 'Mils' Thous yes, because it's calibrated in inches and thousandths of inches.

?


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