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Old 05-05-2014, 05:26 AM   #1
motobene OP
Motoing for 43 years
Joined: Mar 2013
Location: Wichita Mountains SW Oklahoma
Oddometer: 859
Pesky Base Gasket Leak - Cheap Fix

When I redid my 2011 Raga 300, I had to re-use the base gasket because I could not get the required ~0,33mm base gasket to maintain the very-close-to 1mm squish in time to ride an event. Also, I just greased the gasket instead of using sealer, because I thought I might swap top ends again (recall the engine size investigation). Apparently a couple of tiny bits of gasket were left on the cases when I removed the gasket, and they were scraped off in the rebuild, leaving tiny unloaded areas. Silicone 1 adhesive would have been a smarter way to reassemble with a used gasket. The grease lasted quite a while, but was not as good.

The Amarillo event (Comanche Cup) was very dusty, and I noticed some dust clinging to oil seeping from the base gasket in one place about 1cm long. A mere weep, so I didn't pay much attention to it other than checking cylinder-nut torques, which were fine.

Then at the next 2-day event, the Sooner Cup, I was experiencing odd changes in low-speed mixture. The bike would start to go lean in section. I attributed that to a sticking float. The weep was there again, but this time there were two small oily trackways in the dust indicating a larger leak. Curious, Sunday morning I started the motor and spritzed the area with my tire mounting fluid (water + dishwashing soap). Bubbles, and tiny audible suck and blow noises! Gack....

What to do? I had an event to ride. I rode over to Stoodley's fully-equipped trailer to hunt out some sealer for a temporary fix. Sure enough he had a fresh tube of my favorite clear silicone 1 adhesive. I cleaned the area thoroughly with brake cleaner (for good surface adhesion), then applied a thick bead along the base gasket seam, fingering it carefully for surface wetting and so it didn't look stupid. I made sure to cover the leaking area and more., and mounded up to ~8mm depth right over the leak.

I did not have to check on Sunday and had just maybe enough time for the stuff to through-cure before I had to ride. Half an hour before the event, a finger check found the outside cured, but the core of the thick area was still liquid. I repositioned the bike to solar heat the repair area. It just barely cured by the time the rider's meeting horn blew.

The result was quite surprising. Not the repair, but the fact that I had to change the air screw setting a half turn to get the bike to run right! The small air leak that developed had more of an effect than I could have imagined (it was my first base gasket leak, ever). The bike thankfully ran consistently throughout the event, so I wasn't as distracted Sunday by an inconsistency. The repair took so well that I will just leave it as is rather than spend time fitting a new gasket. It looks 'tacky', perhaps, but it works.

Critical to a good repair is a clean surface to promote full adhesion, and a thick enough coating to provide strength against air pressure fluctuations. Silicone 1 is a great repair substance because it is clear and will remain flexible. Later it will be easy to remove. Applied to an oily surface and too thin, the sealer will leak advance and pressure fatigue. Note how after a full event, oil has not started to migrate into the silicone at the gasket interface. I'm thinking it won't.

motobene screwed with this post 05-05-2014 at 06:08 AM
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Old 05-05-2014, 06:32 AM   #2
Joined: Mar 2006
Location: Black Bill Park
Oddometer: 300
Now thats what I call top quality cowboy engineering. I'll have to pick up a tube of that silicone for my travel box that now contains most of a second motorcycle. You have a photo of your favorite brand?
ATGATT: When you fall off your motorcycle, you will be wearing what you were when you got on your motorcycle. Dress accordingly.
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Old 05-06-2014, 07:16 AM   #3
motobene OP
Motoing for 43 years
Joined: Mar 2013
Location: Wichita Mountains SW Oklahoma
Oddometer: 859
Originally Posted by DerViking View Post
Now thats what I call top quality cowboy engineering. I'll have to pick up a tube of that silicone for my travel box that now contains most of a second motorcycle. You have a photo of your favorite brand?
Thanks. My penchant for detail, then these 'cowboy engineering' tricks tend to throw some folks off. But I see labor saving and low-cost solutions as just as eloquent as the fancier solutions.

There is no favorite brand. Dow, GE others. Much probably comes from just a few plants. This is long a commodity item. Once cured it is good for very high temperatures, like around 500 degrees.

Important is knowing there is a big difference between silicone 1 and 2. 1 is the original, in various cure systems, water, platinum, and the more common types that smell vinegar-ish when curing. Silicone 2 is great for construction applications, but I so far have not used it for motor applications. Silicone 1 does not take to paint, and loves to attract dust, so it's not a good choice for exposed construction applications. Clear is not good to use in the sun, because the UV will chew it up over time.

The other major difference is presence of particulate pigments, like black, white, silver, copper-ish, or not (clear), or metal powders. I just use the clear #1 and keep it around in a caulking gun tube, because I use so much of it. I even use it for threaded pipe sealer, and the rare PVC pipe joint that is not permanent assembly but requires future disassembly.

One thing to remember about using it as a gasket sealer is that it is so slick uncured you must not go to final torque on the bolts or screws. You can squoze a gasket out if you do that. Do like 25% of final to set the gasket and cure the silicone for a few hours, then come back and do final torque. This is a trick to have pretty much for sure no leak gaskets.

Another trick is to use silicone one on one side of a gasket, like the better quality primary cover gaskets, on the side you wish to keep the gasket on, and grease on the other side where you want the gasket to separate cleanly.
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