Tri-z base gasket blown out?

[1972mercury]

I got a 85 Tri-z with a fully rebuilt motor with some minor mods from the yamaha wrench report like raising the exhaust port a little and shaving the head with stock carb and dg pipe. Didn't even have 3 hours on the engine until it died on me and noticed it lost almost all compression. Then I noticed the base gasket just below the pipe was blown out. I followed everything to torque specs when putting it back together. How or why would this happen?






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[El Camexican]

Unfortunately that gasket has very little effect on your compression, but it may have lead to the failure. Time for a tear down. Make sure you get the base of your cylinder turned and that your cases are flat. The cause of total loss of compression will have to wait until you get the head off.

Do a leak down test after your rebuild, it is a must to avoid these things.

If the cases use a gasket it should be trimmed flat with a razor and on all types od cases I like to smear a little case sealer at the mating points of the cases as I have found leaks in those spots on more than one occasion.

[christph]

It's hard to believe it blew out like that if the gasket was installed correctly and the base nuts were torqued down. The pressure in the crank case isn't even that high, at least when compared to the cylinder. I would suspect either incorrect installation, loose base nuts, or very uneven surface in that area.

[barnett468]

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WARPED CYLINDER AND CASES

The TRI Z's have a design issue that can allow the cases and/or the cylinder to warp . If either or both are warped enough and the gasket is not glued to the cases, it will in fact blow out which is why it was suggested to have both checked.

It can be difficult to check them depending on where they are warped, especially if they are warped diagonally . One quick way to check to see if there is enough warping in either that it may cause a problem is to first thoroughly clean the gaskets surfaces but be careful not to scratch them . After the surfaces are clean, set the cylinder on the cases without a gasket, then try to gently push a .002" feeler gauge between the case and the cylinder . If it easily goes in at any point, something is warped . If you find an area that is warped, keep installing thicker gauges until you can no longer easily slide them in . If an area is warped around .004", I would consider fixing it . If an area is warped more than that, I would definitely fix it.

If it is warped and you want to fix it, I would start by checking the cylinder for warping first . One way to do this is to put it in a lathe then use a dial indicator and check it for "run out" (warping/distortion) . If it is within around .002" of being flat, it is not necessary to machine it providing the cases are not warped more than around .002", however, since it will already be in the lathe to check it, I would get it machined if it is out by more than .0015" because the flatter the better, plus it shouldn't cost much more to cut it since it is already set up to do so.

One method to adequately check the cases once you have determined the cylinder base condition is to set the cylinder on them without a gasket and make sure it is fully seated . You can install the nuts and lightly tighten them to do this if you want and then remove them . Then try to gently push a .002" feeler gauge between the case and the cylinder as mentioned above . If the cases are warped around .004", I would consider getting them machined . If they are warped more than that, I would definitely get them machined, but this is a major ordeal and fairly expensive.

In general, the thicker the gasket, the more it will compress if the same material is used, and most of the Japanese made 2 stroke base gaskets I have seen will easily compress enough to seal a gap of maybe .004" at the most . If you are on a budget and your parts are warped enough to require machining and you don't have the money to do it, you can use 2 base gaskets instead of 1 . Using 2 gaskets is not ideal or what I would do but it will work unless the parts are excessively warped . It will also lower your compression slightly and alter the performance slightly.

I would also put a very thin layer of gray high temp permatex silicone between all the surfaces after they have been thoroughly cleaned . If you put too much on it can squeeze into the cylinder so much that it might break off and get into something.

I would also not tighten the cylinder base nuts any more than the factory spec and I would recheck them every couple hours at first as they may loosen slightly at first, especially if you use 2 gaskets.


JETTING

Unless you know that it was properly jetted, you must check the jetting . If you don't know how to do that, I and a few others here can tell you how to do that and one tip is too start rich then jet it leaner a little at a time because running it rich for a short time will not hurt anything but running it too lean will cause some damage.


INFO REQUIRED

For now, you can post a photo of the porcelain end of your spark plug from a slight angle and the numbers on the plug if you want . You can also post the fuel and oil and ratio you are using.




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[Dirtcrasher]

All I can say is AKA Ronnie use to tell me to use a center punch and slightly dimple the bottom end where the cylinder rests. The 250R has a thin area I always did.

Barneys got it right though, warped is warped and won't ever seal correctly and cost you lots of parts and repairs...

[John Tice]

Seldom does a 2 stroke cylinder warp enough to warrant facing off the base gasket surface. This is a 400 Maico which we welded on a double DH reed valve unit. This particular cylinder warped around .020” or so. When the base is machined an expanding mandrel is a must.







Case alignment is a different situation. Sometimes a straight edge is a handy tool to check misalignment.
Dimpling the bottom of the cylinder might be a band aid repair at the race track.

[1972mercury]

Thanks guys I'll tear more into it today the jets I was running was a 480 main and a stock 45 pilot the stock main was a 470 it still had the winter jetting in it yet. Here's a picture of the plug










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[3 Wheel Drive]

I have a few heavy duty base gaskets made by Sprock Racing. I bought 5 of them from Mosh back when they sold Sprock Racing parts.

They are thicker than OEM Tri Z gaskets.



$30.00 each, plus the ride. Pm me.

[yaegerb]

Something isn't right. I see ring scoring on the intake side of the sleeve. Was that a fresh plug?

[1972mercury]

Something isn't right. I see ring scoring on the intake side of the sleeve. Was that a fresh plug?

Yeah that was the only plug I had in it was at almost 3 hrs on the engine. I seen the scoring myself on the intake side and some on the exhaust side in the piston too.


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[El Camexican]

I have a few heavy duty base gaskets made by Sprock Racing. I bought 5 of them from Mosh back when they sold Sprock Racing parts.

They are thicker than OEM Tri Z gaskets.



$30.00 each, plus the ride. Pm me.

I still have the one you sold me. It's nice quality, but the thickness is identical to the one that comes in the Vesrah kit, so I used that one.

[El Camexican]

Something isn't right. I see ring scoring on the intake side of the sleeve. Was that a fresh plug?

Looks like he's got a wicked burn on the piston just to the right of the exhaust port. I'll guess the piston is melted away above that dark strip.