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Discussion Starter · #1 ·
Hello, I am putting my 1980 cm400t engine back together after having some work done on the cylinders, and buying some new oversized pistons. I also picked up a complete engine gasket kit. Here is my question, as I was putting the cylinders back onto the crankcase I noticed on the CMSNL website engine diagram o-rings that look like they go between the crankcase and the cylinders. I don't remember seeing these when i took the engine apart, but there are some o-rings in the gasket kit that look like they would go there. With the o-rings in place the cylinders are not able to mate very good with the crankcase (although I haven't bolted anything down yet). I feel like they don't belong there and before I go too far, I thought I would reach out and get some of your thoughts. Thanks.
 

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you need them. it will leak without them.
check the bottom of your cylinders. they are probly there. and realllyyyyy flat and reallllyyy old. unless they are missing.
 

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Yes, you need them. They stop oil weepage up between cylinder liner and cylinder block. You will ideally need a "tube" to install them well. How are the cylinder head inner holes and threads, all good and clean all the way down, 10mm x 1.25 thread and pitch from memory. How are the inner bolts with the built on seals, they are crutial also.
 

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Discussion Starter · #4 ·
Drydreamer, I am referring to the large O-rings that look like they go around the actual cylinder itself? see the attached picture (part #4).


o-ring.jpg
 

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The O-rings supplied in these kits are too large in dia and wont allow the head to pull down. LDR can help with with the right dia and maybe tell you where to find the correct ones. We had a post on this not to long ago you might check the 400 engine section.

Bill
 

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Drydreamer, I am referring to the large O-rings that look like they go around the actual cylinder itself? see the attached picture (part #4).


View attachment 278286
#4 right? Yes, you need them and yes the cylinders will be more difficult to fit with new ones in place. I must ask, did you remove and discard the old ones???:-? Your posts seem as though you did not know of the o-rings until time to reassemble the engine. If the old o-rings are still in there, they will further prevent the new ones from seating correctly. It's worth double checking! ;)
 

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Discussion Starter · #7 ·
Thanks Oupa...the old rings are definitely gone...the cylinders have been gone through bored, and blasted clean (they look brand new :grin:). I think I will wait to see if any suggestions come in as far as the correct size O-rings go.
 

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Have you measured and looked for generic o-rings? That's where mine came from. In fact, measure it (O.D. of cylinder and depth of groove) if you have means to and PM me. Since the min. quantity was like ten, I have extras.:grin:


EDIT - Oh wait! You said you HAVE them in your gasket kit didn't you? Nevermind. :-?
 

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I wil try to get into the shop today or tomorrow . I have a set that came in a kit and several old ones. I will mic them and give you the dia. I have two used heads I will take the O-rings off and see if I can save a good pair.

Bill
 

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Discussion Starter · #10 ·
Thanks Bill...hopefully the ones that came in the kit work OK. Looks like I will need to pull the pistons back out of the cylinder.
 

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O-rings in the kit are 75.8 x 2.55 mm. You need 75.8 x 2 mm . " Partzilla ' has them in stock. $4.29 ea. Part # 91302-283-003

Bill
 
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