Cub Pushrod Tube Gaskets

[old bones]

Could someone help with an answer to my silly question?. I have just purchased a set of the new type of tube gaskets, the set comes with a large black rubber seal which I assume goes in the bottom and two composite type gaskets of different thicknesses . The black one  was a right pain to get in as it seemed too thick but I eventually got it in with the aid of some silicon lube, Now for the silly question; what do you do with the TWO composite gaskets??

I would rather ask the question and get advice before putting it together and it leaking. (My cub has a square barrel and head)

any advice gladly received

[naichuff]

The only type I have seen are the black o ring at the bottom and a red o ring at the top Looked on my parts suppliers sight and that all he listed as individual parts or decoke kits so I assume the square head is the same as the round head since it fits on to a square barrel on one of my engines

[alan]

The push rod tube seals are the same for all cub engines.. and a pretty poor design !!! The triumph design is the black tube at the bottom and a red quad ring at the top. The problem is that there is nothing to stop the top ring popping out if it is over compressed. The "solution" that one supplier has come up with is to use a gasket in place of the quad ring. The gasket should stay in place, but in order to seal it needs to be quite heavily loaded by the compression of the bottom tube when you pull the head down. There is obviously a lot less give in the gasket than the red ring so an element of trial and error is required to get the right set up. Different thickness gaskets are supplied to accomodate the different length barrels and different thickness head gaskets available.

I would suggest you need to do a dry build to determine which thickness gasket to use, however I use a 70 shore viton O ring which is a lot less critcal on set up than the gasket, and a lot stronger than the red quad ring.

[old bones]

Alan, Many thanks for sharing the solution to my problem and also an alternative to it as well. I will do as you advise regarding the dry build and see what top gasket to use ,I must say that the bottom black rubber ring seems very tight so I guess it will not compress very much so perhaps I will start with the thinnest one first.

For the future in case the new type doesnt work that well and as an alternative to going back to the poorly designed originals where would I find a supply for the viton "o" ring that you use?.

Many thanks again.

[alan]

i use a BS124 O ring which is 31.42mm ID x 2.62 dia. In a static application you are looking for between 15 to 20% compression of the diameter. That means on your dry build without any thing on top of the push rod tube you are ideally looking for a gap of between 2.0 to 2.2mm when you compress the bottom tube.

You can trim the tube to achieve this if necessary.

Any decent bearing supplier should have them. Simply bearings here in the UK are good for mail order.

http://simplybearings.co.uk/shop/advanced_search_result.php?keywords=bs124

viton is the best material for this application, nitrile is OK, but avoid the epdm as it is not oil resistant.

Edited April 24, 2016 by alan

[old bones]

Alan,

Thanks for the info regarding the o rings, I seem to remember using simply bearings a while ago so thanks for pointing me in that direction again.

I tried a dry build with the bottom seal in position and nothing on the top, but, when I did up the head bolts  a bit the bottom seal was squashing out and deforming quite a lot and would certainly not seal.

I installed the old seal and could get about 2.5mm gap on the tube, there doesnt seem to be much difference in the old seal and the new size wise, except the old one is more flexible.

Do you use an "O" ring or two for the bottom seal as well as the top?

It seems I have wasted a couple of quid on these new type seals that will NOT fit, so it may be worthwhile experimenting with original on the bottom and an "O" ring on the top? what do you think.

Thanks for your time in answering my query.

 

[alan]

no problem.. the issue is that when you compress rubber you don't change its volume just it's shape. The beauty of an o ring is that it just goes oval, the quad ring and tube have to spread out..

so yes i also use an O ring at the bottom, BS022 25.12 x 1.78 . There are a few choices.. you can trim the rubber tube and use just one, or remove the rubber tube and use up to 3. Do the dry build with the O ring on the top to determine the best way to go. Again you are looking to compress the O rings by around 20% of the cross section diameter .. good luck

[old bones]

Alan, You are a star!. You must have spent quite a while on resolving this problem thanks again for all the info it is much appreciated. I will order the O rings and have a play around with the fitting.

Thanks again.