Leak free gearbox joint

[Jono]

Any tips on the best approach to ensuring a leak free joint between the engine and gearbox? This is an A+ if it makes any difference.

I have noted that the original half moon seal has a metal core whereas my new seal is a rubber extrusion/moulding which is slightly over length when placed into the cut out on the box.

I was going to glue this to the block/crank lip seal using Loctite black grade sealant (for oil), leave overnight then apply another bead before offering it up top the gearbox.

Not sure about the paper gasket, but was going to glue this to the block as well using a smear of Loctite black and another smear on the other side before putting it onto the gearbox.

Any hints and tips as to the best method?

I've replaced the diff out put seals and gaskets as well as the rod change seal so looking for an oil tight box as far as possible.

Cheers

Jon

[Spider]

Apart from the usual checks on gasket faces (flat, no warp, dings, burrs etc), I use Loctite No. 3 gasket cement on the paper gaskets and also on the half moon seal. The other place to check is on the faces where the transfer housing goes. The sump gaskets are usually a little long here and need trimming back, the other thing is sometimes the face between the block and the gearbox are not on the same plane. If there's only up to about 0.005" difference, I'll use a silicon sealant here. Anymore and I correct them.

I can't say I've ever had an oil leak problem in either of these areas ever.

[rich@minispares.com]

I use black rtv and it works really well

The metal core u seals will soon be back in stock.

I had some nos originals and the sample is on test in marks hornet / minispares testmule.....

[minimans]

Many years ago we used to drill and tap the block to gearbox bolts to 5/16 and use Allen bolts. Is this still done? We did it to make the whole thing more rigid (At least that was what was said at the time) but it had the benefit of stopping any oil leaks due I suspect to the two faces not walking about.

[anis]

MED is selling a 5/16 block to gearbox set.

[mk1]

I'm not a great fan of 5/16 conversions.

On an old thin flange S block making really good power it "MAY" just have been worth it, but on a thick flange box, apart from the highest outputs & longest strokes I reckon its a complete waste of time & energy.

Also if you don't use cap head bolts it is almost impossible to do them up & undo them.

[Lakeland997]

Here's what I think the factory were doing on the last production models. This is an MPi box.
Under the RTV sealant lies a normal half moon gasket.
I followed their lead on my latest build and smothered the half moon gasket with RTV underneath and on top before assembly and so far I am leak free.
I did also use the MED 5/16 screws and copper gaskets.

IMG_3782.JPG

[Jono]

That photo exactly mirrors what I found when I cracked my engine (a virgin A+ lump from a 1992 RSP Cooper). Half moon, metal cored seal glued firmly to the box with black RTV, then a smear on the block side/crank lip seal. None on the paper block/box gasket but i will use some.

I'm a bit concerned about the new seal not being a metal cored item so I'm going to seal it to the block first with Loctite black flange sealant and then assemble the next day so it doesn't squish out of place.

As a separate point of interest I found that this engine has an MG Metro cam in it which was clearly the std fitment. Never went that well though, which is odd?

Jon

[GraemeC]

I wouldn't use the RTV and then let it set before assembly - it will stop the seal from seating itself in the correct position.
- Bead of RTV on the main cap, put seal in place (do NOT shorten it, even if it appears too long), second bead of RTV then drop 'box on top and do bolts up.

I've only ever fit paper gaskets between 'box and block with a generous smear of LM grease on each face. Never had that joint leak yet.

[Jono]

...I can see the sense in that Graeme, will take your advice.

My paranoia is based around building a few Lotus TC's and despite the most careful attention I've never got one completely leak free (though it is possible).

Cheers

Jon

[mk1]

For what it's worth, I use the same RTV silicon as Rich mentions above. A thin bead under & over the saddle seal (do not cut, it's designed to be compressed), a very thin layer on both mating surfaces, normal paper gaskets & all bolts torqued up as standard.

In my experience this seam rarely if ever leaks. the usual contenders for oil leaks are; diff oil seals due to worn output bushes, the bottom of the timing chain cover when its been bent out of shape in the past or the bolts have been over torqued or the threads stripped, the oil seal behind the crank pulley when the cover has not been correctly centered.

[abri]

A related topic...a friend of mine races down here in Cape Town and has had problems getting the gasket between the block and the gearbox to stay in place when doing up to 8000 rpm. He's done the 5/16 cap screw modification and tried all sorts of goo to keep the gaskets in place. We've checked the gearbox and block are both true. I haven't been intimately involved with the build but thought I'd mention the problem in this thread in case someone else has experienced similar or has some insight.

Thx

[mk1]

I'm pulling 8,000 or so in mine, & have not had any problems.

2 suggestions I would consider worth trying are Welseal gasket goo from Wurth & / or Copper gaskets.

One or both of these should help ease the situation.

[Jono]

...how about some additional locating dowels?

[minimans]

I've never seen these copper gasket's are they just thin sheet?

[Smiffy]

http://www.med-engineering.co.uk/produc ... gasket-kit

[minimans]

Thank you, answers both my questions there, 5/16 and copper shim gasket.......

[Spider]

RTV sealants can be good, but until they go off, they can also 'lubricate' seals making them slide or displace when squashed up. So you do need to keep an eye on them when fitting up for a few minutes that they are not moving under their own steam.

A related topic...a friend of mine races down here in Cape Town and has had problems getting the gasket between the block and the gearbox to stay in place when doing up to 8000 rpm. He's done the 5/16 cap screw modification and tried all sorts of goo to keep the gaskets in place. We've checked the gearbox and block are both true. I haven't been intimately involved with the build but thought I'd mention the problem in this thread in case someone else has experienced similar or has some insight.

Thx

Has he got plenty of crankcase breathers?

Also, IMO, bigger screws done up tighter on something soft like the gearbox case can cause higher distortion of the alloy case. I've see this over and over. The pressure from the fasteners needs to be spread evenly between the fasteners and not be high spot pressures. While not alloy, an example of this is timing chain covers, we've all seen these over-tightened and leak as a result. For the gearbox to block fasteners, I've only ever used the standard 1/4" fasteners and never found they need more.

[pdb]

the usual contenders for oil leaks are; diff oil seals due to worn output bushes, the bottom of the timing chain cover when its been bent out of shape in the past or the bolts have been over torqued or the threads stripped, the oil seal behind the crank pulley when the cover has not been correctly centered.

Another contender for an oil leak is the small speedo drive oil seal. Combined with a worn speedo pinion things get is even worse