Crank case to block studs

["Slack Alice" Simon]

An engine on loan has come to grief.

The rear stud holding the block to the crankcase has snapped, along with the three on the oil filler side of the engine.

The tappet side and front studs are intact, though some of the nuts were loose.

I cannot find anything amiss in the crankcase - no little end bolt floating around, for example.

The nuts were checked (not by me) not long ago.

This is the first engine I put together with a silicon block to case gasket.


Why would this happen - at the very least I would expect trouble to come first from the tappet side.

I tend to under tighten, rather than over tighten, and the other person used the traditional small spanner approach. 


The only thing I can suggest is that the studs were over tightened earlier in the engine's life - I doubt if I replaced them during a hurried build up.

But why not the tappet side snapping first?


Any ideas?

Simon

[Chris KC]

You may need to provide more detail Simon, e.g. are you using locknuts? Running a blower, or a silly compression ratio? (In my book that's anything over 6:1).

I've never broken one of these studs, but the joint is heavily inclined to separate, one way or another, no matter what you do.

[Ian Williams]

Obviously past history of the case studs could be a factor, however it is my belief the the quality of SOME replacement studs is not good enough. In the past when being lazy I have used off the shelf studs and on occasion experienced rapid failure, I now never use studs where I am not 100% sure of the grade of the steel I have made them from and have not experienced further problems. I have also used a number of silicon gaskets and suffered no problems as a result, even with reasonably high CR and high lift cams. I assume you checked the top of the case was flat, they are often not, and you rectified as required, I also assume the case was not unduly loaded by the chassis?

[Alan]

My guess is they’ll be fatigue failures. The joint is subjected to cyclical loading, and to withstand this the clamping load needs to be greater then the load variation. If it’s not, two things usually happen. First the nuts unscrew themselves. Secondly the bolts fatigue rather quickly. Either of these might happen first. The greatest loading will be on the valve chest side so I imagine these loosen first which makes matters worse by putting a bending load on the others. I’m a bit worried by the resilience of the silicone gasket too - I stopped using them.

[Austin in the Shed]

You want studs of grade EN 16 minimum preferably EN24 if you ask your supplier.
60's  Triumph motor cycles I believe used studs of EN16 as a comparison.

[Speedex750]

Hi SA Simon,

I like most of us I suspect have suffered or experienced something similar. Undoubtedly the quality of the studs can be a factor in this but I've come to my own conclusion that the stud shortness is another factor. Getting some strain into the stud to hold the joint shut without over straining the gripped length is a difficult balance. Undertighten and it only needs a little expansion to lose the tension, overtension and you could put the stud into plastic deformation - and on a short stud that difference could only be a couple of flats rotation of the nut

I'm in the 'glue the block down' camp after my last (disastrous) failure. I also add a spacer collar under the nut to extend the gripped length slightly and Locitite the nuts in place that have been tensioned using a crowsfoot torque wrench. I'm also experimenting with conical washers that extend the amount of strain.

So far so good!

Dave

["Slack Alice" Simon]

Thanks everybody.

It is a standard engine on rubber mounts.

I think weakened/poor quality studs are the answer.

I was wondering about the contribution, if any, of the silicone base gasket.   It seems the jury is out on that.

Whilst looking for more baking sheets for gasket making, I came across some stuff called Viton.

Is this better than silicone?

Simon

[Ruairidh Dunford]

I have fitted over 40 of the silicone base gaskets now - all without issue.

I had problems with poor quality studs about 10 years ago - nothing since. If the studs are not straight (due to dodgy drilling/repairs), they will snap and I gave up using second hand studs in this are of fastening years ago - for the same reason.

[Ian Williams]

Some interesting comments, to clarify mine, my experience with poor quality studs was probably also in the region of 10 years or more ago, I now make my own and prefer 24T steel. I was originally concerned abut the resilience of silicon base gaskets and wondered if we would see increased stud failures due to cyclic loading, you may recall me bringing this up when they first appeared. I had previously been a fan of gluing the joint together, however I have now used a number of them, nowhere near as many as R mind you, and to date they have been a complete success. The one under the most stress will be my Nippy, this is in a 75 type engine still with 3/8 lift cam, driven very hard over 15k miles now and still going strong. Since my accident I have retired from racing so can not comment on their use in those applications, Alan of course may well have experiences in this area behind his comments.

[Alan]

Not at all, Ian. I’ve always glued my race engines together. My comments are based on an engine I recently rebuilt. Admittedly I hadn’t built it in the first place but when I took it apart, all of the nuts were loose and the gasket seemed very soft. I don’t know the provenance of the engine, who built it, or where the bits came from, but it’s enough to make me wonder. That joint is a critical one. I’m not sure that introducing what is essentially a soft rubber sheet into the equation is the right thing to do.