6 replies to this topic

[JonnyAlpha]

Hi;

 

I started a post a while ago in the Bodywork section and hot the answers I needed, the post is here:

 

http://www.theminifo...-repair-panels/

 

I tried ressurecting the post with another question however got no replies and as this is more of a technical question rather than bodywork thought I'd re-post here.

 

I am in the middle of repairing the back end on my Mini and now need to remove the Subframe to get at the underside, it needs a good clean up anyway.

Before I remove it I want to drill the holes through the Boot Floor where the Subframe rear mounting bolts go through however I have hit a snag.

 

To ensure that the holes are in the right place I decided to drill up from the underside through the rear mounting bracket on the Rear Subframe however even with the drill set as far out as possible the chuck hits the subframe on both holes.

 

 

 

Guess I need to buy a longer 8mm Drill Bit!!!

[tiger99]

I think you have answered your own question.  Actually, there is another way. Either use a centre punch which is a good fit through the subframe trunnion to mark the metal, and drill it with the subframe out of the way, or use the drill bit alone and twirl it with your fingers to make the mark. Either way, drill a small pilot hole, say 3mm, in the mark you made, and that will help centre the larger drill.

 

By the way, there are no "8mm" holes anywhere in a Mini, although 8mm is near enough in this instance, if you can actually get the bolts in. The bolts are 5/16" UNF grade 8.8, 1.25" and 2.5" long, and the holes would be a bit larger than 5/16", which is 7.9375mm, to allow some clearance. It would be normal to make them about 8.5 to 8.8mm.

 

Please don't use metric bolts and nuts here, because an M8 bolt has a coarser thread pitch than 5/16" UNF, which brings two disadvantages, it needs more torque to get the same tension in the bolt, and because the thread is deeper, the core diameter is less, so the tensile strength is less. In this case, the difference is rather small, and you would probably get away with it, but in other cases, not so, and it is easier to be consistent. Also, if your Mini, as many have, starts to be populated with a mixture of metric and imperial bolts, future maintenance tends to involve lots of skinned knuckles and stripped spanners and bolt heads. Yes, I know you never said you were using metric bolts, but some people reading this thread might think it is ok, when it is just not a good idea.

 

The trunnions will not rattle about all over the place as a result of having modest clearance in the holes, because with correctly torqued up bolts there is sufficient friction between the surfaces to prevent movement. Most engineering is done that way, not many bolts are directly loaded in shear, and those which are, are "fitted" bolts in reamed holes, not appropriate here.

[JonnyAlpha]

I think you have answered your own question.  Actually, there is another way. Either use a centre punch which is a good fit through the subframe trunnion to mark the metal, and drill it with the subframe out of the way, or use the drill bit alone and twirl it with your fingers to make the mark. Either way, drill a small pilot hole, say 3mm, in the mark you made, and that will help centre the larger drill.

 

By the way, there are no "8mm" holes anywhere in a Mini, although 8mm is near enough in this instance, if you can actually get the bolts in. The bolts are 5/16" UNF grade 8.8, 1.25" and 2.5" long, and the holes would be a bit larger than 5/16", which is 7.9375mm, to allow some clearance. It would be normal to make them about 8.5 to 8.8mm.

 

Please don't use metric bolts and nuts here, because an M8 bolt has a coarser thread pitch than 5/16" UNF, which brings two disadvantages, it needs more torque to get the same tension in the bolt, and because the thread is deeper, the core diameter is less, so the tensile strength is less. In this case, the difference is rather small, and you would probably get away with it, but in other cases, not so, and it is easier to be consistent. Also, if your Mini, as many have, starts to be populated with a mixture of metric and imperial bolts, future maintenance tends to involve lots of skinned knuckles and stripped spanners and bolt heads. Yes, I know you never said you were using metric bolts, but some people reading this thread might think it is ok, when it is just not a good idea.

 

The trunnions will not rattle about all over the place as a result of having modest clearance in the holes, because with correctly torqued up bolts there is sufficient friction between the surfaces to prevent movement. Most engineering is done that way, not many bolts are directly loaded in shear, and those which are, are "fitted" bolts in reamed holes, not appropriate here.

Thanks for the response, your right, as I was writing the post I kind of realised I had answered the question but thought I'd post anyway in case someone had experienced the same problem and knew a better solution but I think the easiest is longer dill bits, I'll have to jack up the rear end to allow clearance for the drill!!

 

I want to drill the holes before I remove the Subframe to ensure that they are in the correct place.

 

Also after I posted I was trawling around the forum and the web for more info on aligning / refurbishing / painting subframes and came across someone else being advised against using M8 bolts. I have already purchased a set of Subframe bolts and bushes from Minispares so hopefully I already have the 5/16 UNF bolts ready to go if not I'll source some. I'll also source an 8.5mm drill bit (I was going to get an 8mm so i'm glad your replied).

 

I also found some info on building Subframe Jigs to help with panel repairs and lining up mounting holes so I'll probably also be building one of my own (already spotted some Angle and Box Steel just need the dimensions if available (think they are in my Haynes Manual.  

Edited by JonnyAlpha, 03 February 2014 - 08:14 PM.

[tiger99]

The jigging dimensions have been posted here on a number of occasions, including about a week ago, so if you search back you will find them. I believe it is an extract from an old BMC manual, as the dimensions for the front subframe are only for the solid mounted type, however the rears are the same on all Minis, except for the extra wheelbase length on van, estate and PUP. I am sure that you will find a jig to be very useful.

[JonnyAlpha]

The jigging dimensions have been posted here on a number of occasions, including about a week ago, so if you search back you will find them. I believe it is an extract from an old BMC manual, as the dimensions for the front subframe are only for the solid mounted type, however the rears are the same on all Minis, except for the extra wheelbase length on van, estate and PUP. I am sure that you will find a jig to be very useful.

Got the jig dimensions now and just bought a long 8mm drill and a standard 8.5mm to drill the rear sub frame mount holes before I remove the sub frame.

 

Thanks

Edited by JonnyAlpha, 09 February 2014 - 04:43 PM.

[seancv1]

that was a really informative reply learn something new every day tiger99

[JonnyAlpha]

Holes drilled, job done - just need to build the Jig now. Thanks

Few questions on Subframe refurb which i'll either add to another thread I have or start a new one.