Subframe tower bolt diameter

[Ollie78]

Hello all, I'm hoping someone knowledgeable can help out.
I'm currently fitting a dry twin bolt subframe in my 67 built mk2 in place of its original hydrolastic type and have a query.
I know the tower bolts for the hydro frame are smaller, 5/16" for the wet and 3/8" for the dry, my question is are the holes for the tower bolts in the bulkhead cross member the same diameter? The dry 3/8" bolts fit snug which would have meant the hydro 5/16" bolts were very sloppy and there was a lot of room for error on fitting the original front subframe. Am I missing something?
Did hydrolastic minis have a different cross member to cater for the smaller diameter bolts, if so why do 3/8" bolts fit?

Thanks, Ollie

[InimiaD]

From a bit of research done a while ago wet Cooper S had the 3/8" bolt as opposed to the 5/16 stud in the standard wet cars.
BMC also used "centralizing" type pin/s which was used to set the subframe centrally through the cross member mounting holes. More of a visual thing.

[rich@minispares.com]

the holes in the bulkhead are the same size

[Ollie78]

Thank you for the helpful reply's... though I'm still not sure I fully understand.

Rich, how can the holes in the subframes be the same size? Am I correct in thinking wet frame takes 5/16" and dry 3/8"? Do you mean holes in the bulkhead cross member are the same size for both types of suspension?

InimiaD, assuming the bulkheads are the same and the 5/16" bolts are located in holes for 3/8" bolts what holds the wet frame centrally in place, just the clamping pressure of the subframe bolts? Is that enough to stop the whole front assembly from moving during driving? It doesn't seem right somehow, if the hydrolastic frame can twist during fitting or later through vibration all the geometry would be way out? Just twisting the subframe 1-2mm or so on the tower bolts changes the wheelbase left to right quite considerably. Do these centralising pins stay in place? Have you got any pictures? I cant visualise where they might go.

[rich@minispares.com]

sorry, I typed subrame when I meant bulkhead - ive edited my post!

[InimiaD]

Here's a pic taken from a BMC manual. The pins are used as a guide to visually check the clearance against the surrounding bolt holes in the cross member.
The pins are taken out after alignment checks and replaced by the proper bolts or studs.



Dry front subbies (up to 76) used a spacer between the tower top and cross member, so in answer to your question, yes, the clamping pressure of the 4 bolts held the top of the subframe in place.

Hope the above helps Ollie.

[Ollie78]

Thanks for taking the time to help InimiaD, that's really useful.
I was wondering as I've had my car on a jig recently and that's one of the things I was wondering about. I was thinking of fine tuning my alignment by wriggling the subframe, theres hardly any movement but I can twist it 1-2mm, if I biased it one way or the other I wasn't sure if the tower bolts and toe board bolts would hold it permanently in that position. I have my answer now as it obvious the hydro cars had loads of possible movement before tightening up.
Cheers, Ollie

[swifty]

Ollie I don't think it's that's critical really . When you think of it the later mpi minis are fully floating and moving the steering wheel when stationary actually moves the whole subframe . The tolerances are so different on any mini . Ken

[Ollie78]

I know Ken I need to get a bit of a life. I was messing about with the mini yesterday, I just like to get things as good as I can and it gives me something to concentrate on.