16 replies to this topic

[samsfern]

building myself another hot engine and gearbox for the mini, ive already converted the block and head to 11 stud, and today i read somewhere about increasing the size of the gearbox to engine bolts from 1/4" unf to 5/16 unf, was just wandering if its worth doing, seeing as its a bare block at the mo. Many thanks, sam.

[bmcecosse]

Yes - worth doing for a very high power unit. Use Allen cap head screws.

[samsfern]

a job for me at work tommorow then thanks.

[Kaj]

I've just done this mod on my engine/box rebuild. The 2 bolt posns at the rear of the block above the diff housing need studs fitted with nuts as there is insufficient space to fit allen bolts. I used manifold studs which do the job perfectly.

Make sure you use a pillar drill to open the block holes out so they are square!

[Wil_h]

No point. No advantage. waste of time.

The larger bolts would be stronger than the case and you wouldn't expect that to just fail.

[Ivor Badger]

No point. No advantage. waste of time.

The larger bolts would be stronger than the case and you wouldn't expect that to just fail.

Not the case at all. I had regular failure of front centre 1/4 unf bolt, fixed by using 5/16. As BMC eccosse says, you need to use cap screws on the front as there is little clearance for 1/2 af heads. This unfortunately means it is a time consuming job getting them in and out. Buy a ball end Allen socket.

[Wil_h]

No point. No advantage. waste of time.

The larger bolts would be stronger than the case and you wouldn't expect that to just fail.

Not the case at all. I had regular failure of front centre 1/4 unf bolt, fixed by using 5/16. As BMC eccosse says, you need to use cap screws on the front as there is little clearance for 1/2 af heads. This unfortunately means it is a time consuming job getting them in and out. Buy a ball end Allen socket.

Can you elaborate on the failure. When did it happen? why only one? I've heard of them comming loose but never breaking. I can't actually see a time when only one can break.

[samsfern]

i drilled and tapped them out yesterday, and when im paid ill be ordering the cap head bolt things from mra. Vizard must have done it for a good reason, ill read up later.

[Paul Wiginton]

Although I did mine, I do agree with Wil.
Not a necessary job, there are some monster power motors that are perfectly fine with the original 1/4"unf's. If inferior bolts are used I can see a problem but Ive observed most of the failures Ive read about on various forums are due to human error rather than the parts. Original bolts are fine.

Paul

Oh, you dont need to fit studs above the diff, bolts fit easily

Edited by Paul Wiginton, 08 January 2011 - 10:02 AM.

[Guess-Works.com]

The main reason I've come across for this being done, other than pub points, is in Miglia racing..

It is a common issue for gaskets to be blown out between the crank case and gearbox, so instead of using a gasket they just use a bead of RTV, increasing the size of the screw from 1/4" to 5/16" means you can increase the torque to which screws can be tightened, which implies the gearbox and crankcase are held tighter together giving a better seal with the RTV.

[Ivor Badger]

No point. No advantage. waste of time.

The larger bolts would be stronger than the case and you wouldn't expect that to just fail.

Not the case at all. I had regular failure of front centre 1/4 unf bolt, fixed by using 5/16. As BMC eccosse says, you need to use cap screws on the front as there is little clearance for 1/2 af heads. This unfortunately means it is a time consuming job getting them in and out. Buy a ball end Allen socket.

Can you elaborate on the failure. When did it happen? why only one? I've heard of them comming loose but never breaking. I can't actually see a time when only one can break.

Used to break the head off the centre front bolt, the one in line with the centre main in a well used S thin flange block inside 100 miles of use. Didn't do it initially, but after about 200 miles of use from build, it used to snap them regularly. After the 3rd one, fixed by changing to 5/16 and never broke another

[Ethel]

That's interesting, it must say something about where the stresses are in the block and casing. I'm dubious that RTV would work any better with bigger bolts, if there's enough flexure to blow past gaskets then the casting isn't likely to be stiff enough to transmit the extra clamping force along the web. Maybe a two stage tightening procedure with RTV would help, so it is compressed rather than squeezed out when fully tightened.

[mini13]

I agree with Paul and Wil on this,

there is a seller on ebay that often has ultra high grade std size gearbox bolt sets for a good price, (and also main bearing sets) using these would be the route I would suggest.

this is the chap...

http://cgi.ebay.co.u...=item25600e58d9


failing that, I have taken mine out to mate in the past, as its easy to get hold of strong bolts in this size, just use socket heads, they are usually 12.9 grade iirc

Although I did mine, I do agree with Wil.
Not a necessary job, there are some monster power motors that are perfectly fine with the original 1/4"unf's. If inferior bolts are used I can see a problem but Ive observed most of the failures Ive read about on various forums are due to human error rather than the parts. Original bolts are fine.

Paul

Oh, you dont need to fit studs above the diff, bolts fit easily

Edited by mini13, 08 January 2011 - 02:38 PM.

[samsfern]

taken from the vizard book, pg 370 "... On long stroke, high revving engines, the bottom of the block tends to flex or conversley the clamping loads between the block and the gearbox are insufficient. Whatever the case may be, the thin flange blocks tend to leak at this point more easily than the thick flange blocks. If you are building a high rpm engine, its best to base it on a thick flange block to cut down the likelihood of oil leaks. Even with thick flange blocks, drill out all the 1/4 unf screw thread holes in the block that are used to bolt the block to the gearbox casing and tap them to 5/16 unf. You should then substitute standard bolts for 5/16 socket-headed bolts."

well, ive already drilled and tapped mine, just need to drill the holes in the gearbox casing when i rebuild the transmission. Allen key type bolts (forgot the name, lol) will be coming from mra minis, so should be good quality. Thanks.

Edited by samsfern, 08 January 2011 - 02:50 PM.

[Ethel]

As Wil said, the weak link will be the female threads in the block, stronger bolts won't do much unless there's more thread engagement. It might also be worth considering the benefits of the waisted head studs - thinner shafts mean better elasticity relative to the clamping force.

Thanks for the Vizard reminder Sam, if he says revs (not torque/bhp) are the cause, then the source must be the reaction forces from holding the crank. Perhaps the reinforcement webs on the A+ are a better remedy that wasn't available to those engines when DV wrote that section?