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HK1837 Online
#1 Posted : Tuesday, 14 May 2019 11:40:45 AM(UTC)
HK1837

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This may have been covered before. I have also spoken to gm5735 on this at length as well. But I am still confused.

On the surface it looks like simple geometry: GMH simply used a taller mount adapter on the driver's side to effectively tilt the SBC design engine so it fitted (in combination with the sump and passenger side exhaust manifold). As far as I am aware there is no difference between a HD-HR and a HK-HG with regards to where the engine/suspension crossmember has its engine mount stubs welded wrt to the centre line of the car and wrt to being aligned horizontally. Basically there is no apparent offset in the engine bay of a 6cyl or a 253/308 in one of these cars which you'd expect if the actual welded on mounts were anything but totally symmetrical.

HOWEVER. When I approached both the makers of Tuffmounts and the fabricators of the aftermarket gearbox crossmembers both gave me a big spiel how the engine is tilted and offset in the engine bay? My replay was "yes I know that, but I want to centre it hence why I'm trying to buy 2 x passenger side Tuffmounts and a gearbox crossmemebr with the transmission mount centred". Apparently it cannot work that way?

Right now I have a 307 and Powerglide in a HK GTS up on the hoist. It actually has 2 x tall (driver's side) adapters in it with an original HK V8 Powerglide gearbox crossmember and rear mount. The engine almost sits level going by the bottom of the sump. I'm pulling the driveline and crossmember out soon, and I plan to stick a bare SBC block and TH350 in there on the Rodtech front crossmember so I can get the steering shaft sorted and find extractors, block huggers etc (even Corvette 2.5" Ramshorns) that fit it properly. I will just stick 2 x short (passenger) side adapters in there and move the TH350 across on the factory rear mount to centre it so I can see how it all looks. However I wanted to at least order an aftermarket gearbox crossmember with the box centred so I can look at exhaust clearance for dual 2.5" pipes, and not get grief from the supplier about it not working etc.

So just wondering if someone has actually levelled all this up and taken measurements as to whether it is really as simple as I think or if there is some other hidden trickery going on as per the Tuffmount maker's claims? Note that I want to actually use 2 x short side Tuffmount mounts eventually, but if they do not place the engine as per original this may not actually work. I did notice there were some others around too like Rodtech if Tuffmount don't work, Rodtech's sit claims they sit the engine in the factory position so I should be able to use 2 x short ones of their design.
https://www.rodshop.com....-type-engine-mounts.html

Lastly, do HK-HG with Saginaw or V8 Powerglide use the same rear mount as an Aussie 3/4spd or Trimatic? The Tuffmount rear mount description says they have a shorter one for TH400 (which I know in HQ-HZ is shorter than the 3/4spd or Trimatic mount) but say the V8 Powerglide one is common to Trimatic and Aussie 3/4 speed. Is the TH350 and V8 Powerglide plus Saginaw mount shorter as well or is it the same as a Trimatic one? I've never really taken notice before, but a HZ parts catalogue doers not help as 6cyl and V8 3/4spd and Trimatic are different part numbers to each other, TH400 is different again and so is TH350 different number to all others. And there is some weird stuff with HT where the mount used on V8 Powerglide differs between 253/308 and 307 and 350 (all different) but it does appear that Saginaw 4spd and Trimatic use the same rear mount for 253/308 and 6cyl. What s very interesting though and probably answers my question is HK Brougham uses the same rear mount as 6cyl Powerglide 7432236 which is the same as HQ-HZ V8 3/4spd and V8 Trimatic, yet the rest of HK V8 Powerglide uses a different rear mount 3870184 which is the same mount as HK 307/327 4spd and the same as HT 307/350 4spd or 307 Powerglide. So I guess if I use full height 743226 mount under the TH350 on the I will need the spacers between the x-member and the body, but if I use the shorter TH400 mount I can delete the spacers and thus have a tiny bit more clearance around the crossmember, bearing in mind it will be one of these:
https://www.ebay.com.au/...d4761:g:2UMAAOSwfO1cy27U

Edited by user Tuesday, 14 May 2019 2:20:30 PM(UTC)  | Reason: spelling

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Ausjacko Offline
#2 Posted : Tuesday, 14 May 2019 5:58:26 PM(UTC)
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I will only comment on my experience with a HT SBC 4sp. In my Saginaw car the gearbox mount is the same up to VL commodore. I had no issues with this mount re clearance along the floor pan or height into the console.
Dr Terry Offline
#3 Posted : Wednesday, 15 May 2019 7:45:52 AM(UTC)
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As far as I can tell the only difference between the various rear mounts is in the 'stiffness' of the rubber. Obviously there are exceptions like the TH400 & the Opel etc., but many of the others are interchangeable.

In other words many will fir other applications, because they are the same physical size, but some will transfer more or less vibration through the mount than others.

There are several different part numbers of rear mounts which all cross over to the one Mackay mount for replacement purposes.

Dr Terry
If at first you don't succeed, just call it Version 1.0
HK1837 Online
#4 Posted : Wednesday, 15 May 2019 10:02:49 AM(UTC)
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Thanks guys. I think I’ll try using a TH400 mount under the TH350 on a HK V8 Powerglide crossmember. This should lower the box at the rear but without the spacers it also will raise a few mm too. With the engine sitting maybe 10mm lower on the driver’s side by use of two passenger side mount adapters it should all then sit square and centre with an aftermarket crossmember with mount centred.

I measured the HK this morning and the centre of the gearbox rear mount as measured to the inner sills is 25mm shorter on the passenger side, meaning the mount sits 1/2” offset to the passenger side (or close to that). You can clearly see the tailshaft angle when it is on a hoist.

Edited by user Wednesday, 15 May 2019 12:01:37 PM(UTC)  | Reason: Not specified

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Dr Terry Offline
#5 Posted : Wednesday, 15 May 2019 1:49:24 PM(UTC)
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Originally Posted by: HK1837 Go to Quoted Post
I measured the HK this morning and the centre of the gearbox rear mount as measured to the inner sills is 25mm shorter on the passenger side, meaning the mount sits 1/2” offset to the passenger side (or close to that). You can clearly see the tailshaft angle when it is on a hoist.


The tailshaft is also off-centre at the diff, because the axles are of equal length, placing the pinion to the right.

Dr Terry

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HK1837 Online
#6 Posted : Wednesday, 15 May 2019 2:01:01 PM(UTC)
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This will be a 9” too so will check if it sits centre or not.
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Dr Terry Offline
#7 Posted : Wednesday, 15 May 2019 3:22:10 PM(UTC)
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Originally Posted by: HK1837 Go to Quoted Post
This will be a 9” too so will check if it sits centre or not.


This is the trick to making a good tailor-made 9".

If you make the 2 axles of equal length as the old Holdens were, the 9" has so much pinion offset that it places the tailshaft even further off-centre, so far so that in early Holdens the rear uni hits the side of the tunnel.

If you make the axles unequal in length (the same amount as in a Ford), then this places the tailshaft dead-centre, again most likely hitting the tunnel on the other side. This is because the Holden tailshaft tunnel is off-centre

You need to do some calculations to place the 9" diff yoke in the same position as a Holden diff (i.e with less offset).

Dr Terry
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HK1837 Online
#8 Posted : Wednesday, 15 May 2019 4:30:32 PM(UTC)
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Just did some measuring.

It looks like by a tape the pinion centre on the banjo is roughly 30mm difference from the side of each leaf which if measured properly probably works out to be 25.4mm so 1/2" offset of the pinion to the driver's side to match the 1/2" offset tunnel.

My 9" by rough measurement (as it is still in plastic and the centre is not in it) is about 30mm different from the centre of each spring perch to the approx. location of the pinion centreline so it too is offset to the driver's side by that ballpark 12-15mm mark.

So by me moving the back of the gearbox back centre to the sills I only have a sideways tailshaft deflection of 1/2", not the 1" as per original HK V8.

Once I get the engine/gearbox dummied up (centred on all 3 mounts) I guess I can ensure the crankshaft angle is as close to parallel to the diff pinion angle as I can make it. Rodtech hopefully can tell me what pinion angle they use. I have heard that standard carbied SBC's crank centrelines are tilted back at 4deg and the diff pinion is at 4deg too, so will be interesting to see how this all works.
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Dr Terry Offline
#9 Posted : Wednesday, 15 May 2019 5:06:54 PM(UTC)
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The figures that I have put a stock Holden diff at 3/4" (19mm) off-centre & a 9" at 2" (51mm) off-centre. Assuming equal length axles in both cases.

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HK1837 Online
#10 Posted : Wednesday, 15 May 2019 5:15:31 PM(UTC)
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As I said my measurements are rough, and by a tape so could easily be 38mm difference on the banjo in there now from the centre to the leafs on each side (which would be 19mm offset). I measured about 30mm give or take.
Again measured roughly the 9” I have looks to be about 30mm difference too same measurement.
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If we all had the same (good) taste, who would buy all the Fords?
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