I know what you mean about the red motor pistons being cheaper.  You can pick up fairly cheap red motor piston sets on eBay (although 149s might be a little rarer).  Most grey motor pistons will be more expensive, but if you go with the red motor pistons you will have to add the cost of having the little ends of your grey motor rods honed out to fit the red motor gudgeon.

Tortoise.

Hi fewithrb30,

I assume you are talking about 149 pistons from a red motor.  Pistons from a 149 red motor are 3.25" diameter and (because of the 3.125" stroke of the grey motor crank as opposed to the 3.000" stroke of the red) will give you 155 cubic inches in a grey motor.  The red motor pistons may have a different compression height (distance from the centre of the gudgeon pin hole to the top of the piston) and therefore MAY give you a different compression ratio.  You will have to measure them - or someone on this site may know the difference.

The gudgeon pin is also larger in the red motor pistons, so you may have to have the little ends of your grey conrods honed out to match for an interference fit (grey motors were a small diameter fully floating gudgeon whereas reds were a larger diameter and interference fit) - although this has been done before.  For all the stuffing around, and cost, you may do just as well using oversized grey motor pistons and your grey motor rods.  Grey motor pistons are commonly available in 3 & 3/16" (3.187") bore (for a displacement of about 149 or 150ci), and less commonly available in other sizes.  I think JP Engineering in SA still make varying grey motor pistons, and can even make dome-topped 12.5 to 1 compression ratio pistons for race applications.

Old FX & FJ hotrodders used to bore their grey motors to 3.25" but because of poor quality manufacturing in the early Holden blocks, they would sometimes risk boring into the water jackets, and have to scrap their block.  The later model grey motor blocks were slightly improved in this regard.  If you have a 'J' prefix engine number on your block you are in the best shape.  Generally 'No prefix letter' = FX/FJ; 'L' prefix = FE/FC; 'B' prefix = FB/EK; and 'J' = EJ.  The more common 3 & 3/16" bore grey motor pistons would probably be your best bet, would still give you about 150 cu in (not much difference to 155), be more reliable in regards to the block, and even give your block another life if you ever wanted to rebuild it again.

The largest bore size that I have heard of in a grey motor without special sleeves being pressed-in and welded (a big job), was 3 & 1/4 + .030" (3.280") giving a total displacement of 158 cubic inches.  This was done a couple of times by racers in the early 60s, and the engines generally held together, however it must be pointed out that in the 60s, grey motors were relatively new and wouldn't have suffered much water jacket corrosion at that time.  The grey motor blocks now are around 50 to 60 years old.  Racers more commonly bored their grey motors to a maximum of 3 & 1/4" (155ci) or even 3 & 3/16" (149 to 150ci).  These sizes were more reliable.  More exotic experiments were done by re-sleeving with bigger sleeves but these had to be welded into the block top and bottom to give some rigidity back to the block.

Lots of people overbore their engines to the limit, but the extra few cubic inches do not usually make that much of a difference.  If you are using the same head, cam, carbs and exhaust, the percentage difference in the power between a slightly overbored engine and a hugely overbored engine, is nowhere near the percentage increase in the cubic capacity.  For example, I did up a 3 & 3/16" (149 or 150 cu in) block to replace the old stock (3" bore) 132 in my FE.  The stock block was still in good nick and was holding compression well, but I wanted what I thought would be a reliable increase in horsepower.  I used the same head, cam, carb & exhaust from my stock-bore FC engine.  The difference was hardly noticeable.  Now I only bore my engines to the next available oversize - and give my engines many more lives at the same time.

I don't know what the difference in compression ratio will be, but the standard ratio for an FC was about 7:1 and an EJ about 7.5:1.  I don't imagine it will increase the ratio much above 8.5:1.  Red motors with a standard ignition timing curve will handle a maximum of about 10:1 on premium.  I imagine that the grey motor head being less advanced would probably handle something less than that - maybe 9.5:1 (but I am only guessing).  And I also imagine that this would be a good compression ration for a mild cam. 

If you are going to go straight gas, you can increase the compression ratio heaps.  You could probably even use those special dome-topped 12.5:1 compression pistons.

Hope this helps.  Tortoise.

Thanks everyone for your help. 

I spoke to a diff modifier recently and he told me that once you get an axle re-splined, the result is not as strong as an orginal Holden fine spline axle because you've cut away the hardening treatment on the factory splined end.  He said that very few modifiers will give the ends a hardening treatment as good as the factory treatment.  He suggested that re-splining defeats the purpose of swapping to fine-spline axles, because a factory course-spline axle will generally be stronger than a re-splined fine spline axle anyway.

A shortened 9" would be nice, but the strength is probably overkill, and I don't really want the weight.

So its back to the drawing board for a bit more thinking.  Maybe after what Craig A said, I'll just stick with the diff I've got.

 

Thanks for the posts. 

Craig A, your Torana sounds like a pretty wild ride.  I didn't realise course spline axles would handle that sort of power.  I have been told the Borg Warner rear ends handle plenty of punishment, but as you indicated, you like to change between 3.55 and 3.08 - that's not something you can do with a B/W, well not without pulling the whole rear end out, or setting up a new diff with the rear end still in the car.

EffCee, thanks for the info on the length of the LJ rear ends.  You mention cutting off the existing mounts and welding on FC mounts - will this cause distortion?  Would I have to get the housing set up on a jig to align it back straight? 

fb delivery mentions that all HZs have fine spline rear ends.  Even though HZs are way too wide, it got me thinking about how much it would cost to get one cut down, FC mounts added, and axles shortened and re-splined.  The other benefits would be that they come with tapered bearings, and HZ brakes.  I don't know if HR brake backing plates are interchangeable with the HZ housing ends, or whether I would have to use HQ to HZ brakes after that.

fchoon, you mention a Centura rear end - are they a B/W?  Or are they completely different?  Are they a lift-out centre?  How wide are they compared to a standard FC rear end?  What ratios are available?

Tortoise.

Hi 87 velo,

I believe the HQ Racers have to use the standard red motor rods (not the blue motor ones).  I believe the red motor rods use the 5/16" rod bolts.  The blue motor rods use the slightly larger 11/32" rod bolts (the blue motor rods use the same size as a small journal small block Chev rod bolt).  ARP sell rod bolts for both red and blue motor rods.  I understand that one of the mods the HQ racers do to their engines is to use the 5/16" ARP rod bolts.

Just be aware that the ARP bolts are a higher tensile bolt, so when you torque them up, they may need slightly more torque than a standard bolt.  I was told once by an engine reconditioner that if you don't torque the ARP bolts up enough to get the correct amount of rod bolt stretch, they may come undone.  If you do them up to their higher torque setting however, this may alter the 'out-of-round' at the big end.  So he said that if you fit ARP rod bolts you should have them closed and honed after fitting the new rod bolts, to suit the new torque setting.

I have seen the red motor rod bolts for sale on eBay a few months ago - they were no cheaper than the blue motor bolts.   I think they were around $140 per set.

Tortoise.

Hi guys,

I was wondering if anybody can assist me with some knowledge about fitting a fine spline diff into an FC?

I own an FC with a red motor in it and an HR rear end & course spline diff.  I want to run a fine spline diff and axles.  Is there any easy method fitting a fine spline rear end?  Does any other Holden have fine spline axles the same length axles as an HR or FC?

I know that some LH, LX & UC Torana rear axle housings had fine spline diffs, but I have heard the housings are wider than an FC or HR by about 6cm.  Does anyone know if this is the correct difference in width?   Is there any other Holden rear end that uses fine spline axles that is the same as, or narrower than the HR/FC housing?  Does anyone know how wide an LC or LJ rear end is?  Do LC or LJ rear ends have fine spline axles?

Tortoise.