cross dressing astrons...

[GESigman]

hi all, so far ive been able to find the answers ive been after by hiding in the background and reading old posts, but now the time has come to get some feed back..

So, The scorpion is running low on compression and the time has come to have a go at engine building and use a few of the parts ive collected. stripped down a spare scorp A2 motor and found some nasty looking bores and a need for some machining, however the GH Siggy motor i stripped has beautiful smooth bores and so a plan was hatched
The plan so far: A1 Sig block, A2Scorp crank rods and flat-tops, Twin 45 dcoe webers, Crow cam #701 Lift .470", Duration 280deg (212@.050"), 37/63/65/35, lobe separation 104deg (specs on the box) and a head with the usual fruit

The question i have.... the A1 head... can it be used with the flat tops from the A2? haven't been able to find a straight answer. been told that it would bring the compression up to ridiculous levels and wont work, and found bits and pieces of info that suggest it will work just fine and not at too high a comp. anyone who can give me a definitive answer? if i can then the increased compression should add to the fun, and if not ill just have to use the scorp head

[Superscan811]

Astron 1 pistons were typically dished, where Astron 2 pistons are typically flat top.

To compensate for the dished pistons, typically the Astron 1 heads have a much smaller size compression chamber.

What this all boils down to is:

Dished pistons (A1) + Astron 2 head = lower compression engine than standard (good for low boost applications)
Flat top pistons (A2) + Astron 1 head = Higher comp engine than standard. (good for a normally aspirated engine)

There are several types of bot Astron 1 and 2 heads and pistons so unless you know exactly what you are going to use, it's a guessing game,

Hope this helps.

[GESigman]

thanks for that, I thought all A1 heads were the same, as with stock flat tops (knowing that A2 heads are all over the show) So in theory I should be alright using the A1 head with the Scorpions flat tops... I might just have to bolt it up and see what happens, hoping it doesn't go bang

[GESigman]

If anyone here who has done it successfully, any tips / hints that make life easier, or things to watch?

[C_Fernance]

I have done it in the past at least 10 years ago. Blew my engine and needed to rebuild it ASAP so I could use the car to get to work. Dad and myself made a bitsa up with what ever parts I had laying around.

It ended up using the original GK Scorpion Astron II block and crank, a set of flat top pistons and the rods out of a TM or TN Magna, and an Astron I head and inlet manifold. Carb, cam and so on were all standard parts as well.

It was very much a bolt together and hope for the best scenario. We bolted it all together, turned it over a few times by hand to make sure nothing hit, then started it up. The engine ended up outlasting the car, then being transferred into another Scorp and was still going like that when I sold it about a year later.

The biggest issue I had with the engine was what fuel I could use. It had to be 98 octane, and even then certain brands ran better then others. Shell would ping whenever I put my foot down hard, another brand (Caltex?) would ping under heavy load on hot days, and the 3rd brand I tried (BP?) was the only one it didn't ping on.

I probably wouldn't do it again. The high comnpression made filling up expensive and a pain in the ass to try and find the right fuel it needed, and it didn't make any more power then a standard engine. Part of this could have been due to the standard cam, carb and so on, but it could have also been because the Astron I head has smaller valves and ports so they don't flow as well. A better option would be to use an Astron II head for the better flow potential, then shave a millimetre or two off the face to bump the compression up.

[GESigman]

Thanks for that, that's pretty much the information I was hoping to find. I find myself wanting to believe that the lack of power was indeed because of the stock cam and carb, it's what the brain wants to think anyway. Thought about having the A2 head shaved instead of using the A1, but not knowing how much I need to take of to get the result I'm after, aside from not wanting to spend anything I don't have to, makes me want to try with the bits I have. I'll never have a use for it otherwise haha

[cheaterparts]

The plan so far: A1 Sig block, A2Scorp crank rods and flat-tops, Twin 45 dcoe webers, Crow cam #701 Lift .470", Duration 280deg (212@.050"), 37/63/65/35, lobe separation 104deg (specs on the box) and a head with the usual fruit

I wonder what Crow recommend in the way of C/R for that cam - I did look at there site but they dont seam to list astron cams these days - I'm guessing around 10 to 1 C/R for a short duration after market cam but the lobe sep is quite tight thinking a 280 deg cam would be closer to 110 deg
the tighter lobe sep may like a little more C/R --

[GESigman]

thanks Cheaterparts, i might just give the guys at crow a call and see what they say. then i guess its just a matter of working out how to get to the suggested c/r. head shaving im assuming, the question is, how much ?

[Pianca]

Hi mate.

First post but i saw yr response and was hoping to share some experience.

I just finished rebuilding my Scorpion motor.

Used the astron 1 head, flat top pistons (1mm o/s), shaved the head 20thou and put in a big cam and single 45 delhorro side draft. I don't get any pinging...

In order to make the most of the astron 1 head i got the big valves put in it from the astron 2 type (3mm larger) and done a port and polish.

Works unreal and have shaved approximately 2 seconds down the quarter from its stock configuration.

I hope this helps...

[GESigman]

helps a lot there actually, i think i might have a plan after all. so much for what the workshop reckons then hey, my faith in them is fast running out

[Pianca]

One last thing.

If you put the bigger valves in the head, you will need to get the head worked slightly to fit the right valve seats (as they are bigger) and the new seats pressed in. I had that done by a head shop, however, the port and polish I done at home.

My compression calcs were estimated to be somewhere in the region of 10.4 - 10.7 (not super high, however, massive improvement from the stock 8.8 it made running my oversized cam drivable).

Also, Aston 1 head is adjustable mechanical tappets so it makes fitting big cam's somewhat easier than the hydraulic (which I believe is in the astron 2 head, though I may be mistaken).

All the best.

[GESigman]

good shit, thanks for the info boys , looks like the plan will work out after all