A7 to A10

[Blackcat]

Hi!

Back in 2006 on the British Only Forum Lucas from Cuba mentioned that he had rebuilt a 1956 A7 with an A10 crank and used two Daewoo Tico car pistons in the A7 cylinder block bored out to 69 mm to compensate for the longer stroke of the A10 crank.

I?ve got a nice set of A7 crank/rods and am considering giving this a go. Is it possible to use the A7 crank with an A10 cylinder block in a pair of late A7/10 cases topped by an A10 alloy head? Anybody tried this or a similar modification? If Lucas reads this then I?d be interested to know how he got on with his rebuild.

Martin

[muskrat]

G'day Martin,
                  It is possible but would have very little compression. The pistons would be too low in the bore at TDC. Hubbie found that problem recently.
Cheers

[wilko]

Perhaps the Daewoo pistons had the gudgeon pins sit lower bringing the pistons level with the top of the barrel?

[muskrat]

With an A7 crank & rods with a 69 bore A7 barrels would only bring it up to 542cc With A10 barrels and daewoo pistons = same.
Cheers

[South Oz Dave]

This is more or less what I'm facing right now.  I'm stripping a motor from what I was told was an A7 (and the number stamped on the case certainly confirms that AA7 2953) of about circa 1950.... but....

According to the trusty '49-'53 spares manual numbers stamped on the components seem to bare no resemblance (living or dead) to the manual.  But I digress.

What I have found is... a cylinder head number 67-381 (not in aforementioned manual), a barrell (sleaved) numbered 67-253[ A10 of vintage definately), conrods 67-391 (vintage?) and polson pistons 385.

The barrell and pistons match (sort of) in terms of bore - pistons measuring at 2.610" which I think is within original A7 spec, and infact they are an australian manufacture.

What may be important to the original question though was the massive build up of carbon on top of the pistons and within the squish area of the head. Depths went from about 3mm to 5mm across both surfaces.  There is also discolouration in both the barrells and crankcases which may indicate pretty significant heat, but if this is related I don't know.

Ulimately I hope the experts here may have answers for us both, but whoever built this motor before I got it, I suspect enjoyed it for only a short period of time.

[trevinoz]

Dave,
            A7 plunger head is 67-380, cast number 67-381.
Barrel is 67-378, A10 is 67-253. A7 barrel has 7 fins, A10 has 8.
Rods marked 67-391 are A7.
All should be small fin.
Trev.

[muskrat]

G'day Dave, welcome to the forum.
                                                So it sounds like you have a '51 A7 motor with '50 A10 barrels sleeved back to A7 size with nearly no compression. Do the barrels have cut outs in the skirts ? The carbon build up would be from poor burning of the fuel/air due to low comp.
All you need to do is find some A7 barrels and pushrods to bring it back.
Brian is over your way, maybe he could shed some wisdom.
Cheers

[South Oz Dave]

Cheers, then A7 barrell and push rods it is. 

Anyone got any (or know of any) for sale?

Also, with the bike came a spare head marked 67-25.  is this a twin carb or early one with separate manifold for single carb. I saw a beautiful one on ebay last night for $495 US, but is it relevant to the A7, as it certainly doesn't seem to suit the A10 barrel.

[muskrat]

G'day Dave,
                sounds like an early long stroke head that mates to a spigot on the barrels.
Put up some pics so we know what you have got, and go from there.
Cheers

[chaterlea25]

Hi Dave
I have a set of  A7 pushrods, if you fail to find some locally,
I'm rather far away from Australia though!!!
HTH
John O R

[trevinoz]

67-25 is the early long stroke A7 head.
Trev.

[South Oz Dave]

Thanks Fellas.

So the early long stroke head is (I assume) a single carb jobbie and worth putting on the shelf as a curiosity?

As for the push rods, give me a couple of weeks to get some employment (aka coin) and then I can start to look seriously, and if nothing turns up locally, I'll be approaching you John on bended knee.

As an asside, I was intruiged by the depth of the dome in the 67-25 when compared to the later head (0.334" or 8.44mm) deeper than its later cousin. Why doesn't it carbon up or is the early A7 barrell shorter than its later cousins to compensate?  I would have thought a compression issue would have occurred here as well?

[muskrat]

G'day Dave,
                that reflects the spigot on which the head sits. So in essence the top of the barrel is in the head.
I might have a set of alloy pushrods, if I can find them their yours. Will look tomorrow night.
Cheers

[South Oz Dave]

Cheers mate, much appreciated.  I have started to download some pictures on the 'newbie' thread, which will give you a better indication of the motors condition.  I'll add to it as time goes bye.  thanks for your generosity.

[trevinoz]

Dave,
               The long stroke head can be either single or twin carby, depending on the model.
Trev.