3a rebuild to 4a

[jamiec]

Why not rebuild your old 3a engines to 4a size?
i have done just that lots cheaper then paying for
a 4a that cost alot more.you can sometimes get
3a engines for free.just have it bored out to fit 4a
pistons makes cheap 4a and still have the standed engine
block the first 4a engines were just overbored 3a blocks
anyway, thats what toyota did.the wreckers in Australia charge
lots for old 4a motors bored ours out +40 thousents over 4a
make it about a 1620cc

[keith]

That would be an option, there is a set of .030 over pistons and rings on ebay pretty cheap ($10 + shipping). But I just bid on a reman 4A.

I was not aware that Toyota's could take an overbore. Toyota engines generally have about the thinnest walls around.

[Typrus]

Ummmmmmm.....

3AC- BorexStroke - 3.05x3.03 inch
4AC- BorexStroke - 3.19x3.03 inch
4AGE= Same bore and stroke as 4AC, and it reads 3.19x3.03.

Is .14 inch overbore really that good of an idea? Thats over an eighth of an inch.... We aren't talking 14 thousandths, we're talking 14 hundreths. 140 thousandths.
Just examined the block in the garage. I have this to say- If you do this, I will call you foolish. The cylinder wall can't be much over 1/8 inch thick. So by boring out a little over 1/8 inch, you'd effectively halve the wall thickness. If you look at the lip on the deck of the head, its much thicker, but you have to notice that the lip tapers off along the cylinder wall.
You know, 1/16 inch of cylinder wall might put up with 70 horses, but I doubt it'd go a whole long time. That, and none of us who does the swap wants to just settle with a simple swap. We want the carb. Some want headers, cams, porting jobs....

Just hit up your local scrappers they can't charge much over $100 for an old 4AGE or 4AC block, or maybe even whole engine.


77.5mm vs. 81mm 2.5mm. Doubt if thats safe.

Unless you could find 1mm undersized pistons. Doubt it though.

[jamiec]

well the overbore works very well i have done it with
bigger cam modifide head and exhaust. this motor
has been in every day service for two years then we
sold it.had done lots of offroad work and towing trailers
no problems.like i said before toyota did this to make
there first lot of 4a motors they were the same block.
i was concernd about the cylinder wall thickness to.
i had about 4 spare 3a blocks so could try boreing 1
cylinder to see. the engine rebuilder said it was fine.
it was bored out +40 thuogh = 1mm over the standed 4a bore size.
that motor was better than my standed 4a powered tercel.
yes i know the wall tapers off and i guess i must be foolish
but it works fine.

[Typrus]

Yes, I would accuse you of being foolish for trying. If it worked, kudos to you, I must say.
So I suppose let me put in my caution. If you are going to try this, MAKE ABSOLUTELY SURE that there are no visible casting flaws in the cylinder wall outer surface. Any casting "dent" could cause some serious compromise.

Jeeze... I must say I admire your balls for trying a roughly 3.5mm overbore. Wow.

So then here's a thought- do the hyper overbore on the 3AC, then attach a 4AC head? Slightly better flow and the likes. Would need to be a later head though to make better power.

Well, I suppose I can see that.... 83-85 versions of the 4AC making 70HP and 85ft/lb versus later 74/86. Not much difference, but I'd imagine a "simple overbore" would be a little less efficient.

I suppose it might weigh a little less than a 4AC.

I dunno... It just makes me a little nervous to overbore 3.5mm.... "shrug" I guess if you kept it fairly conservative and kept it well cooled and lubed.....

Oil cooler, high volume oil pump, and that fancy electronic water pump in order? Heh.

[ColSwede]

Does the 4AC or 4AGE bolt up to 6 speed manual 4x4 Trans? Same motor mount placements?

[BaileySims]

4ac will bolt up no problem with no modifications, the 4age requires a ton of mods.

[The Professor]

I would be interested to know the difference in thickness (if any) between a 3A and 4A cylinder walls. My guess is that larger liners were installed in blocks destined to become 4As, making the OP's mod that much less appealing.

[Petros]

there was a technical service bulletin out warning dealers to watch for wall break through when overboring the 3a engine, the "fix" was to sleeve the engine bores. they made no service bulletins for the 4a block. so it appears some of the 3a at least will not tolerate an overbore.

though I do know of someone that overbored a 3ac to 4ac, but not recommended because of the risk. Also, the 4ac head, almost identical (same valve train and externals) has an improved water jacket for better coolant flow around the combustion chambers. Also, if you over bore that much and still us the same head, the compression ratio will get raised to above 12 to 1, which may be too much for pump gas.