4age crank - smaller lighter or larger heavier?

[rallysprinter]

I’m in the process of planning and collecting parts for a replacement engine for my AE86 rally car and I’m undecided about which way to go in relation to the bottom end.

I have to run a bigport head, but the regulations for the bottom end is basically free of restrictions other than me wanting to stay under 1600cc.

Do I go for the later larger heavier bottom end or the earlier smaller lighter bottom end?

I want a strong engine that will run between 4000-7500rpm all day in competition that will result in about a ~11.3:1 compression ratio. I’m not chasing sheep stations, so I’m not going to buy really expensive brand new parts, when I can make do with a cheaper alternative. But I want to make the right choices and not just go with the general opinion of the internet, which appears to be the larger later model bottom end, without real justification.

So, to quote Bill “Both cranks are extremely strong, and only break with quite a bit of mis-treatment. The 40mm crank weighs about 11kg and the 42mm crank about 11.7kg.“

I have 2 blocks, and both appear to me to be 7 rib blocks.

Block 1 came with the car and I believe it was originally from the head that is currently in the car.

It doesn’t have the external oil drain but it contains a visibly smaller crank with small big end journals (My research suggested that all 7 rib blocks hat 42mm big end journals but I have one with 40mm big ends.) The bores look a little rusty but I think would clean up with a hone.

Block 2 I bought as a parts package early in the year and is a small port block. The crank has a few scratches on the big end journals but the bores look pretty decent and obviously has oil squirters.

One of the normal reasons given for going with the later model crank is to use the matching factory 10.3:1 pistons. But I bought a set of used HKS pistons on Ebay that are supposed to be “high compression”. They suit an 81mm bore and have 18mm wrist pins but no specs other than what I can measure but they will fit up to the early model light crank/light rods.

From left in photo above -HKS aftermarke 18mm pin, Toyota original piston 18mm pin, Toyota original 20mm pin.

A search of HKS 4age piston for specs only finds this reference
It lists a 81.00mm 8.0:1 or a 81.25mm 11.4:1 piston so can anyone tell me some more details about what I have? I’ve measured my set and it appear to be 80.90mm which I assume is to suit 81mm, but it doesn’t look like a low compression piston. I’ve measured the height of the “dome” from above the pin and it is similar height to that of the outer ridge of the big port piston.

Now, can anyone tell me any reason why I couldn’t use the early crank in the later small port block. What about the oil squirters? Would the HKS pistons need clearance for the squirters? Can they be modified for a decent price? If the oil squirters are removed, do the oil galleries leading to them need to be blanked off to keep oil pressure up?

Thoughts?

Cheers
Craig