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Main Cap Girdle Design

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Hey all. I have a small cnc and im able to design/machine a crank girdle for my 4cyl build so I thought why not. Its not a typical common performance motor so nothing off the shelf for it (Isuzu 4ze1)

Just wondering if anyone has any tips for design? Will any basic shape that links the caps together be good and better then nothing?

I will be doing it from aluminum so do I need to do anything to allow for the different rates of expansion (Iron block/caps)

Or is this something that needs to be carefully designed and tested and can do more harm then good and best left alone?



hello sorry I'm not a machinist but i would be under the thought that if its an alloy block then it would not be an issue but if it is cast iron I would be doing a lot of research hopefully someone here has some experience on this

Regards ross

Yea the motor I think about it the more I think Aluminium would cause issues, as it heats and expands I think it might actually push the caps out of place, especially the ends ones.

I could make it from steel, just my machine is more of a router then a mill so not super rigid. I have done steel before but I have to take it very slow. altho this will be a one off piece for myself so if it takes a long time its not really a problem...

I was able to find some pic's of the bottom end and it appears to be quite beefy - is there a known problem with it, or are you just doing this as a project, for fun?

If the former, you should be able to design a full girdle, if you have sump clearance (or it can be modified to clear). I would expect it to be better to fabricate from steel, not so much because of a slight difference in expension rates, but because it would be much more rigid - perhaps doing the design and fettling, etc, in alloy or even a fibre-board, and having a machine shop make the finished product from 'plate - they can machine it flat, too.

Besides the aforementioned sump clearances, you will have to allow for the crank and big end clearance and thread engagement with the block - use studs so you have full engagement, and can use nuts for the actual retention. You will need to carefully measure, and have correctly sized spacers between the caps and the plate - they may have slight variations that will affect the installed clamping force on the caps for the same torque - from what I have found, you're going to be looking at less than a thou, if possible, and you may do the final sizing with engineers' blue and a fine file. Regardless, a lot of work for a questionable return.

My personal view would be, unless I knew it was a problem, having the bottom end crack tested, cleaning the block threads with a proper cleaning tap, and using suitable ARP studs and nuts.

Nah it was more a case of Im putting the motor together soon. I have a CNC, would be sorta cool and if it makes the engine stronger then why not :)

Normally its the pistons and rods that are the weak points which ive replaced with forged items.

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