I would not worry about cinder blocks, I would worry at what point is the car supported.

 

Yes at the place you normally jack up the vehicle or use jack stands along the flat frame rails as long as its supported evenly.

 

I would worry what the neighbors are going to think about old cars up on blocks in the front yard.

 

Many years back, did the same as you propose, late one afternoon with the intention of doing the project the next morning. One cinder block broke, front driver side hub was one the ground. Never again will I use them to support a vehicle.

I started a re-do of the brake system on my pickup. Started last fall, delayed by life and medical issues.....

All 4 corners, 8x8x24" blocks of PT wood covered w/a 1/2 plywood to hold them together.
6 ton jack stands I got on sale at Tractor Supply.
It don't move.

HTH...Don

 

For the cheapness of jack stands, and you want to use concrete blocks??!! If you need it higher, use 6x6 cribbing. It amazes me you have lived as long as you have.

 

Common CMUs (AKA cinder blocks, though they are now made from dry-cast concrete rather than cinders) have a compressive strength around 2500 psi, so with a 2x4 across the 1.25" center web can hold about 1.25*3.5*2500 = 10938 lbs.

That's assuming the block is upright, with the openings on the top and bottom. On its side, the capacity is about the same, but only if the load is centered on the block. It will only hold a fraction of that weight if the load is on the shell between the webs.

 

but only if the load is centered on the block. It will only hold a fraction of that weight if the load is on the shell between the webs.

And that includes people using ramps to drive the car up onto the concrete blocks. Puts lots of stress on the one side of the block.

Im trying to remove a rear end and curious cinder blocks with 2"x4" across the top on the cars frame rails should hold the car up very stable just inside each tire

Cinder (concrete) blocks are iffy enough. Could at least put a 2x8 on there instead of a 2x4. A 2x8 would probably reduce the chance of being killed by 25%.

 

I'm not sure if you mean actual 'cinder blocks' (cement and wood ash) or just using it as a term for concrete block (CMU). A lot of cottages and season properties up here used to be built on concrete block piers, so they can support a fair bit of static weight. I certainly wouldn't be trusting a piece of 2x lumber to carry the weight of a vehicle, unless you are just using scraps as a pad between the vehicle and blocks. How high do you need to get it to haul a rear end assembly out? How are you planning to drop out the old rear end and jack in the new one?

 

I had them crumble and vehicle on the ground. Don't risk your life. Use jack stands.