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Just finished putting the transmission and subframe back in after fixing a input shaft bearing, but most of the suspension (lower control arms, knuckles, ...) aren't installed yet. The front of the car is sitting on jack stands (on the jack points behind where the front wheels would be) and I can put a 3 ton jack under the front center subframe jack point for extra support. Would it be OK to start the engine in this configuration? That way I can check a few things and it would be a lot less work pulling everything apart again if something's wrong. I won't be getting underneath the vehicle while it's running.

It's a front wheel drive car, a Honda Civic.

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The jack points of a motor vehicle are the design locations to allow one to lift the vehicle without creating undue stress in inappropriate locations. This is done for service lifts for the purposes you have described. It's also for changing tires, rotation, etc.

Jack stands are considered one of the many safe methods to support a vehicle for such service. It's very difficult (but not impossible) to apply sufficient force to topple a stand, although the amount of effort to do so is substantial.

You haven't stated if this is front wheel, rear wheel or all-wheel drive, but if it is rear wheel drive and the front wheels are elevated, consideration should be made to ensure that the drive wheels will not be engaged while in this state.

You would not have to have the centerpoint jack other than for your own peace of mind, as the jack points can be considered, for your purposes, to be the same stress points as the wheels when in contact with the earth. As such, you'd not be driving with a fifth contact point for the vehicle.

You state you won't be getting under the vehicle, which endorses the acceptable level of safety for your objective. I would have no fear to be under a vehicle supported in the manner you describe, if all the other aspects of safety have been implemented.

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    It's a front wheel drive car. Honda Civic.
    – Unimportant
    Mar 14 at 21:54
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    @Unimportant you might want to edit that information into your question.
    – SQB
    Mar 15 at 9:03
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If a vehicle is supported correctly via jack stands, I wouldn't have a problem starting the vehicle. The main thing you need to worry about with the knuckles and front suspension not is place is, there's nothing there to support the drive axles in your front wheel drive car. If the axles are not in place, you'll most likely be leaking transmission fluid out the holes where the drive axles go, so not a good thing there. If the drive axles are in place, if it should go into gear for any reason, you'll be spinning the drive axles around without any support on the end, which means they'll be flailing around and causing damage.

If the front end was together, I'd say it wouldn't be an issue. With it apart, I'd suggest it is problematic at best and probably not a good idea.

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    I have 2 old sawn off axle "stubs" to plug the holes.
    – Unimportant
    Mar 14 at 22:01
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I'd like to add that the cheap stamped metal type of jackstands available at the automotive discount stores fail an order of magnitude easier than the beefy cast iron jackstands. It also helps to have the jackstands on flat level concrete, if they're on grass or similar you'd better have a chunk of 3/8" plywood at least 18" square under each one.

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