engine mounts keep cracking - help

Yeah, you can weld up some heavier motor mounts, but then something more expensive will probably bust, something doesn't sound right here.

Is the engine and transmission solid mounted at all three points?

If so, that's no good, you need some compliance somewhere to have some give as the chassis moves around under load.

Generally a stock rubber mount at the transmission tail along with sold motor mounts works ok.

Good luck-

Paul T.

What you want is a very ductile weld, so it can take the vibration and shock loading without stress fractures. Definitely go to a 3/16" plate.

You haven't specified what your settings or wire size was for the weld.

You might want to try welding it with 6013 stick electrode. It is a very ductile weld.

Fatigue cracking, often closely related to vibration and stress concentration.

Hello, sounds like a design problem? The reason that some sort of rubber mount is typically used is that engines do vibrate, and allowing them to vibrate means they don't tear things up, while trying to rigid mount them simply vibrates the whole mounting structure until some part of it breaks. If you modify the current mount until it stops breaking, some other part will break.

First off, your weld is too big in proportion.

3/16 is ~1.5 times as thick as the base plate, and ~3 times as thick as the tube wall. That much weld metal there will create a hard spot and stress concentrations. Engineering-wise, any time the fillet leg is greater than the thickness, there is too much weld. When you fit these things back up again, grind the base plate flat and flush (no pits/ bumps), and square up the tube. You are looking for good fitup, with a perfect connection between the two before you weld. No notches, no sharp corners, all smooth.

My suspicion is that since he's using a Kent 1600cc motor, it's in a Formula Ford or Sports Racer of some sort which typically uses a rigid mount transaxle. Rubber mounts won't be an option.

Peter

its a rotating ineria, you have a strong motor with very little give through out the drive train, thats the hole point of building a race car dilvering the engine to the pavement the tighter the tolerances the less loss of power, it has to be snappy to shave a second, this is why soild engine mounts arose, it takes the engine rotational eneria and gives it to the tranny, drive shaft, rear end and tires, if these are with tight tolarnces the power exchange is instant, the stress levels are very high on every componet. in my colloge days i seen a few torn motor mounts sheer bolts, bent frames, one guy had a mustang with 1/4 soild motor mounts and a grade 8 chain to stop the the tearing, he had to replace the chain every couple weeks it would streach the links, 2 inches between the frame and engine, and your sitting at 9 inches with a cantilever, thin gauges lol snap, the more performance improvements you do, the reinforcing also has to be compensated in the attachments upgrading the bolts, locktight, helicoil threads, tieing the bolts with wire, upper cross member engine supports, frame torision bars, when you tighten things down it pulls to the next weakest link, if you have the room in your engine compartment build a frame extension from your frame to your engine mount, the wider it sreads across the frame the less pulling in one spot on the frame, once the engine mount is secure and the frame is reinforced the power will go to the tranny and tranny mount

Rodney, would you be interested in some periods? I have some I'll give you... I tried to read all this in one breath and had to get resusitated by my wife. But I think you're right. Steve

"SPR" wrote: Rodney, would you be interested in some periods? (clip) I tried to read all this in one breath and had to get resusitated by my wife. (clip) ^^^^^^^^^^^^^ I tink YOU are right, SPR. I have some period furniture, and, when I sit in it, I never get out of breath.

I have another thought about the racing engine mounts. Granted, as you make the mounts stiffer, to get power to the road quickly, you have to make it stronger to keep it from breaking. Don't you eventually reach a point where it weighs so much that you start to lose the advantage you were trying to gain?

Fillet size - I think there is a more obvious and easy-to-measure way to specify this, which is a much usable when doing the weld. Tell me if I am wrong. I reckon the fillet face width needs to be twice the material thickness.

This comes from simple trig. The working can be seen in the last of the three pictures at

By trig., if the minimum weld metal thickness is the sheet metal thickness, which will be measured at the 45degree angle from fillet root to fillet surface, then this makes the fillet face twice the sheet metal thickness.

I arrived at this idea when wanting to avoid "burning" defects on the back of TIG fillets on stainless steel sheet (a heat issue), but it works for this "racing car engine mounting plate" (a strength and compliance issue) equally.

Richard Smith

I've always used thickness = leg size as the maximum fillet. The weld dosen't have to be as thick as the base metal, since it is stronger. Using the thickness as the leg length, the face = sqrt(2)*t = 1.41*t ~

1.5*t.

There have been some good comments here, I would only add one thing: Look at the sources of vibration. On your next rebuild, weigh each piston, connecting rod, valve stem, etc., milling or drilling to equalize. Spin balance flywheel, drive shaft, etc. For some engine builders, these steps are SOP, but assume nothing if you contract it out.-Jitney

I am really surprised that nobody said any thing about the .060" tubing going to the plate. It is sometimes hard to get a good weld on thickness' that are not the same. When we used to fab up race car parts we would make them out of a little thicker material and then drill out the additional weight, while not giving up structural integrity.

inhale now quickly read ..... lol In most cases inline engines are easier to bring to a balance in harmonitics, its your v-'s that require fine tuning as you have roatating and centrifical balances. Most race engines are based on being blueprinted and balanced, yes weight can be a slight hinderance when reinforcement is concerned, anothor solution is aluminum, my minivan, 92 Dodge Caravan, for instance has all its engine mounts suspension members in aluminum very neatly designed, as well there are different areas where weight vs power can be be a give and take and allowances in the rule books, aluminum drive shaft, aluminum flywheel, aluminum master cylinder, plastic gas tank, aluminum rad, aluminum water pump, plastic electric fans, aluminum heads, pistons, rods cavlair body, endless list of bits of tweaking. Yes drilling holes in componets will take some weight loss as well as I recall from my college days. A training video back in the 30's they wanted to use a diesel race engine, so to improve its lap time they drilled the frame full of holes, it did perform better than the gas engines, yet the tires they had at that time could not handle that much torque, it shreadded the tires and lost the race. The rule book is the finial word as to what you can get away with.

ey" wrote in message news: snipped-for-privacy@posting.google.com...