Machinist types...need some advice:

[makenzie71]

I love my 1991 RX-7.  Had a great big oil leak on the front cover...spewing a quart of oil in about 2-minutes at anything above idle. Just found another, though....

The pass. side motor mount, where it bolts to the motor, the bolts are stripped out and oil is seeping from them. There's also about a 1/4" gap between the mount and the block at the smallest bolt (the one on the end)...is there supposed to be something in between the blocka nd motor mount here?  Surely all you GS'ers would know...just run out and check your RX-7.

Biggest question, though, is would it be safe to tap the holes one size bigger? They're 12mm bolts now...if I step it up to a 1/2" bolt would I encounter any issues? Going to go with a standard size because all of our taps are standard. Would there be enough material to cut threads to go from a stripped out 12mm bolt to a 1/2" bolt?

Other suggestions, except helicoil (too much $$$), will be greatly appreciated!

[delineator]

an m12 fastener has an outer thread diameter of just shy of 12mm, so the stripped hole is probably nearly 12mm in diameter.

a 1/2"-13 fastener calls for a hole of 27/64" that you then tap. that is approx 10.72mm.  so if your m12 hole is completely stripped, the hole is too big to get good threads tapping for 1/2".

helicoil it, or better yet use a threaded insert, then you can use a standard m16x2 tap. helicoils use odd taps. you can get them here, mcmaster is awesome for just about everything.

i would personally prefer to keep everything metric, so

www.mcmaster.com
metric inserts:
$4.96 for 1  search for 97120A260
$12.96 for 5 search for 97084A260

and a tap would be about 30 bucks from them if you dont have access to one. drill hole to 14mm then tap.

or do the same philosophy as above for a 1/2" fastener since you have access to inch taps. for a course thread 1/2" bolt, the insert would want a 5/8"-11 threaded hole and use these inserts:

90259A163 at $8.25 for a 5 pack.

just a few bucks and you'll be sorted.

[makenzie71]

I just got a 9/16x18 tap, a 33/64 drill bit, and some bolts.

[delineator]

yeah, that would work too... probably the best idea, actually, if not too worried about keeping it metric.  have fun.

[makenzie71]

Actually I had to go back and change some things due to availability of tools and the dimensions of the engine I'm working with.  I only had a tapered 9/16x18 tap, but the hole I'm cutting threads into is only about 9/16" deep.  So there wasn't enough room to cut the threads fully.  I had to go back and examine what I have available to me.

I have a 9/16x12 shallow taper, and a 9/16x12 bottom tap.  Not fine thread stuff, but there's no reason why it wouldn't hold.  I'm going to drill the holes 1/4" deeper and hope I don't hit a cooling passage.  Then I'm going to set studs into the block with some good Loctite and set the mount up with nuts and lock washers.  then I can finally move away from oil leaks.

[Paulcet]

There's oil behind that mount?  Maybe the gap is a seal that's missing.  If there is an oil passage there, you might want to be careful not to get any chips in it.  Though I don't know what you could do...   

[makenzie71]

nah part of the oil pan is sandwiched between the motor mount and the block.  With all the bolts stripped and loose the seal wasn't tight and oil was seeping.  The way the rotary is made there are no oil passages really...only thing I have a chance of hitting in this particular area is the water-jacket, and if that happens I can seal it with silicon and a good bolt.

[delineator]

yeah, studs is a good option too, just be sure you never mess those up or then what do you do?!

[makenzie71]

Poop on myself and sell my GS750 to buy a new motor, then set this one on fire.

I'm considering selling my GS750 anyway...it'll save $500/year on insurance, registration, maintenance and parts and could cover the cost of a TII transplant.  Then I could sell my current motor (with the mounts repaired) and make $500 of that back.

[ohgood]

If its cast iron, helicoil, if its cast aluminum, weld and tap.

I wouldn't mess with going any deeper into the bock. Rule of thumb is 1 1/2 times the thread WIDTH for how deep it needs to go for holding strength.

Soo, 1/2"-13 = 3/4" deep for the tapped hole is fine. Add 1/8 for drill relief and chamfer. Total is still less than an inch.

[makenzie71]

I got the two "interesting" bolt holes tapped...they actually go all the way through to another external portion of the block.  I just couldn't see it because of all the crap in the holes.

The inner mount is pissing me off, though.  They sheared that bolt off and it looks like they already tried to use an ease-out and broke a taper onto the broken bolt.  I'll be trying it some more tonight, though.

[ohgood]

I got the two "interesting" bolt holes tapped...they actually go all the way through to another external portion of the block.  I just couldn't see it because of all the crap in the holes.

The inner mount is pissing me off, though.  They sheared that bolt off and it looks like they already tried to use an ease-out and broke a taper onto the broken bolt.  I'll be trying it some more tonight, though.

get a sharp pointed punch and lightly rap on the remaining bit o the bolt. once there is a dent in it, start trying to tap it counter clock wise with the punch. heat. kroil. heat. kroil. over night. heat. kroil. then start the tapping with the punch again. very very few bolts can resist this approach. take your time.

if and when it begins to free itself - STOP. more heat, more kroil. then start again. the heat helps to break down the corrosive crap holding it in place. the TAPPING with the punch counter clockwise helps to break the bonds too. key is tons of taps, not tons of force.

you can dremel a screw driver flat on it also. then get a impact driver to try and loose it.

if the crap they TRIED to free it with falls out in the process ... STOP. get a drill (left hand is best) and drill it out just enough to beat in an old (flatten the end of it so it will broach the remaining bolt) allen key into the hole. rap on the allen key while turning gently. i've used this method a ton, works like fun bags and ass for solving problems.

got a welder ? weld a nut on the bolt... tap n turn. if it breaks you still have the tap n turn with the punch.

orrrrrr take it to a machine shop, pay them $40 or 50 and just don't care. they'll get it out for ya. or they don't deserve to work in a shop.

good luck !

[Paulcet]

works like fun bags and ass for solving problems.

I don't know.... fun bags and ass could cause more problems than they solve.

[makenzie71]

Works like fun bags and ass...lol....that's great.

I didn't want to go through that trouble, though, so I punched, drilled and tapped the siezed bolt for an M8X1.25 bolt.  Worked great.  The load requirements are pretty minimal so this will handle it.