Refurbishing an M90 and converting from L-Jet to M1.3

FlyingBrick · Post by **FlyingBrick** » Sat Feb 11, 2012 3:12 pm

Mike W. wrote:Just use M30 ones, a M90 is really just a specific M30. If work was done that recently, it would have good ones if they were replaced, but since you don't know...

Copy that.

Careful, I see mission creep. A shop can't do much more with an assembled short block than you can. Unless you're planning to take it apart completely and have it hot tanked. O ringing the block is unnecessary unless you're going to turbo it, and even then it's a luxury. As far as the head, location and the individual shop makes a difference, and I strongly suggest you look for the best place in town, not the cheapest, there's still a lot of good 'ol boys who only want to work on cast iron and will only begrudgingly work on ferign stuff. I would guess $3-400 to break down, valve job (which would include installing your seals) and reassemble. In case that seems expensive, and it used to to me, it's not. There is a lot of time involved in breaking the head down and reassembling without breaking anything. Oh, they're going to want to sell you valve guides too. They will be worn, but unless they're really, really worn, don't generally cause a problem. But that'll be another hundred bucks.

I don't want to fully rebuild the entire thing, but I'm wondering about the best little things to do. I firmly believe that if something's worth doing, it's worth doing right. Plus I'm particularly OCD about my car. I was thinking:

-Diassemble short block myself
-Have machine shop hot tank short block
-Replace crank bearings myself
-Replace piston rings (I believe M90 are NLA, are there M30 ones of the same diameter?)
-Maybe have machine shop o-ring (would like the option to go FI down the road and have peace of mind if so)
-Have machine shop diassemble head, clean, clean valves, install B35 cam, replace valve seals

I would definitely find a reputable place, and $3-400 would be reasonable to me for the peace of mind. Depending on initial cost, I would consider the valve guides if they were about another $100, but I will weigh it based on your advice.

Thanks for the great help so far.

GripGreg · Post by **GripGreg** » Sat Feb 11, 2012 7:29 pm

How many miles are on the bottom end? They usually last two head re-builds before new rings are needed. I've heard of these bottom ends lasting up to
400,000 miles with frequent oil changes. Meaning 3,000 to 4,000 miles between changes. Did you check the compression before the tear down?
Do change the valve guides 'cause you're right there. Plus, I've also heard that in the eighties, the material was differant compared to the material used for today's valve guides. And, they're at least thirty years old! I think age is most of the reason. Just my 2 cents.
Good luck,,,,Greg

wkohler · Post by **wkohler** » Sat Feb 11, 2012 7:32 pm

M90s are a different kettle of fish though. Due to the large bore and shorter stroke, they tend to oval the bores over time. It's worth having them checked.

GripGreg · Post by **GripGreg** » Sat Feb 11, 2012 7:41 pm

Good catch, Chris! These guys are E12 gurus. I'm just a pest.

FlyingBrick · Post by **FlyingBrick** » Sat Feb 11, 2012 10:14 pm

Thank you for the input Greg.

wkohler wrote:M90s are a different kettle of fish though. Due to the large bore and shorter stroke, they tend to oval the bores over time. It's worth having them checked.

The piston rings or the cylinder bores? Or both?