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Login to post Forums » Grassroots Motorsports » Freshening a tired 180k Miata motor

Oct. 5, 2010 11:25 a.m. amg_rx7 HalfDork

I recently installed a low mileage 1.6 motor into my 90 Miata replacing the original 180k mile short nose crank motor. I did this partially b/c that 180k mile motor was burning oil and smoking at startup and partially b/c I got the donor w/ 90k miles cheap.

This car is going Spec Miata next season.

So, now I have this little lump of motor sitting in the garage and I'm wondering what to do with it. I'm considering a mild freshening and use it but I've never rebuilt a motor before so I'm not sure what parts would be necessary and what kind of costs would be associated.

I wouldn't want to go overboard with all new parts. This would need to be a budget build otherwise I'd just buy a crate motor. I was thinking maybe replacing the wear items - like bearings and piston rings and whatever other parts typically wear out in these motors. I'm much more familiar with rotaries and the parts that rotaries wear out but piston motors are something I never focused on.

Thoughts?
Oct. 5, 2010 11:47 a.m. Keith SuperDork

Flyin' Miata engine rebuild kit

That'll give you an idea of what you need. Lots of gaskets and seals will be replaced. If you're a member of Mazda Motorsports, you can get factory parts really cheap.
Oct. 5, 2010 12:07 p.m. 4cylndrfury SuperDork

Ive honestly never met someone who was more comfortable with spinning triangle wizardry, than a traditionally opposed rotating assembly...if anyone belongs on this board its you

All I can add as far as advice is that if youre married, the dishwasher is not as great a parts cleaner in practice as it is on paper...
Oct. 5, 2010 12:11 p.m. nocones Reader

Keith wrote:

Flyin' Miata engine rebuild kit

That'll give you an idea of what you need. Lots of gaskets and seals will be replaced. If you're a member of Mazda Motorsports, you can get factory parts really cheap.

You should probably note that it also doe not include the Kitty!
Oct. 5, 2010 12:13 p.m. triumph5 HalfDork

Keith wrote:

Flyin' Miata engine rebuild kit

That'll give you an idea of what you need. Lots of gaskets and seals will be replaced. If you're a member of Mazda Motorsports, you can get factory parts really cheap.

While you're at it a new oil pump. Cheap insurance.
Oct. 5, 2010 12:25 p.m. FlightService New Reader

+1 on the new oil pump, foolish not to do it while you are in there.
Oct. 5, 2010 12:27 p.m. amg_rx7 HalfDork

Yeah, I'm a rotary nut. Have owned many RX7s over the last 20 years and never had an engine that needed replacing. I have known people that have needed rebuilds and enjoy watching and reading about them. This whole piston thing is new to me.

I am a registered Mazda racer so I get really nice pricing on factory parts. Gotta love Mazda!

Couple of questions: - This motor is a short nose crank. Does that matter with regards to replacement parts? I noticed the FM kit is for the 91+ motors. - What would need to get done to the block (if anything)? I read about people sending the block to the "machine shop" when rebuilding but have no idea what that means.

Thanks.
Oct. 5, 2010 12:27 p.m. FlightService New Reader

I just read the kit and one phrase stuck out.

"right down to a tube of RTV to seal the oil pan and spark plugs."

So that is why I always get beat, I am not using RTV to seal the spark plugs.

Maybe they should re-phrase that sentence?!
Oct. 5, 2010 12:37 p.m. Keith SuperDork

Maybe "they" should I'll talk to "them".

Agreed, an oil pump isn't a bad investment, but it's not that unusual for people to leave the stock one alone. So we don't include it in the rebuild kit. Nor do we include the water pump, even though that's usually replaced. I'm not sure who came up with that mix of parts, honestly. That part number is really one that we use internally when rebuilding engines.

The biggest difference between the short nose and long nose engines is anything attached to the front of the crankshaft. The oil pump and front crank seal are different. Get the 1990 parts diagram from Mazda Motorsports and that will give you the part numbers.

As for what needs to be done to the block - if the cylinder bores have worn, you'll have to get it bored out a bit more and install larger pistons. A Miata motor with 180k is liable to be in excellent shape inside though.
Oct. 5, 2010 12:49 p.m. Kendall_Jones Reader

Also note, you cant overbore a SM engine.

In fact, take a lot of measurements when you take it apart (plasigage the bearings), You may not even need to change the bearings. We did a SM engine for a 1.6L that had 50K of race miles on it & the bearings were still well within spec. We didnt change them (loose motors are faster). We did however ring it & do a sweet head job on it. - It turned out to have 3 HP less than a "Pro Motor" during break in :)

Also, one of the SM "tricks" is to use 010 over rings on a STD bore & file to fit. I'm not a big fan of that (I like low tension rings).

The latest SM rules incorporate a lot of minimum / maximum measurements. read & incorporate that into your build.

Kendall
Oct. 5, 2010 12:57 p.m. amg_rx7 HalfDork

Thanks Keith.

I guess one can check cylinder bore (and piston) wear using some kind of tool and check the specs against the FSM to see if the cylinder bores needs some love.

The FM kit makes it seem like rebuilding would be a fairly inexpensive affair at ~$600.

How much does the cylinder bore honing typically cost if needed?

I'd probably need to get the head freshened/rebuilt as well. I read that this is best done by a competent machine shop. Any thoughts on how much this typically runs and it is indeed best left to a machine shop?

Thanks.

Edit - Kendall, I'm not sure I'm smart enough or experienced to file or profile to fit. I'll take a look though and see if I can make some sense of the SM specs in that regards.
Oct. 5, 2010 1:31 p.m. Keith SuperDork

I expect machine shop costs will vary around the country. Give a call to a couple of local shops, they can give you a pretty accurate price quickly.
Oct. 5, 2010 1:58 p.m. Kendall_Jones Reader

I think my local shop charges about $500 for a really good (my specs) head job (mill surface, valve job, assembly). You may want to invest in new valve springs & retainers as those can ruin your day if the valve drops.

Also, you can get decent quality cheap gasket kits on ebay. I'd spend the $30 for a mazda head gasket but the cheapo ebay gasket kits are OK (and they include copper crush washers, etc that the mazda kit does not). And, if you plan to run nasa, you have to use mazda parts (no non mazda parts allowed).

You can probably get away with having a machine shop hone out the bores. The FSM gives a maximum value that the standard bore can be. I've done 150K mile engines and not needed to overbore.

kendall
Oct. 5, 2010 2:21 p.m. RandyS Reader

about 4 years ago I freshed a 107k 1.6 with new rod bearings, valve stem seals, manually lapped the valves, head gasket, front/rear main seals, cam and CAS seals, and shimmed the oil pump relief spring with two washers.

In my experience the smallish 45mm rod bearings go first before rings in an otherwise good running Miata engine.

Last I heard the engine still runs as good as new with 30 psi idle oil pressure/60 psi oil pressure at speed and they have put another 40k on it.
Oct. 6, 2010 12:29 p.m. FlightService New Reader

In reply to Keith:

I noticed "they" changed the wording on the description.

Now I have 2 problems

1 I have lost my excuse as to why I get out run.

2 I have RTVed spark plugs.

LOL
Oct. 6, 2010 12:43 p.m. rjones33 New Reader

Keith wrote:

Flyin' Miata engine rebuild kit

That'll give you an idea of what you need. Lots of gaskets and seals will be replaced. If you're a member of Mazda Motorsports, you can get factory parts really cheap.

Hahaha: Oh Hai, you can haz headgasket nao.

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