Miata engine building for dummies (a learn me thread)

While I'm still looking for a used engine for my Miata I'm contemplating rebuilding the engine in my car. It's going to be more expensive by the looks of it, but at least that way I know what went in it. The downside is, I never rebuilt an engine before. Obviously, I'd farm out the machining and would "just" do the disassembly/assembly myself.

One of the options would be to just rebuild it completely stock. I'm not sure that's worth the effort, so I'm thinking about the following:

• Bump the compression and displacement using a set of Supertech 85.5mm pistons. As it happens, it looks like someone on CR is selling a set of 9.5 compression pistons
• Get the head checked and probably worked over a little by someone who knows. Either that or drop on a 99 head and intake manifold.
• Replace the usual suspects like oil pump, water pump, the service stuff. Obviously, use all new seals and bearings
• Maybe get the rotating assembly balanced, while we're at it

Is this feasible for a beginner whose only support network would be a bunch of Internet forums to build an engine like this? Obviously I'd probably be able to get some recommendations for good local machine shops, but this is V8 country out here.

Also, can I run an engine like this off the stock engine management for a while or would I paint myself into an MSPNP-shaped corner?

I'd probably just replace rings/bearings, have the head looked over at a shop, then slap it back together.

You should read this though:

http://www.motoiq.com/magazine_articles/id/1347/frankenmiata-part-i--freebuilding-the-engine.aspx

One test I advocate whenever you are dealing with low compression is a leakdown test. The gauge is inexpensive (less than $100) and it will help you understand that compression is escaping.

You might find out that a valve job is all you really need.

If you do decide to do a full rebuild, then you need to take it all apart and actually measure what is worn or not.

Miata engines run forever as a rule, so I would be surprised if you need a bore. New bearings would go in just cause it is apart. Once again, measuring will tell the tale. Unfortunately, by the time you buy all these nice measuring tools, you could have paid the machine shop to assemble it for you using all their neat measuring tools that they actually know how to use!

One test I advocate whenever you are dealing with low compression is a leakdown test. The gauge is inexpensive (less than $100) and it will help you understand that compression is escaping.

You might find out that a valve job is all you really need.

If you do decide to do a full rebuild, then you need to take it all apart and actually measure what is worn or not.

Miata engines run forever as a rule, so I would be surprised if you need a bore. New bearings would go in just cause it is apart. Once again, measuring will tell the tale. Unfortunately, by the time you buy all these nice measuring tools, you could have paid the machine shop to assemble it for you using all their neat measuring tools that they actually know how to use!

Leakdown test is actually on my to do list. The main reasons I think I have to dig deeper than just taking the head off and do a valve job are:

• Since the last PDX, oil pressure is noticeably lower than it was before the PDX. Now this might just be a pressure relief valve that is stuck open, but I somehow doubt it. Oil pressure is just a little low when the engine is cold (albeit running 15W50 for track use atm) and a lot lower (to the tune on 10-15psi) when the engine is hot.
• The engine just sounds like it's clattering/rattling more than a Miata engine should and it's not collapsed lifters (already checked that) even though the noise comes from either the intake cam or below it.
• The cams looked like the engine had been neglected at some point in the past (skipped oil changes and all that) so I might be in for an interesting surprise or three when pulling the head off.

I guess letting the machine shop do what they're good at is probably a wise decision, even though at least part of this work would be for the learning experience.

BoxheadTim wrote: I guess letting the machine shop do what they're good at is probably a wise decision, even though at least part of this work would be for the learning experience.

If you just want to build it yourself, cool. I rebuilt three or four BMW M10s when I was younger, it's quite satisfying.

One thing to look out for, though. Get the micrometers, etc. first and learn to use them--and make sure you can find a machine shop that will do exactly what you want them to do. The first one I did, I got the block & head back, put everything together, and had no compression. Took it back apart, had a friend teach me how to measure everything..and it turned out the machine shop had just cut the cylinders to 2nd overbore without being asked. At least the crank was in good shape, and they just polished it..

It's just an engine inside. You might be in V8 country, but measuring bearing clearances and setting ring gaps are the same.

BTW, I don't recall what year your Miata is, but if it's a 1994-97 then the compression ratio is either 9:0 or 8.8:1. Bumping to 9.5 will be nice but not enough to freak out the computer. Same with a 1999 head. If the bore is good, a set of stock 2001-05 pistons (10:1) might be a nice drop-in upgrade.

As for your oil pressure, I see a lot of problems with the senders as they get old. Partly corroded electrical connections on the exposed spade connector on the sender, partly (I'm assuming) gunk in the sender. Still, if the engine's clattering more than before that's an indication.

Miata is a 95 with (I think) the original engine. I retrofitted the OEM oil pressure sender and gauge from a 1.6, both were "known good" 5k miles ago but I guess these things have a finite lifespan. The oil pressure never was stellar, though. My 1.6s with similar or higher mileage in the UK had better oil pressure than this 1.8 ever had.

My thinking was that 9.5 with the bigger bore would probably give me a nice little bump in power but still leave the option of FI at some point if I discover the pot of gold at the end of the rainbow.

I guess one question is if boring the engine for 85.5mm pistons is "safe" as there seem to be varying opinions on that?

BoxheadTim wrote: I guess one question is if boring the engine for 85.5mm pistons is "safe" as there seem to be varying opinions on that?

My understanding is that it varies with the individual castings, some do, some don't, and really the only way to tell is to try is and see if it works. I definitely wouldn't go that far if you want to put a turbo on at some point (stuck with 84mm pistons on mine).

WHERE are you??

Even a solid rebuild should claw back 10-20% more power... just from everything being fresh.

Simple head work that almost always pays dividends... 3 angle with back cut valves, and matching bowls to seats.

Might be tough to find a head gasket for 85.5 mm bore. I ran 85 on the Targa engine, but that's getting pretty big. I wouldn't want to boost an engine with that big a bore.

codrus wrote:

BoxheadTim wrote: I guess one question is if boring the engine for 85.5mm pistons is "safe" as there seem to be varying opinions on that?

My understanding is that it varies with the individual castings, some do, some don't, and really the only way to tell is to try is and see if it works. I definitely wouldn't go that far if you want to put a turbo on at some point (stuck with 84mm pistons on mine).

I'm not really overly keen on turboing it, hence the quest for some more NA power. Looks like I managed to bag the parts, let's see how it goes...

Keith wrote: Might be tough to find a head gasket for 85.5 mm bore. I ran 85 on the Targa engine, but that's getting pretty big. I wouldn't want to boost an engine with that big a bore.

OK - boost is out then. I didn't want to boost it anyway .

The guy who is selling the parts is also throwing in the 85.5mm Cometic head gasket that you need to use with the pistons, so at least that part is covered.