So I just took my engine to the machine shop to get work done to both the engine block and cylinder head. This will have a MK Turbo kit and will be swapped in a 1990 NA seen here https://www.miataturbo.net/build-thr...vt-swap-92987/. This will be mainly a street car with an occasional autox maybe track day. My power goal is 250-300 whp. I wanted to get quality parts for the engine build so I went with the following.

On the block I am going to be installing:

• Supertech 8.6 CR 84mm pistonsw/ Wiseco rings
• Manley H-beam rods
• ACL Main and Rod bearings
• ARP main studs.
• new freeze plugs

On the head I'll be using:

• Supertech +1mm oversize intake and exhaust valves
• new valve guides
• supertech valve seals
• Supertech double valve springs
• MMD SUB lifters.

The machinist is going to do the following:

Engine block:

• Hot tank block
• remove and install new freeze plugs
• deck block
• bore and hone
• install pistons onto rods
• install and gap rings for boost
• line hone for main studs
• install main and rod bearings using ARP hardware

Cylinder Head:

• Hot tank head
• 4 or 5 angle valve job
• install new valve guides
• cut valve seats and install bigger supertech valves
• mild porting in bowls and runners
• install double valve springs
• install MMD SUB lifters and clearance with lash caps
• probably have to tip the valves for proper clearance according to machinist

He mentioned getting the crank balanced and said he would ask his dad whether he thought it was needed. What do you guys think? Should I get the crank balanced while I'm already there getting all this work done? I'd like to rev this high so I think balancing would be a good idea. Another thing he asked was if I wanted the block to be torque plate bored/honed. I did a little bit of research on that and read different opinions, but what do you guys think? Should I get this done? If I remember correctly, he said it would be like $50 or $75 extra to do it with that torque plate.

Also I was thinking about gasket matching the intake ports on the head. Will it make a big power difference considering that I am going to have a built engine with boost? I want to do all I can while it's already there so any suggestions/recommendations are welcomed. They gave me a quote for $1495 for all this work. I supplied all the parts besides the valve guides and lash caps. He will be getting them from SI valves and the caps from Supertech. This shop works on a lot of different motors and has experience in building performance miata engines. It was recommended to me by another miata owner who has gotten work done there. When I took it there, I didn't really request any specific clearances, I did however mention it would be boosted. I trust they know their stuff since they've built performance miata engines before. Is there any specific tolerances I should be asking for when building a forced induction engine? Any and all comments about this are appreciated.

Thanks,
David

 

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Wow that was quick. Thanks for the link. Lot's of helpful info. The machinist did say that the tolerances are better on the looser end so I'm assuming he'd be using factory spec tolerances on the max side, like you said. For the Supertech pistons, he said he would go with what supertech recommends to use. They have installed these pistons in BP engines so again seems like he knows what he's doing.

 

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He showed me around the shop and they have this neat machine where you put the crank and you can bolt on the rods/pistons to the crank so it can be balanced while they are on the crank. Again he said he would talk with his dad and see what his opinion was on whether it was needed. I feel like they know what they're doing with the boring and clearances, but I could ask what they're normal procedure is when boring out cylinders.

 

If they bolt bob weights to your crank, they dont know what they are doing and you should leave with all your parts.

 

I'm not exactly sure how they're going to get it balanced. If I remember correctly, they had a Sunnen crankshaft balancer. I was told by the owner's son that they do work for Track Speed Engineering so I trust that they know what they're doing.

​​Is it bad to balance your crank using Bob weights? I'm not really sure how that works.

 

bob weights are not used to balance an inline 4 cylinder crank.

 

I see, I'm going to do a bit of reading on that. So what do they do when they say balance the rotating assembly? Do they only balance the rods and pistons and leave the crank as is?

 

Okay so I read up on balancing the rotating assembly. Like Twibs said In-line cranks do not require a bob weight to be added for balancing. From what I read they weigh and balance the pistons and correct them if they're off. They do the same for the rods and also like Art said, they balance the big end and small end of the rods. So what about the crank? If they don't use bob weights, then do they just put it on the machine by itself? If that's the case I don't think it would need balancing, I would guess that Mazda did that at the factory, but what do I know. Same thing with the rods and pistons. The rods are new Manley H beams and I believe they are already weight matched pretty close to each other. I do remember seeing a paper inside with the weights of each one. The Supertech pistons I think would also be weight matched, not sure on that one. I bought these used, so would it be a good idea to get them check and weight matched if needed? What about the rods and crank? I'm not sure if I should even get this done.

 

When I was having my machine work done on my engine my builder told me that an inline 4 usually does not need to have the crank balanced and only need to balance the rods and pistons(which i did myself). I believe the theory is like you said, the crank is a flat plain, so even with the masses of the rods and pistons changing, it should not affect the balance of the whole assembly. I would assume that the only reason to put it on the balancer is to improve on Mazdas original balance or if you knife edging/ bull-nosing which is not common. He recommended I didnt as it was so expensive for such little gain, and having about 900miles on the engine I agree as this engine is about 10 times smoother than any stock BP or B6 engine I have driven.

 

I spoke with the machinist today and asked that he weight match the pistons/rods and leave the crank as is. I don't think I need to improve on the Mazda's original balance like you say, but I'd like to at least check the pistons and rods since they are lighter than stock and since the pistons were used.