Cylinder head milling & hone, measure block

Automotive
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Hello all,

Rebuilding a 1.8L Miata engine. Does anyone has experience and know-how to mill small Miata 4 cylinder block? Also interested in honing, measuring piston bore. I was going to being to a shop, but wondered if any of the fellow members had done this work before.

Thank you.

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Based on my own personal experience:

  • Honing is something you can do here, we have cylinder hones and ring compressors.
  • If a cylinder needs boring, I’d send it to a shop because you’ll want tolerance of the bore to match the whatever the oversize pistons are. There’s a fairly narrow band you need to be within.
  • Having a shop bore it will ensure proper alignment, straightness, roundness, and diameter. Plus their tooling will put the proper wall finish on it.
  • While there have them deck it so it’s flat if needed, usually the aluminum head is what needs to be straightened rather than the block. Be sure to know how much material is removed so if there are any potential valve clearance issues you’ll know in advance.

If crank needs to be reground, again, send it out and have it ground to match the oversize bearings available. Same for camshafts.

Rebuilding an engine is fun (and aggravating), but certain machining operations are just best to send out and have done. Modern engines have much tighter tolerances than the last engine I built, which was when dinosaurs roamed the earth.

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Talk to @Brandon_Green
The block for the lemons car (240) was sent to a machine shop for ~150 and the head was surfaced here

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Tom is a a good source of advice - I’d defer to knowledge.

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Miatas are cake to work on but there are some things that are still best left to the pros. David (@Photomancer) is steering you in the best direction.

Raymond
(2013 Club Edition PRHT)

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Need to keep in mind how boring, decking the block, and milling the head affect the static compression ratio. Have shop cc the chambers.

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Art - what if the head has no chambers ?

Then you use a thicker head gasket…depending on the amount of material that was removed from the head (within factory specs).

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As in it’s in the piston - as in some diesels?

That’ll work. Probably good idea to o-ring the deck or head.

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not just diesels…
This is a photo of a similar head off of a V6 engine - (V6 with one cylinder head) I have:

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VR5 or VR6?

need 20 characters

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6 in my case, matches the engine block in my avatar…add a another bank and you have a W 12 for Bentleys, add couple for cylinders and another bank and you have a W 16 for a Bugatti Veyron.

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I’d suggest like (Photomancer did above) have a good machine shop vat, measure, hone, surface and start with a clean bare bock then use the facilities of the Space for assembly.
Some procedures and or processes are better left to those that have to tooling set up for specific tasks.

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Forgot that they will vat it for you. Comes back looking new plus the scale on the water jacket is removed so more efficient for cooling and oil galleys are clean.

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Not the first rodeo. As a member I am interested in promoting the space and doing the work myself as much as possible. Bridgeport is a class I have yet to take, but good to hear we have the equipment. The block is perfectly fine; considering re-ringing but do not have expensive tools to measure for sizing purposes.

Would anyone be around this weekend to help milling the head? The head is stock height and straight. Just need to skim the top to remove old gasket and slightly increase compression rate.

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Photomancer would be your guy for decking the head and giving a tutorial on checking the bore on the cylinders.

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The lesson is easy: I find a friend that has had it done locally, ask if they would recommend them, if so, have them do the work. Here’s why

It’s not that our tools couldn’t machine the head, but when done it will be a custom one off design: variable compression (as in each cylinder would be slightly different). The trick to milling a head FLAT and SQUARE is getting fixtured up flat and parallel to the bed of the mill. Assuming the head 24" long, and you can get it to within 1/4 of a degree of parallel, the difference would be 0.100" different end to end, if it’s 10" wide it would be off 0.040" side to side.

They have fixtures and that will align it to less than that. I’m not sure how much difference a 0.100" of would make in compression because I don’t know the chamber size, but I’d guess probably half a point of compression so the engine wouldn’t be balanced from a compression standpoint.

We have no tools capable of boring the cylinders.

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Surely the Bridgeport table is flatter than that…

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The table is flatter that. But the surface you want to machine won’t be resting on the table. It has to be positioned so that surface is parallel to the table.

Now if you believe the head is not flat or true now, what do yo use as reference points to determine it’s parallel? The top of the head, where the cams, etc. are will be the side facing the table, so you fixtures will have to support the head so they are not in contact with surface of the table.

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Cam bearing bolt holes? (that surface). Those (presumably) should be flat when the head is mounted to the block.

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First, know nothing about car engines. This is just a question.

Could you not use the cylinder surfaces as your reference surface? Run a dti on their vertical surface and adjust until you have all grouped cylinders vertical. Averaging out any error among them?

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