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Author Topic: Pistons??  (Read 736 times)

ZJCapri91

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    • 91 Capri N/A 05 Excursion V10 06 F150 5.4 V8
Pistons??
« on: July 12, 2021, 05:17:19 PM »

Howdy everyone,
I've run into a big problem with an engine rebuild I'm having a machine shop do. My current Capri engine broke off the crank end, due to an improperly installed key on the harmonic balancer. This was no problem, as I had a spare engine (don't we all ;D) that I was going to just put in the car. This resulted in the realization that the cylinder wall on one cylinder was rusted in that engine. No problem, Ill have a machine shop hone the cylinder to get rid of the rust.

The machine shop instead told me it needed bored. Oh well, no problem. The shop called me eventually, and says it needed a .030 bore to get the rust out. This is a big problem. I cannot find any pistons for this engine, let alone for an .030 bore. I read about miata pistons in these motors here, but when I had the shop check it out for me, they said they wouldn't do it, because the piston in the miata is domed?  They also said that unless they can replace most parts in the engine that broke the crankshaft, per the engine builders request, they would not be willing to put the crankshaft into the good motor.

Does anyone have any suggestions for this? Are miata pistons actually good? Has anyone tried them? Has anyone found good pistons that are meant for an .030 or higher bore? Or should I just find another engine that is running and take it?
Any and all help will be appreciated, and yes I know the machine shop seems like a PITA.
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WashiestSnake

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    • 1991 Mercury Capri
Re: Pistons??
« Reply #1 on: July 13, 2021, 06:28:53 PM »

Miata pistons will work just fine, there is absolutely no problem with using them, so long as they are stock compression for your model.
Heres the Compression Ratio for each Model
NA Miata: 9.0:1
Base Naturally Aspirated Model Capri with Manual Gearbox: 9.4:1
Base Naturally Aspirated Model Capri with Automatic Gearbox 9.0:1
XR-2 Turbocharged Capri with Manual Gearbox: 7.9:1

The bore on the all of these pistons stock from factory is 78mm

The problem is your going to have a hard time finding a .030 bore, right now its very easy to find aftermarket pistons in .010, and .020 sizes both of which are available from Weisco.

What they have done is a +.75mm bore, and with this engine there isn't a large water-jacket, so even if you could find pistons(which you can have Weisco custom make for you), you could have problems with needing to sleeve the engine cylinder due to the jacket being so close.

 Also the machine shop is in the right here, much/all of these old parts need to be replaced, you cant just put in old rod bearings, and piston rings and the rest back in and call it good when you put new pistons in. All of these components wear just like your brakes do, and in this case they can be completely ruined when the crank pulley snapped.

You absolutely need to rebuild the whole engine if you want it to be reliable, I realize this is not a cheap option, but its a much better option then buying a used engine from another Capri. Another thing you need to make sure is if you will go down the route of rebuilding this engine is you need to make sure you have a long nose crank on the new engine, if you don't you can have this happen again down the road, there are fixes involving locktite, but its unlikely the engine builder will be willing to do them for you. Overall the Machine shop is trying to save themselves and you the problem of the car breaking down in 100 miles, because they did things the wrong way. It may seem like they are bullying you but that is not the case at all, everything they want to do inside that engine is reasonable. The only thing they are wrong about is that the Miata pistons work just fine. What compression ratio was the pistons you asked them about? if they are higher then  stock 9.4:1 you can and will have problems with leaning out the stock ECU.

Another thing when you do this is your going to want to get the head checked, resurfaced(if needed), replace the oil-pump, water-pump and get new seals throughout. I would also add I would replace the throw-out bearing, and have the flywheel replaced of resurfaced, and a new clutch put in. If you do all of this you are likely to have less issues down the road.
« Last Edit: July 13, 2021, 07:57:11 PM by WashiestSnake »
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1991 Mercury Capri 68,xxx Miles--Project(AWD)
1996 Lincoln Continental -- The Boat(Given to my Brother)

Rocketman

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    • 91 BPT AWD Capri & 1991 XR2
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Re: Pistons??
« Reply #2 on: July 16, 2021, 07:36:28 PM »

Rockauto is showing pistons for the Miata (1.6L, same engine internals) but only at 0.020" over: https://www.rockauto.com/en/catalog/mazda,1991,mx-5+miata,1.6l+l4,1190887,engine,piston+&+piston+ring+kit,14008

You could custom order pistons from many places, tell them it's for the 1.6L mazda.
There is a difference in compression ratio as WashiestSnake pointed out, none of the Miata pistons are "domed" though, even the 9.4:1 are dished, just not as deep.
I mean I have seen domed pistons for the 1.6L but that's for crazy high compression N/A builds

The machine shop does seem like a PITA. Do you have another option??
I'm not sure what I'd do if it were me, I'd probably order some new bearings and seals and take a weekend and smash the crank into the old motor.

There's also a good junkyard locator site, www.car-part.com
You can punch in your zip and see if there is an affordable motor nearby. Keep in mind you can use a Miata longblock as well, you'll have to use your FWD manifolds, valvecover, timing belt cover, etc - there is an extra coolant port in the head that will need a freeze plug installed into it. (Miata has the thermostat on the timing belt side)

Best of luck keep us updated!
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1.8L Turbo All Wheel Drive Capri... the "GTXR2"


greywolf27030

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    • 1991 XR2, 1992 XR2, 1993 XR2
Re: Pistons??
« Reply #3 on: July 17, 2021, 08:58:06 AM »

Miata pistons will work just fine, there is absolutely no problem with using them, so long as they are stock compression for your model.
Heres the Compression Ratio for each Model
NA Miata: 9.0:1
Base Naturally Aspirated Model Capri with Manual Gearbox: 9.4:1
Base Naturally Aspirated Model Capri with Automatic Gearbox 9.0:1
XR-2 Turbocharged Capri with Manual Gearbox: 7.9:1

The bore on the all of these pistons stock from factory is 78mm

The problem is your going to have a hard time finding a .030 bore, right now its very easy to find aftermarket pistons in .010, and .020 sizes both of which are available from Weisco.

What they have done is a +.75mm bore, and with this engine there isn't a large water-jacket, so even if you could find pistons(which you can have Weisco custom make for you), you could have problems with needing to sleeve the engine cylinder due to the jacket being so close.

 Also the machine shop is in the right here, much/all of these old parts need to be replaced, you cant just put in old rod bearings, and piston rings and the rest back in and call it good when you put new pistons in. All of these components wear just like your brakes do, and in this case they can be completely ruined when the crank pulley snapped.

You absolutely need to rebuild the whole engine if you want it to be reliable, I realize this is not a cheap option, but its a much better option then buying a used engine from another Capri. Another thing you need to make sure is if you will go down the route of rebuilding this engine is you need to make sure you have a long nose crank on the new engine, if you don't you can have this happen again down the road, there are fixes involving locktite, but its unlikely the engine builder will be willing to do them for you. Overall the Machine shop is trying to save themselves and you the problem of the car breaking down in 100 miles, because they did things the wrong way. It may seem like they are bullying you but that is not the case at all, everything they want to do inside that engine is reasonable. The only thing they are wrong about is that the Miata pistons work just fine. What compression ratio was the pistons you asked them about? if they are higher then  stock 9.4:1 you can and will have problems with leaning out the stock ECU.

Another thing when you do this is your going to want to get the head checked, resurfaced(if needed), replace the oil-pump, water-pump and get new seals throughout. I would also add I would replace the throw-out bearing, and have the flywheel replaced of resurfaced, and a new clutch put in. If you do all of this you are likely to have less issues down the road.

I"ll second that.
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Jack Byrd

ZJCapri91

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    • 91 Capri N/A 05 Excursion V10 06 F150 5.4 V8
Re: Pistons??
« Reply #4 on: July 17, 2021, 10:17:58 PM »

Miata pistons will work just fine, there is absolutely no problem with using them, so long as they are stock compression for your model.
Heres the Compression Ratio for each Model
NA Miata: 9.0:1
Base Naturally Aspirated Model Capri with Manual Gearbox: 9.4:1
Base Naturally Aspirated Model Capri with Automatic Gearbox 9.0:1
XR-2 Turbocharged Capri with Manual Gearbox: 7.9:1

The bore on the all of these pistons stock from factory is 78mm

The problem is your going to have a hard time finding a .030 bore, right now its very easy to find aftermarket pistons in .010, and .020 sizes both of which are available from Weisco.

What they have done is a +.75mm bore, and with this engine there isn't a large water-jacket, so even if you could find pistons(which you can have Weisco custom make for you), you could have problems with needing to sleeve the engine cylinder due to the jacket being so close.

 Also the machine shop is in the right here, much/all of these old parts need to be replaced, you cant just put in old rod bearings, and piston rings and the rest back in and call it good when you put new pistons in. All of these components wear just like your brakes do, and in this case they can be completely ruined when the crank pulley snapped.

You absolutely need to rebuild the whole engine if you want it to be reliable, I realize this is not a cheap option, but its a much better option then buying a used engine from another Capri. Another thing you need to make sure is if you will go down the route of rebuilding this engine is you need to make sure you have a long nose crank on the new engine, if you don't you can have this happen again down the road, there are fixes involving locktite, but its unlikely the engine builder will be willing to do them for you. Overall the Machine shop is trying to save themselves and you the problem of the car breaking down in 100 miles, because they did things the wrong way. It may seem like they are bullying you but that is not the case at all, everything they want to do inside that engine is reasonable. The only thing they are wrong about is that the Miata pistons work just fine. What compression ratio was the pistons you asked them about? if they are higher then  stock 9.4:1 you can and will have problems with leaning out the stock ECU.

Another thing when you do this is your going to want to get the head checked, resurfaced(if needed), replace the oil-pump, water-pump and get new seals throughout. I would also add I would replace the throw-out bearing, and have the flywheel replaced of resurfaced, and a new clutch put in. If you do all of this you are likely to have less issues down the road.

I appriciate that. I want to make a minor correction on the replacing stuff part. They wanted to replace the crank was really what I was getting after. Otherwise they "couldnt stand behind their work" I knew the rings and bearing needed replacing. Just to clarify, this is the spare engine, without broken crank, from a later model capri.

This was the information I needed, along with rocketman as well. I trust the machine shop understood what they were doing with the .030 overbore, but hey, professionals can be wrong. Mostly worried about the water jacket at this point.

I'm going to try different means, and by that i mean a different machine shop, who would be willing to polish the crank, make sure it's good, and put it in the other engine. Otherwise, custom ordered pistons will have to happen, but at least I'll get better N/A performance  ;D
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ZJCapri91

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    • 91 Capri N/A 05 Excursion V10 06 F150 5.4 V8
Re: Pistons??
« Reply #5 on: July 17, 2021, 10:25:22 PM »

Oh, and one more thing. The head was just rebuilt as of 2 years ago, with less than 2k miles on it. I'm going to install all new parts that you suggested. I basically gave them a stripped block. I'm definately going for reliability here, as she is the daily. Kinda sucks though with it being down for as long as it has been, prime convertible season is almost over here in Ohio.
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