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Ford Tractors Discussion Forum

Bore size

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tommy59

11-15-2023 05:54:23

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Hey Everyone,

I have a Ford 850 with a 172 and i am looking to order an engine rebuild kit,

my question is they show 2 different bore 4 & 3.9 will both of them work in my tractor or do i need to measure the current bore.

I am planning on changing sleeves.

Thanks!

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RBnSC

11-16-2023 05:56:28

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Re: Bore size in reply to tommy59, 11-15-2023 05:54:23

Consider this, buy oversize pistons and have the block bored. Save on the cost of sleeves and installing them and having to hone the sleeves after the install. Unless the sleeves are loose then you will have to address that anyway. I have done it it works. You can have it sized to the piston to cylinder wall clearance you want or what is recommended. Ron

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showcrop

11-16-2023 05:35:54

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Re: Bore size in reply to tommy59, 11-15-2023 05:54:23

2X what Tim posted. You have maybe a 50-50 chance of being able to use parts that you order blind. In addition, once the machine shop has inspected and measured your parts and tells you what machining your block, head, crank, and cam will need they can order your correct parts for you. In addition, since they are doing this every day they can source them from the best places and even save you money!

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Tim PloughNman Daley

11-16-2023 04:35:01

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Re: Bore size in reply to tommy59, 11-15-2023 05:54:23

I highly advise not to go out and buy any parts until you get the block stripped down, boiled, magna-fluxed, inspected, and precision assessed as to what exactly will be needed and the correct sizes. Don't assume a previous owner hasn't modified the bore in the past. Take the block and head to a good, reputable machine shop who knows old FORDs and have them do that grunt work. As mentioned, the 800 Series used the 172 CID, OHV, Red Tiger Engine, with a 3.9 Bore diameter with a 3.6 Stroke, in standard production. The OEM Owner/Operators' Manual and the I&T F0-20 Manual has all the SPECS; don't know where you got 4 from. Whatever. Unless you have all the experience, correct tools, equipment, and/or own your machine shop, it would be futile to try and rebuild the engine without using a qualified shop. It is well worth the investment. Your mechanic can also supply your parts kit as well as he will get the correct sizes and parts.

Tim Daley (MI)

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tommy59

11-15-2023 16:42:03

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Re: Bore size in reply to tommy59, 11-15-2023 05:54:23

Thank You!

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Leroy

11-15-2023 07:57:48

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Re: Bore size in reply to tommy59, 11-15-2023 05:54:23

The over bore larger will be a thinner sleeve and are designed to add horsepower. Think most will want the extra power.

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pomester

11-15-2023 06:09:25

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Re: Bore size in reply to tommy59, 11-15-2023 05:54:23

3.9" is the original bore, 4" is often used because more is better, right?
The kits will come with pistons, sleeves, and rings so it doesn't matter which you choose. Both will fit within the 'dry block' bore.
An unsolicited suggestion, when you get your kit, take it and the rods to a shop and have the rods reconditioned with new wrist pin bushings sized to the new pins. Often overlooked, it can make a big difference in reduced engine noise and increased engine longevity. I'd have the shop instal the new sleeves also, they are set up and do sleeves regularly. It can be a challenging job in the home shop.

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