flat top pistons

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seahaul
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flat top pistons

Postby seahaul » Fri Jan 01, 2016 10:08 am

I have a 154 that has low compression (70-80) and decided to replace the rings. Found out that it has 0.040" oversize pistons with the flat top instead of domed. Will using flat pistons with a head made for domed ones reduce the compression to any large degree?

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Re: flat top pistons

Postby Eugene » Fri Jan 01, 2016 11:02 am

The domed piston head with flat head pistons will reduce the compression ratio. How much, unknown. Probably take an automotive engineer and a stack of formulas to figure out an approximate answer.

If the head has been installed, conduct compression tests. If compression tests are satisfactory, use the tractor as is.

The least expensive modification would be to change the head to one for the flat head pistons. This will provide a compression ratio of about 6.5 to 1. The numbered Cubs with domed pistons and head have a compression ratio of about 7.5 to 1.
I have an excuse. CRS.

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bob in CT
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Re: flat top pistons

Postby bob in CT » Fri Jan 01, 2016 1:52 pm

A 1950 head is just under 33.5 cc. A 1976 head (P/N 355 691 R2) is 30 cc, so using an older head would further reduce the compression, not raise it. It would be an interesting exercise to measure the displacement of the pop-up. I need beaker or something with enough accurate graduations to get a "good enough" number on the cc's displaced and then there are plenty of online calculators to get the actual number on compression. I have a late Cub and ran into the same issue. It was just rebuilt, but I pulled the head just to see what was what and it had 0.040" over cast iron pistons. I bit the bullet and bought the correct factory pistons at $130 each.

Did you do a wet/dry compression test?
Last edited by bob in CT on Fri Jan 01, 2016 3:35 pm, edited 1 time in total.

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Re: flat top pistons

Postby ricky racer » Fri Jan 01, 2016 2:50 pm

Check your bore diameters and compare the bore to the piston. If the wear in the bore is within spec, run a ball hone through the bores, install new rings, put it back ttogether and enjoy.....
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Re: flat top pistons

Postby BigBill » Fri Jan 01, 2016 10:07 pm

Cut the head .010" / .015" just to clean it and make sure it's flat.
I'm technically misunderstood at times i guess its been this way my whole life so why should it change now.

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Re: flat top pistons

Postby outdoors4evr » Mon Jan 04, 2016 8:59 am

My 184 had a similar story. At some point it had been rebuilt with .020 over pistons that were flat topped steel pistons.
Cleaned up cylinder walls with a ball hone and put in new dome top aluminum .020 over pistons. After break-in compression tests range from 115psi to 120psi.
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seahaul
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Re: flat top pistons

Postby seahaul » Mon Jan 04, 2016 9:25 am

Maybe since the block was bored and 0.040" pistons already installed, the increase in cubic inches will offset the slight reduction in compression and the engine horsepower will be the same??
I was suspicious about the engine power because the former owner told me it just did not have much power. This tractor was equipped with a front end loader. But since the compression numbers are so low, I figured the problem was in the worn rings rather than piston/head configuration. The ring end gap was 0.034" so I know the rings were worn a lot.
I'll install new rings and see what I have.
Thanks for your help.
Charlie

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bob in CT
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Re: flat top pistons

Postby bob in CT » Mon Jan 04, 2016 9:45 am

Do a valve job too and I recommend you have exhaust seats installed. I asked about wet/dry to call attention to the valves as a source of some compression loss too, but if they are even, more likely it is rings.

You can check your compression using an online calculator and compare the difference between standard and 40 over. Probably not much.

http://www.csgnetwork.com/compcalc.html

If you need the complete engine specs, this service manual will cover the engine specific items for you.
http://www.farmallcub.com/[ Sorry, direct links to manual section is not allowed. ]/files/Service%20Manuals/Blue%20Ribbon%20Service%20Manuals/GSS-1411%20W-Rev%201%20Cub%20and%20Lo-Boy%202-76%20OCR.pdf

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Re: flat top pistons

Postby TurboRoadster » Wed Jan 27, 2016 9:54 am

cheapest fix is to have head milled and trued, bump the compression up and give the old girl some nuts.

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Re: flat top pistons

Postby outdoors4evr » Tue Feb 23, 2016 7:43 am

@seahaul - What did you end up doing with this?

I am very curious as to what happens to a block once it has been bored .040 over and is worn out.
I guess that you could replace the rings and put in dome pistons and shave the head to increase compression, but the reality is that this block is very worn.
Is this block at the place where you have it bored for a sleeve? Or is a replacement block the way to go?
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seahaul
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Re: flat top pistons

Postby seahaul » Tue Feb 23, 2016 9:55 am

I installed new rings on the flat top pistons and compression went up to 115 on all cylinders. It runs great now. The previous owner stated that he was selling because it did not have enough power to use the front end loader. It just needed new rings.
The cylinder bore measurements indicated very little wear since it had been bored out to 0.040" over sometime in the past, so I would not call this a "worn out block" at all. I'm sure that eventually the cylinder walls will wear to the point that the block will have to be replaced, but it will probably not be for many years. In the meantime, I have a really great running tractor.
I was also encouraged to use this block because the crankshaft showed very little wear and had never been turned; it still had standard main bearings and rod bearings.

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bob in CT
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Re: flat top pistons

Postby bob in CT » Tue Feb 23, 2016 12:37 pm

Change the oil frequently and you will be all set for many years.

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Re: flat top pistons

Postby gitractorman » Tue Feb 23, 2016 5:59 pm

Great to hear that rings did the trick. The cylinder walls in the C60 engine are pretty thick so I'm sure your engine will be fine. I was thinking the same thing as Bob. I'd keep good oil in her and change it often. Also, I'd use good, no ethanol gas if you can find it and run a can of Sea Foam through it once a year. If you stick to this kind of service, it will outlast all of us!

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