Advice needed on engine rebuild: piston mystery

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Vermond
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Advice needed on engine rebuild: piston mystery

Post by Vermond » Sun Sep 02, 2018 7:25 pm

Thanks for letting me join your tractor forum! A short introduction why I need your help: A friend of me lends me his Nuffield tractor so I can mow my grass :-). In the last plusminus 5 years it probably ran not that much and now I have it in use for mowing my grass it probably has to work a bit harder. We have encountered some engine problems, and I have quite some experiences working on cars, but not on tractors, and I can use some advise. The problems in chronological order:

(Engine: 4DM (I think :) ))

1) Coolant disappearing very fast and rising oil level: After removing the head I found two cracked cylinder liners. The other two were worn as well so I have removed them all.
2) Worn bearings on conrod big-end bearings
3) Maybe worn main bearing crankshaft

I have changed a lot of heads on car's, but I have never had to check whether the crankshaft bearing-dia's are still within tolerances. At the moment I cannot measure the main bearings because they are too big for my micrometer, but I have measured some of the crankpin diameter's and I have made a graph to visualise my measurements and available bearing sizes.

Image

The first two 'lines' represent nominal ranges for the crankpin dia and the big-end bearing dia, including the nominal clearance. I found out that the bearings in the big-end are 010 ("?) bearings. I have added these nominal dia's as well in the graph, and I have added my measurements of the crankpin's: 61.52mm (phat dashed line). My thoughts: The crankpins are worn more than allowed and I have to go to 020" bearings, and I have to refinish/turn the crankshaft?

I can of course visit an engine rebuild shop, which I will probably do this week, but I'm curious if my reasoning is correct.

Some pictures of the tractor:



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Last edited by Vermond on Sat Nov 09, 2019 3:46 pm, edited 1 time in total.



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Jeroen Sleijpen
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Re: Advice needed on engine rebuild

Post by Jeroen Sleijpen » Sun Sep 02, 2018 8:38 pm

Hi there, welcome on the forum.

Looks like your crank journals are worn. I take it you measured all of them? Best is to take your crank to the local engine rebuild shop and have the crank machined to the appropriate undersize. It's not just the wear that determines the next undersize, but also any damages to journals that need to be cleared by machining.

Keep us posted!

Jeroen
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Vermond
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Re: Advice needed on engine rebuild

Post by Vermond » Sun Sep 02, 2018 11:32 pm

Hi Jeroen, thanks for your quick response.

I agree and I will take the crankshaft to an engine rebuild shop this week. I was wondering, are all the bearing-sizes still available for all journal-undersizes? What is your experience with machining the crankshaft, are all dimensions and tolerances known at these shops or do I have to do my homework and supply the shop with possible undersizes and so on?

Rick Cobley
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Re: Advice needed on engine rebuild

Post by Rick Cobley » Mon Sep 03, 2018 12:34 pm

Hi, Best way of going about this is to find out what sizes are available and get crank ground to match. Charnleys or Tractor Spare Parts will have them.

Regards Rick Cobley

Vermond
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Re: Advice needed on engine rebuild

Post by Vermond » Mon Sep 03, 2018 12:46 pm

Hi Rick, thanks for your response and advice on shops. The website where I bought my cylinder-liner etc only had 0.010" as "undersize", and now I see there is a lot more still available. I will make a list on available sizes so the rebuild-shop knows to which diameter to machine the crankshaft.

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Jamie Davis
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Re: Advice needed on engine rebuild

Post by Jamie Davis » Mon Sep 03, 2018 2:00 pm

A list of parts suppliers is here:

viewtopic.php?f=24&t=984
Jamie Davis

Past - Nuffield 10/60
Present - Nuffield 460 & Nuffield 4/65

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Jeroen Sleijpen
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Re: Advice needed on engine rebuild

Post by Jeroen Sleijpen » Mon Sep 03, 2018 8:19 pm

The most common undersizes are 0.010 up to 0.060 but some manufacturers have some odd sizes in between.
Just go to your machine shop and have a look what is necessary and have the crank machined to the available undersize.

Jeroen
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Vermond
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Re: Advice needed on engine rebuild

Post by Vermond » Tue Sep 18, 2018 11:47 am

Update: the crankshaft is checked by the machine-shop and confirmed to be straight and the crank journals have to go to -0.02" and the main bearing journals to -0.03". At the moment my supplier is finding out if they can get those undersize bearings.

The bearings fitted were already -0.01" and -0.02" for respectively the crankpin journal and the main journals.

Vermond
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Re: Advice needed on engine rebuild: piston mystery

Post by Vermond » Sat Nov 09, 2019 3:59 pm

I have ordered some new piston rings, but I can't seem to find the correct ones :), so I am in need of some advice.

First I ordered a set for a 95mm bore Universal Four engine, but I got variant (b). Than I noticed the first pistons had a parallel top ring, and I ordered the tapered ones. After receiving the parallel rings (c), I got a lot of play, and they are certainly not the correct ones.

In the image below you can see the two incorrect rings (b) and (c), and the original one: (a). Does anybody knows what top ring I have to order? It is around 3.14mm thick.

The piston has the following number: 521 201. If I google this, I end up with a piston for a Austin 702 7ton truck (which has a 95mm bore, so it could be possible). But I was wondering, how to get a top ring for this piston? :?


Image

diggerdeano
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Re: Advice needed on engine rebuild: piston mystery

Post by diggerdeano » Sun Nov 10, 2019 5:56 pm

hi some truck engines were fitted with hardened top ring grouves so its possible your new pistons are of this type the thing i cant remember is if the rings were different or not

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