Rod Bearings

[Dexta Jan]

I am working on my engine of a Dexta.

I wanna replace my bearings from the rod's.

But i don't know wich bearings i need to replace.

The number on my bearings is: 31131201 (4324).
I guess that the first number a perkins number is.
Is this the STD (standard) bearing or a oversize?

[Emiel]

Hello Jan,

Why do you want tho change these bearings?

You should measure the crank shaft and then decide whether it is usefull to fit new bearings, or that the shaft has to be grind round and to size again, before fitting new under size bearings.

Regards

Emiel

[Dexta Jan]

The bearings are in the middle copper coloured.
and one set bearings has a scratch.

The crankshaft and the rods are in good condition.
An old man who had a MF garage recommended to use the bearings wich are used in the engine.

He tells me that the numbers are probable Perkins.

On the main bearings i have number 31121221.
I found this on internet as a Perkins bearing.

[YorkshireDextaMan]

Hi Jan, If your shell's are worn down to copper i would imagine there's more of an issue to it than just replaceing the shell's.
First of ,most people on here will tell you that this is not a true Perkins engine and so any numbers on there you can disregard. The shell bearings are for a Dexta and thats it, the only marking you need to be aware of is wether it says +20. +30 , or STD, and you can get them anywhere, e-bay is where i have found best. Now for the other things you need to watch out for, you should also check the Mains, Check the con-rod big ends, where the shell's fit because if the shell is well worn it moves in the rod and damages the rod surface, and there aint nothing you can do but replace the rod.
Emiel's right about the crank, if it's scored it would need work ,but if it is fairly smooth you can light dress it with emery, light i said, lol.
it should be ok for light work,
And last replace the oil pump.
Jim

[Nick]

The bearings on my dexta havent got +10 +20 or std at all on them so how do i decide which ones to buy new? The crank is not scored and the bearings are not worn out but are very lightly scored.

[Tubal Cain]

If you intend to replace the bearings, the first priority is to measure the crankpins to establish whether or not they are within spec. The original crankpin diameter is 2.2485” to 2.249” (57.112 – 57.125 mm).

Althought the castings for the Dexta were made by Fords the engines were assembled by Perkins and are basically a Perkins A3 engine.

According to one of the main parts suppliers the con rod bearings for the Dextas which they supply also fit the Perkins A3.144 and A3.152 engines.

Gerald

[Nick]

The crank measures 57.127mm, which is the worst, the other two are right on the limit. Im not going to bother re-grinding the crank, but its these shells that are bothering me because there is no size on them.

[Tubal Cain]

You can fit standard shells.

Gerald

[russelm]

51.127mm tells us that the crank is within the original tolerance so they are standard bearings.

[Nick]

Yes, i am actually amazed that its still in tolerances, although judging by the rest of it and the low hours on the clock i dont doubt it. It works out (i think) that each journal on my crank still measures 2.2485 all the way around, how chuffed am i! So ive now coated them in thick oil and clean cling film until i get new bearings. While im on this subject, would it be an idea to slightly hone the bores? Ive got a bore honer, which grit do you reckon i should use? I think i have 180 or 240.

[russelm]

I would hone the bores only if you are replacing the rings.

[Nick]

Yes i am replacing the rings. Basicly the engine was seized from standing, i got it unseized and it then had no compression, so I took the pistons out at the weekend and most of the rings are stuck in and one was broken. So ive got a new set of rings and im getting a new set of shells and small ends, i just thought whilst its all apart i may aswell hone them ever so slightly. The bores are also amazingly within tolerances, there is hardly any wear at all that i can see

[russelm]

If fitting new rings then the bores will need to be honed to break them in.

[JC]

Make sure that the bores aren't worn before you fit new rings, Nick. If there's a ridge at the top it can break your new top compression rings. Also, don't be afraid to hone the cylinder walls more than "slightly". You need a good cross-hatch pattern for your rings to seat ( I guess that's "bed in" in British English ). You can hone a lot, before you change the bore diameter much.

[Nick]

There is no lip at the top of the bore, so what grit should i use to hone them? I think ive got 180 and 240 grit, i reckon i will try 240 first and see what its like, then do 180 if necessary. How do i tell when ive done it enough?

[blue32]

The lip on top of boring is mostly on the left side of cylinder (if it is any) this lip is bad for ring,s with old stock pistons and rings. (becouse of spill between the gap in piston and stock-tired rings, ok??) with new rings in old piston the risk for broken ring is not so big anymore...becose the new ring is tighter in piston even in stock pistons. But the best result is with new piston too, the spill between gap and rings is mostly eliminated, so therfor I prefer new piston with rings, and honed bore.
With change of new std. rod bearings, I do prefer additiv in the oil for slick an good running in engine, at the first ten houres running. And then shift into new oil again. This has newer failed for me. When the new rodbearing,s are to be mounted, I,d put oil film on the crankside on it, but not any oil on the bacsides of bearing. Mostly of repaires who engines is "going to thight around" after mounting is becouse of the fact that it is some oilfilm also at back of the bearing, and that is not good.

Blue32