Hi Folks, Please confirm or refute my assumptions for size selection for a main bearing replacement. I am not in a rush, just thinking through options.
I have no idea of my crank’s re-grinding history, if any, and I have not seen the crank yet.
The reason for considering the job is that the oil pressure is not at all spectacular when hot. 25psi at 3,000 rpm, with a mechanical gauge I am using Mobil 1, 5w30, high-mileage. (I previously used Castrol 20W50. I tend to agree with those who think that a high-viscosity oil like that is just masking issues and is lacking in protection at start-up. I could make a cup of coffee waiting for the pressure gauge to rise from zero. OK, I exaggerate.)
Again, for a roll-in replacement, engine in situ, I am supposing that:
the existing bearings will have a mark on the back if they are under or oversized (+.020, whatever) or whatever? How are they marked so I can correctly read what I have?
when doing a roll-in replacement, one checks the apparent condition of the journal, but does not actually try to mic it?
Plastigauge will play a part after selecting bearing size, to confirm a good choice was made, but that plastigauge has no role in the original choice?
Thanks for your comments. As you can see, this is a new area for me.
The oversize will be marked on each shell, otherwise, STD. A lot can be ascertained from the condition of the old shells and it’s a good idea to mark where they came from as you remove them.You will need to use the same size as you remove* so there’s no choice involved unless you decide to regrind the crank. Plastsigage will indicate the final clearance.
*Unless of course they’re the wrong size, which isn’t unheard of.
Just wondering why you would want to go for this. What mileage is your engine. For me whilst a ‘roll in’ sounds simple I would bite the bullet and take the engine out and undertake a total refresh given the incremental increase in scope and overall benefit.
Just my opinion.
Best Regards
Hi Liam, I expect that you have come to this conclusion after finding that oil consumption and cylinder compressions are fine - hence concentrating on bottom end. Having recently had my 3.8 rebuilt I have a few thoughts - you may have had same/done below:
Get an oil sample done - looking for wear elements (copper, chromium etc) and combustion carbon - maybe you might be better placed with a full rebuild - per Phil’s comments?
Check the bypass valve/spring in the oil filter head - not a big job. Poor seating and/or failed or tired spring will oil bypassing at lower pressures than ideal.
The XK 20w50 hot guide is 40psi at 3000 rpm - expect your oil will run lower than this by its nature - but how much?
Our XJ6 4.2 took a worrying while to build oil pressure after a week lay up. A spin-on conversion cured this as modern filters include an anti drain back valve.
Yes, I’d start with the gauge. I recently had erratic readings from a new 0-100 PSI “Smiths” mechanical gauge after start up (new engine rebuild). Slow to build pressure, and it wouldn’t move above 30psi. I plumbed in an old Stewart Warner and pressure was as expected. Then I pressure checked the new “Smiths” gauge on air compressor and confirmed that it was erratic and inaccurate. Returned the unit for a refund and found a genuine, used 0-60 psi Smiths indicator on Ebay. Pressure readings matched the Stewart Warner.
Hi Liam You might find this of some help. Jaguar recommends 20 - 50 weight oil for use in the XK engine, and specifically states in the manual that lighter weights such as 5W/20, 10W/30, and some 20W/40 should not be used in worn engines. I suspect they are not speaking about winter driving. I also suspect that they know what they are talking about as to weight recommendations.
The oil pressure gauge is electrical and does’t respond quickly - nowhere near as fast as a mechanical gauge, so I wouldn’t make too much of this. This doesn’t mean you shouldn’t check it’s accuracy. I use 20W/50 in my engines with a mechanical gauge and pressure is virtually instantaneous on start up. Also the 20W refers to the weight of the oil when cold.
You don’t say what your oil pressure is with 20W/50 but if it is 40 psi more or less when fully warmed up 3000 rpm, I don’t think there are issues being masked - given that it’s a recommended viscosity…
I wouldn’t worry about wear at start up. There is oil in the bearing to journal space at rest, unless the car has been sitting for a long period. The protection comes from a “wedge” that forms in the oil between the bearing and journal when the crank starts to move and that doesn’t require oil pressure to form.
I’ve done a number of roll in bearing replacements on my E Type that I raced - it was an end of season ritual, done if the crank journals looked ok. It’s not too difficult though it is time consuming. You might as well do the rod bearings as well.
Terry, maybe you have a later manual with different recommendations, but my '64 3.8 owners manual recommends SAE 30 for 32F to 90F, and a few multigrade 20W30 oils too, winter and summer. Recommends SAE 40 for temps over 90F. Duckhams 20W50 is also recommended, but the basic recommendation is SAE 30, OK maybe SAE 40 in much of the US in summer.
The oils not recommended for worn engines requiring overhaul (i.e., pretty seriously worn, not “normal” wear) are all oils with winter weights of 5 or 10. The impression I get is that multigrade oils were new then and the lower W weights were a little scary. I don’t think their recommendation was based on long experience.
Modern SAE 0W40 oil seems to me to be perfect for the engine today.
I am confident that the pressure relief valve works OK and I am pretty sure the gauge is not utterly wrong, but I may be mistaken. The pressure rises to 60PSI when cold and never above that. But it then begins its steady descent to 25 - 30 PSI when hot. This data is true in my car for Mobil 1 5W30 and for Castrol 20w50. The rate of decay in pressure as the engine heats is the difference. Castrol looks nice on the pressure gauge for longer. My point about my mechanical gauge being slow is that at start-up, especially in colder months, the Castrol 20W50 is decidedly slower to register on the gauge. Mobil 1 registers a much quicker reading. The Castrol is, apparently, thicker stuff and it resists flow. The latter is, by definition, what viscosity is all about.
My reason for considering a roll-in replacement is one of marital harmony and feasibility as much as bearing metallurgy. My removal and replacement of the IRS last year was not greeted with joy by my wife. Big lumps of iron or aluminum laying around my modest garage this year will not help. I would love to remove the engine and overhaul it properly. My wife and son believe I spend too much time under it and not enough time driving it. (I don’t live too far from Lippincot’s Garage in PA and that would be a nice approach if I could hide the expense of a full overhaul.)
I do not believe I have good compression. It is all over the place between cylinders (130 - 150), so I do not believe that is at all ideal either, but I am unclear on how that directly effects oil pressure. Would someone enlighten me, please? I know that also points to a rebuild. I get that.
Hi Jerry My manual is for the 3.8l, then it has an addendum for the 4.2l with nothing there about oil viscosity. The manuals (and handbook) came with the car when I bought it in 1972. The drivers handbook which was specifically for Ser 1.5 and is original says the same as the 3.8 factory manual as far a viscosity goes, except that it describes the lighter oil as not recommended for engines needing overhaul, instead of “worn” engines as in the 3.8 factory manual. Curious. My point was that he shouldn’t be afraid to run 20W/50, and particularly if it gives him the oil pressure he wants. (Just read his last post in which he says it doesn’t) Modern engines run much tighter clearances and need lighter oil. The mains clearance specified by Jaguar for the XK engine is 25 to 42 thou. which by modern standards is huge. If you look at this article by Mahle:
they suggest 20 to 28 thou for a 2.75" journal. I believe that cars built in the factory today have even tighter clearance, as they have better control over machining. It’s very difficult to cut a crankshaft as long and heavy as the XK one straight - that is having all the journals perfectly concentric with each other. This makes tight bearing clearances a real problem. This was a chronic issue with building a racing engine. Jaguar of course used machine tools that Sir William bought from Standard when he first started out. Standard sold them because they were worn out - Jaguar used them to build XK engines for as long as they built them. Yikes!
I attended a lecture on oils by a Shell engineer a few years ago and he was ecstatic about the oils that have 0 viscosity cold, but they are all either full or partially synthetic as I recall. Certainly in a place like where I live they give flexability for changes in temperature.
Liam poor compression has no effect on oil pressure. If your engine runs fine, isn’t knocking, or fouling plugs, why worry about a rebuild? Unless you are going to put a huge number of miles on it, nothing your engine is doing will lead to a catastrophic failure.
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The mains clearance specified by Jaguar for the XK engine is 25 to 42 thou. which by modern standards is huge. If you look at this article by Mahle:
Terry, the factory service manual? Page A.18 is a basic copy of the serial number plate oil recommendations. S2 recommendations are on p. A.Y.s.4. The S1 recommendations there don’t even mention the single weight oil, only the multigrade, and same recommendation about very worn engines.
I say roll-in a new set of bearings, if for nothing else, to inspect what is down there. The Haynes manual recommends change-out of the mains and rods, at like 50K & 30K, respectively, for lengthier healthy engine life (I posted about this not too long ago). AND, if you have someone will to help, under there, that in non-union, I say go for it. Just be sure to use “soft stuff” (wood, plastic) around the journals so as not to score, and lots of pre-lube on the journal side and as liitle as you can on the “back” side.
I wish I had, 20 years ago, done this when I was just inspecting inside the sump. It may very well have saved some grief and would have been at a time in my life when getting up and down from the floor was easier.