How to do an engine assessment on my wifes 81A?

[david.skinner]

Next job - I would like to assess the state of the engine in my wife's 36 pickup. I think that the engine is a '38 81A, as I can see a 81 stamped on the block (but no 'A') and the engine is a 24 stud with no raised intake manifold so from my desk research this puts it into the 38-41 bracket.

My concern is driven by the way the oil pressure drops as the engine gets hot. I get around 60 lbs on startup but it will drop to 15-20 with around town driving revs and down as far as say 5 on hot idle.

I thought I would start with a compression check. Can I assume that these are done hot with wide open throttle and the primary side of the ignition coil disconnected?

I was also thinking of dropping out the oil pan and checking for sludge and looking at the oil pump intakes but happy to take any advice available.

My objective is to get the car up to level of reliability that makes it practical to go on vacation in it and use it regularly around town.

Regards
David

[deuce_roadster]

What you describe could fall into the normal range depending on the viscosity of the oil and how slow your idle is. What is the other general description of the engine? Does it have high miles on it, does it smoke? The reliability issues generally have to do with the entire fuel system and ignition and charging systems.

[Talkwrench]

I too think that sounds normal. Yes do a compression check as you stated, attach a vacuum gauge as well. If you haven't seen the oil in it yet then drop it. Reliability as stated above, Lawrie has done a few miles and you will see his issues where minor, he was even towing. Silly things can still happen, I rebuilt my trans only to have a bearing fail a few months later pfft what do you do ??!

[david.skinner]

great advice. I have no idea of the miles on the engine but it is very tidy externally and doesn't smoke at all. The previous owner changed the oil last year and used Penrite HPR 30 Oil looking at the receipt. I don't have a tacho but the idle is pretty slow as I adjusted it up just a bit from stalling. It appears to run very well from my limited experience. What do I test with the Vacuum gauge btw? much appreciated.

[Talkwrench]

If you know how to use a vacuum gauge you can tell a lot, also how to tune. Greatest instrument you'll own .. [apart from your own]

This should get you started

[david.skinner]

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Wow this is so cool - more toys and interesting ones at that. I'll let you know how I get on.
Many thanks.

[david.skinner]

Did the compression test today - interesting results which I would like feedback on please. My procedure was; warm up, remove fuse to electric fuel pump, disconnect coil primary, cable tie throttle to wide open, remove plugs 7 cranks on each cylinder then retest wet after 5ml oil each cylinder. I include this in case I have this wrong

Results:
assuming 1 closest to front on RHS and 5 closest to front on LHS
dry wet
1 100 119
2 103, 116
3 100, 114
4 100, 110
5 80, 89
6 78, 89
7 80, 88
8 82 92

Different heads on each side so can I assume different compression ratios or can something else cause this? next steps? btw the engine runs very well now with a Skip distributor in it.

[WestCoast]

a compression check is good for finding out the condition of the upper end, rings and valves, but will tell you nothing about the lower end, mains, rods or cam bearings, compression readings are good on one side, but as you said with different heads on one side could account for the lower readings on the other side, but they are all very close to each other, so I think every thing is all right on the top end, ford never gave a pressure reading at idle, all the book says is 30# at 30 mph, seems like you have a pretty good engine, any oil pressure at idle is good on those engines

[41ford1]

Those numbers look OK. The difference from side to side is most likely caused by the different heads. Can you post the head numbers? The "low" oil pressure is common on these. My 59ab in the avitar runs about the same. It's been like that for the 7 years I've owned it.

[david.skinner]

Head photos attached. the 1st photo one is the higher compression head

[41ford1]

I don't know if the C7RA-A head is equal to the 77 head listed in service bulletin 6000. If it is, that CR is 7.5:1. The 81T head has a CR of 6.2:1. My info comes from SB 6000 dated Feb. 6 1942. Both heads are for domed pistons.
If this engine runs well and has no strange noises run it as is. No need to fix something that ain't broke.

[david.skinner]

thank you for your advice. I'm not too unhappy about the results. Wifey has expressed some interest in aluminium heads but there is no rush on this. I'm more interested in getting to know each of the systems and their quicks. Is there a non invasive way of checking the state of the bottom end. I'm intending to drop the oil pan anyway as I don't know whats in there.
Regards
D.

[Bassman/NZ]

The C7RA-A head is from a Canadian 46-48, ie: a C59A engine. I have those, they are higher compression than an 81 head. I think that is your answer. Your oil pressure sounds OK to me, Penrite HPR30 is a damn good oil. I think you're good to go.
BTW: Most engines in NZ are Canadian, unless they've been recently imported from the US.

[swoopNZ]

See how far the front of the crankshaft extends out.
Only an inch or so means it's a '38 with the smaller diameter crankshaft.
Sticking out a couple of inches makes it a 39-41 .

[david.skinner]

Thanks swoopNZ, there is about 1/2" between the crankcase and the belt pulley (or mre accurately the front of the oil pan and the belt pulley) then about 2" to the manual crank plate thingy? Does that help? would love to get it ID'd.

[david.skinner]

Hmmm - if the Canadian head is from a 46-48 wouldn't that make it for a 3 3/8" bore whereas the block should have a 3 1/8" bore?
Is it time to pull the heads off and do some measurements?
Haven't done this with a flathead before - any tricks? just a set of head gaskets in parts?
regards
David

[38 coupe]

Sounds like you have the 39 and later crankshaft. We did a long thread on engine IDs a while back. Check it out: https://www.fordbarn.com/forum/showth...5301&showall=1

If it was me I'd find another car head so you have matching car heads. You will get a little bit better performance and gas mileage. I personally prefer cast iron heads to aluminum since you don't have the corrosion issues with cast iron.

[ford38v8]

David, off topic, but it looks like you have a copper fuel line connected directly to the carburetor without a flex line. This is wrong on several levels!

[41ford1]

Originally Posted by david.skinner

Hmmm - if the Canadian head is from a 46-48 wouldn't that make it for a 3 3/8" bore whereas the block should have a 3 1/8" bore?
Is it time to pull the heads off and do some measurements?
Haven't done this with a flathead before - any tricks? just a set of head gaskets in parts?
regards
David

The heads will generally interchange between 221 ci and 239 ci as long as both engines are 24 stud. The Canadian head will have a CR 6.75:1 if it is the same as the US 59 head. If the Canadian C7RA-A is more common in New Zealand replace the 81 with one. I personally would not go into it to measure things until it is time to replace the head. A flathead is a pretty robust piece, don't go breaking it trying to fix something that is not critical. You will probably find more important things that need attention first. Like the fuel line.

[david.skinner]

no worries on the fuel line - next on the list, however there is a flex hose at the firewall.

I shall go on the hunt on the other thread. Many thanks. Got no problems at all replacing the heads just want to know exactly what with.

Regards
D.