|The MGA With An Attitude
WORN OUT ENGINE Considerations - BE-104
At 09:51 PM 11/4/05 -0500, Steve Semko wrote:
>"Having trouble starting, and it runs bad in cold weather. The compression on cylinders 1 and 2 were around 75 lbs and cylinders 3 and 4 are running around 50 lbs."
No wonder it runs bad. Compression should be 125-140 psi, and all within a 10% range. Anything under 100 psi will make it hard to start when cold. More then 10% variation between cylinders can give a rough idle. (More than 145 psi would be a special high compression engine set up).
>"Well, I bet you know what I'll be doing this fall and winter."
Fall AND winter? This is not a complete restoration, just an engine overhaul. Lemme see here. 75 min to remove engine, 60 min for disassembly and inspection, which takes care of Sunday afternoon. 90 min to deliver block and head to the machine shop and order up machining on Monday morning.. Shop will procure new cam bearings and hardened valve seat inserts as needed. If you have a good relationship with the shop a rebore can be done and size known so you can order matching pistons by Wednesday afternoon. Second day air delivery gets you all the required parts by Friday afternoon, when all of the machine work should be finished. With due diligence it can be back together by midnight, in the car and running in time for the Saturday morning fall color tour with the club, which makes a good break-in run. If you get 500 miles on it on Saturday you can re-torque the cylinder head and take it autocrossing on Sunday.
>"I am now thinking that the holding back feeling that the engine shows while accelerating through the gears and the loss of power on up hill climbs is directly related to the low compression in all cylinders. "
Yup, very likely.
>"One question that puzzles me though is the fact that I do not see any oil burning smoke coming out of the exhaust."
No problem. An engine can burn up to a quart lf oil in 200 miles, 8 gallons of fuel (32 to 1 gas/oil mixture), with very little smoke. If the valve guides are excessively worn it can blow a quart of oil out the tail pipe as little droplets in 50 miles with very little smoke at steady speed (don't follow close behind it). Shifting gears gives a puff of white smoke when you put your foot back on the throttle after overrun.
Blue smoke is too much fuel, which is corrected with carburetor adjustment. White or light gray smoke is excessive oil getting past worn valve guides. This is fixed with a valve job and new guides. If the rings were bad enough to cause pressure that low, it would be blowing lots of dark smoke out of the crankcase breather pipe and oiling up the air cleaner in a matter of minutes.
An engine can be excessively worn in all respects and even smoke like crazy, but still run fairly well. When compression is under 100 it can be hard to start in cold weather, but may still have reasonably decent power. When compression is under 75 it can be hard to start a cool engine even in warm weather, and it may miss on a couple of cylinders for a minute until it warms up a little, then run well but with noticeably less power. With compression of 60 on all cylinders it might still run decent but with power low enough it may not want to do 60 mph into a head wind. With compression of 30 on all cylinders it may still run, after a push start, but you would be hard pressed to get it up a small grade on the driveway into the garage.
>"I do have to add about a quart of oil every 500 miles "
500 is border line normal for an MGA, nothing to worry too much about, as oil is still cheaper than internal engine work. 700-800 per quart is honorable mention, and 1000 per qt is very good. 1200 per qt is very unusual, and maybe only with a new engine just run in. Anyone who claims to go 3000 miles between oil changes with stock MGA engine without adding oil is lying.
A quick qualifying note here. Later model MGB engines with a vacuum drawn on the crankcase via emissions controls may not leak oil at all. Then an engine in good condition may exhibit lower oil consumption. Even then more than 1200 miles per quart of oil is very good for a vintage MG.
>"but I am attributing that to a bad leak around the timing chain cover, poor gasket seal for the oil pan and a small leak around the Spin on oil filter adaptor. I figure that if I was burning that oil I'd have blue cloud 100% of the time. "
Nah, it's not that bad. A decent running engine in proper tune can burn a lot of oil in place of a little fuel without much smoke.
>"I don't even see any smoke on a cold start up! What gives with this situation??"
Have someone follow you when driving around town, and see if it doesn't do a puff of smoke out the tailpipe when shifting. Foot off throttle draws vacuum an sucks oil into the cylinders. Foot back on throttle then gives the short puff of smoke.
>"As I begin thinking about my plan to get the compression back to where it should be I would like your thoughts on a few things. First, I believe that the engine needs to be removed from the car. Correct??"
Not necessarily. You can pull the head to do a valve job, and pull the pan to replace crankshaft bearings and check the oil pump. Pull the radiator to get at the front end to replace timing cover seal, and install later type cover with rubber seal if you still have the original felt seal style. In a pinch you can even R&R the pistons, hone the cylinders and install new rings with engine in place. You would have to remove the engine to R&R the crankshaft or to have it re-bored, or to service the clutch or gearbox.
>"Second, While replacing the head gasket a few weeks ago all of the cylinder walls were shiny and smooth. Will they need to be rebored? "
They are supposed to be shiny and smooth. That in itself is no indication of any problem. Before removing the head do a blow down test to see where it's leaking compression. Bring a piston to TDC on compression stroke, apply 20 psi air pressure to the spark plug port, and listen. Hissing at the carburetor is a leaky intake valve. Hissing at the tail pipe is a leaky exhaust valve. Hissing at the oil filler cap is leaky piston rings blowing through the crankcase.
If you don't have smoke out the draft tube on the side, and the front oil filter is not getting oiled up, then the cylinders and rings may be okay as is, and a valve job may put it back in good order.
>"The pistons that are now in there are oversized already at .030". Would it be better to resleeve?"
Hey, this may be good news. It is a little unusual for an engine to be worn out and clapped out the second time around after a rebore and new pistons (unless you drive as much as I do). This is sounding more like a "simple" valve: job. When you have the head off check to see if there is any appreciable ridge at top of cylinder due to wear of the cylinder walls below the top ring. Only do a re-bore if it really needs it. You can get a dial indicator telescopic bore gauge for under $50.
>"Third, While I am redoing the pistons "
Whoa there. You just put the cart ahead of the horse, which can get expensive. Rebore and new pistons is money you may not need to spend.
>"what other things should I examine and assess for possible replacement?"
Well, check everything of course. Always do the diagnostic work first to determined the exact problem and specific needs before you go throwing money at things which may not need to be fixed.
>"I have noticed that at idle my oil pressure is at 30 lbs with the trans in neutral and the clutch engaged, 20 lbs if the I have the clutch pedal depressed. While running in gear it is showing 40 lbs."
That's a little low, but not criminal (or terminal). You would be amazed at how nicely the oil pressure recovers just by replacing the crankshaft bearings. While you're under there, also check the condition of the oil pump.
Minimum oil pressure required is commonly stated as 10 psi for each 1000 rpm engine speed, up to the relief pressure setting. For me, when it gets down to 20 psi at idle when hot, it's time to change the con-rod bearings. An engine with bearings and oil pump in good condition will normally give 40 psi at idle when hot, and may hit relief pressure by 2500 rpm.
>"Four, Is this a I job that I can tackle or would it be better to get it farmed out."
The more you do yourself the less you pay someone else. Anyone with a moderate assortment of hand tools can do a lot of R&R, and only farm out the machine work on the block and head. You may pay for a few new tools, parts as required, and machine work only. The rest you can do yourself if you have a mind to.
>"He charged $800 and I supplied the parts that included new pistons and cam shaft."
I'm so tight I squeak when I walk. I'd be hard pressed to part with that much money for labor only. The only labor I pay for is for machining to install press fit valve seats and re-bore cylinders. I may put as much as $400 into the cylinder head, including hardened steel valve seats and bronze valve guides, and a little clean-up porting work. I usually do with under $200 for the engine block rebore and new cam bearings. If there is a badly leaking rear crankshaft seal, then line boring the block might cost as much as the cylinder re-bore.
>"I do have the MGA workshop manual and the Haynes MGA manual and I am fairly mechanically inclined but I have never tackled anything of this magnitude before."
First time for everything. When you have rebuilt the engine once, there is nothing mechanical left which you cannot do, and you could mechanically restore the whole car bumper to bumper.
>"Engine R&R, Brakes, Electrical, cylinder head R&R are about the extent of my experience so far."
So you have done most of it already. Engines and gearboxes are just large Tinker Toy sets you can take apart and reassemble at will. A clapped out engine may need some machining on the head and block, but otherwise engine and gearbox work is just replacement of worn out parts. It's not like body work and painting which may require some coordination and skill to have good results.
>"Five, what would be a fair price to pay for an engine rebuild if I supplied the parts? People around here are charging $50 - $60 an hour labor."
If you give the engine to John Twist at University Motors it can come back rebuilt and painted for about $1200. If you give him the whole car it can come back with rebuilt engine for $1800-$2000 completely detailed, and you don't even have to lift the bonnet. The more you do the less you pay for, and the less it needs the less parts you buy.
>"Six, Any other words of advice, assurance or condolences would be greatly appreciated."
You can do it. E-mail and hand holding are free. Do the blow down test first. If the rings are okay, maybe just do the head work and put it back on the road. How much do you intend to drive it in the next 10 years. Minimal usage does not justify the same whole hog rebuild you might do for one you intend to drive another 100,000 miles.
I have been thinking about putting together a web page for complete engine rebuild. Most of the engine work information is already on my web site as short articles about specific components. It's just not all tied together in a single chapter.