• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Cheap 1275 rebuild

Share this page

I have to agree with Trevor and ABFish about tinkering! A friend of mine likes to tell me that he thinks Spridgets are the most eviromentaly friendly cars on the road since I'm "always working on them and never wasting fuel by driving them"!!
I love to tinker but I do some driving too!

Kurt.
 
Fred, that is correct, you’re looking at a 1275 rod bolt that has started to tare. Just a few more exuberant revs to 6500 would have created a catastrophic failure. I guess this say’s a lot about the quality of bolts that are used. Car manufactures are in business to make a profit, every corner that can be cut will be cut. Their only interested in providing protection until the warranty has expired, why should they worry about thirty years later? Parting lines are the weakest points of any engine, whether it’s the rods, mains or cylinder head, every precaution must be consider in protecting these weak points. If you decide to choose one of the best you can look at ARP, if you choose to go back standard choose BP North West to purchase your OE bolts, studs, nuts and washers. Best of luck on your cheap rebuild!
 
Alan:

Did that 1275 rod bolt head rotate in the rod when you removed it? The rod bolt head "tab" on the RH side looks like it might have passed by the edge of the rod bolt head relief in the rod during removal. I've had this happen numerous times during teardowns and sometimes even during assembly. When comparing the current rod bolts to the older BLMC factory stuff, it looks like some of the aftermarket producers have gotten the rod bolt dimensions just a little bit off for those "ears". ARPs seem to be quite a bit more like the original BMC/BLMC factory size.

Wonder if it rotated, there could have been some bending moment that found that place in the rod to start the crack????

Mike Miller
 
I sure did mike, I see that from time to time also, not very much material support in the tabs aera. Frankly I feel that BMC really design the rod bolts for a one time torque anyway. If the rod bolt is a little loose in the rod when torquing you sure can twist a rod tab off. I keep that bolt as a souvenir to remind me to always change the rod bolts. Now days I just use ARP or Carr so I can sleep well at night...
 
Thanks for the tip Alan.

Remember though, this is for a mild street motor which will rarely see revs in excess of 5000. The car will simply tool around town to car shows and Sunday drives. The most thrills will be going a bit fast through the twitsies.

Fred
 
Fred, I did some quick calculations of what kind of stress that a rod bolt takes at 5500 RPM. Remember your rod bolt take this kind of stress every time it rotates.


Rod Radius: 0.8775
Linear speed: 523.6
Angular speed: 5500 RPM
Mass: 575 grams?

Centrifugal
Acceleration: 291092 inches/S squared
Centrifugal force: 4251.39 N(kg-m/s squared

So you can see what kind of stress is applied to your rod bolts. This is why I choose the best to maintain the integuarity of the parting line. When you calculate the heat cycles into this formula you would see what age and stress heat can and will do. Good luck!
 
Too much to read through so I don't know if anyone said it yet. Drag a pennny across the bearing journals and if it leaves copper, polish it.
 
Hmm, never heard of that penny test. Thanks.

Sounds like a pretty simple thing, but wouldn't it take a pretty bad journal to scrape copper off a penny? Mine looked fairly pristine with only the most minimally detectable visible scoring on the rear main. Felt mirror smooth to the touch though.
Can't imagine a penny would give up any copper to that.

Fred
 
Mike, brings up a good point about ring land wear in the piston, I as well as Mike tear down and build alot of 1275, we're building two 1275s right now, one for the street, one for race. It's pretty common to see broken rings in a 1275 worn engine, they stay in the ring land captured, even pump a little bit of compression, but eventually if pushed will destroy the ring land in the piston, and if that happens then they normally tear up a cylinder wall.

Another thing I see, 1275 crank that look decent as far as journal wear, but measure .0002" -.0003" under spec, now if you want to get by on budget and you see this, then maybe you just polish the crank, but have the connecting rods resized to small end of housing bore spec range, so you tighten back the clearnces with the rods vs the crank journal, getting back to closer to normal clearence will do a couple of things, lessen the chances of bearing spinning in the future, and give you better oil pressure, and resizing rod is about 1/3 to 1/2 the price of grinding a crank, if you see deep scratches you can catch with your fingernail, face reality and get the crank ground. I always tell folks you often here of budget rebuilds that get done and get back on the road, you just don't hear alot about on the forums when they blow up :smile: I always tell folks a engine tell you what it needs, not you telling the engine what it needs, often people just don't measure and built it cheap and hope for the best, that's what the guy did with my MGB street car, I traded him for, he took alot short cuts, fooled himself into thinking his eyes were a micrometer and the engine chunked a rod on me at about 20K miles, I guess 20K miles wasn't bad for this type of "look and hope" rebuild, but I wouldn't term it as good either, and now the crank, rods and block are scrap metal.

So in closing if you do a budget rebuild, be smart about it, get everything measured, go about machine work as to get the most for your money, like the rod resizng I talked about.

As for honing, I'll share a little trick of mine with you all, if you got one of those three stone hones, they by themselves are pretty useless, but before you chunk it in the garbage can, go a get a couple of sheets of 3M Scotchbrite #7447 (burgundy in color) and wrap it around the stone hone and hone the cylinders with that with alittle oil on th cylinders, it will give you a amazing finish that promotes ring seating, I do it to every engine I build, and I consider it the very best finish money can buy.
 
I might add to Haps comments, after honing with the 3M Scotchbrite thoroughly clean cylinders until no gray residue is left on a clean white cloth. When assemblling your rings use a 90 degrees staggered and use automatic transmission fluid for assemble. Do not use engine oil, transmission fluid burns off quicker and allows rings to seating faster.
 
BlueMax said:
use automatic transmission fluid for assemble
Click to expand...
...seriously?
 
Excellent advice Hap and BlueMax!
I love the tip about the ScotchBrite pad.
Do you think I should take out my piston/rod assemblies and re-hone?
The engine is on a stand without the head or oil pan bolted on yet. I'd hate to get a bunch of honing residue in it now though.

Fred
 
TulsaFred said:
Sounds like a pretty simple thing, but wouldn't it take a pretty bad journal to scrape copper off a penny? Mine looked fairly pristine with only the most minimally detectable visible scoring on the rear main. Felt mirror smooth to the touch though.
Can't imagine a penny would give up any copper to that.

Fred
Click to expand...

You'd be surprised.

Hap Waldrop said:
if you see deep scratches you can catch with your fingernail, face reality and get the crank ground. , you just don't hear alot about on the forums when they blow up :smile:
Click to expand...

I wish I had taken pics of how bad mine was. You didn't catch it, it caught you. It actually cut my finger! I knew it should have been turned. My thoughts were: if it had been rattling on cold start for 10K and was still running good now, it should only run better with new parts. I was just trying to buy some time. I never dreamed it would go this long.

Believe it or not Hap, you are the one who inspired me to put it back together this way. You once said you've never seen a LBC motor that didn't need to have the rods resized. That tells me everything out there that hasn't been rebuilt recently has out-of-round rods but is still running good. The oil film doesn't know if it's the crank or rods out of round, so why not put my out of round crank back in? I am shocked it's still going as good as it is. I plan to sneak a peak at the bottom end soon just to see.

Don't worry Hap, you'll know when mine blows up 'cause I'll be ordering parts from you the same day. :yesnod:
 
kellysguy said:
Don't worry Hap, you'll know when mine blows up 'cause I'll be ordering parts from you the same day. :yesnod:
Click to expand...

And I be glad to welp you anyway I can.

As for what I use on cylinder walls on assembly, I use SAE 30wt motor, I massage it on the cylinder wall, and into the piston skirt, but put none on the rings, I want a good ring scrub on pressure build up cranking, been doing that on many engine for many years. Between my scotchbrite honing, my light use of 30 wt oil, I looking for good and quick ring seating, chances are this way, you're 90% seated in the garage before the engine ever pulls under it own load, and when it does, the ring fully seat in short time. I never, ever dunk my pistons in oil, that just makes everything harder to seat, nd makes a big mess. What I'm doing is pretty common among professional builders. Never ever use any sort of assembly lube or synthetic oil on cylinder wall for assembly, you'll never seat the rings.
 
Back
Top