General Tech Connecting Rod Bearings

[KVH]

Maybe some folks here will have an opinion about connecting rod bearing clearances. Please see attached pic.

I'm using .010 undersized bearings, and my journals measure right on the mark at about 2.076 (stock is 2.086). Sounds near perfect, but my PlastiGauge shows barely .0020 clearance and I'm supposed to be at least .0028 according to the Shop Manual. I say I should torque things up and go for a nice ride.

Any thought this is just too tight? I thought so at first, but then wondered whether a variance of .0008 could really mean that much--assuming PlastiGauge is all that accurate.

Thoughts?

PS I'm ignoring that wide smear to the left since a re-torque did in fact eliminate that aberration. The right half is accurate.

 

[sp53]

I am not sure, but what you are saying sounds good to me because there are going to be little variables everywhere plus small general human error. The last time I did a crankshaft, I took it in and had it polished and used what they suggested, I think 20 over. They did take the rods and caps and bolt them together and trued up the round hole on the connecting rods. I plan to do the same thing again this winter as I work on my motor.

steve

 

[Sarastro]

Plastigage is not all that accurate. It's good for showing whether you have a problem, but you can't take it too literally.

My engine did about the same thing. I used a lighter oil for break-in (10W30) and everything seems fine, after about 1700 miles at this point.

 

[CJD]

As long as you can turn the crank, I would go with it. If you plan to race, then a loose engine is needed, but on the street .002" will run just fine, so long as you can still turn the crank.

 

[Rod-H]

I agree with the above, .002 is fine for these engines. Use a lightweight break-in oil, to ensure oil gets to everything early on. Plasigage is not the most accurate way to check clearances.

 

[sp53]

Hi John I am trying to get an understanding of tight and lose motor, so would a new motor actually feel tight when driving. For example, would the new motor feel faster and slow down quicker when the gas is let off. Whereas a looser motor would coast smother and slow down slower.

steve

 

[KVH]

Just another update. Hastings is discontinuing the TR4 rings I bought years ago. Grant may be one of the few good options for new rings. But sheesh those three-piece oil rings are a pain and I kept getting "hop" and overlap while I assembled them. I am so bothered by the tedious nature of the retainer rings and prospect for overlap that I actually installed my pistons then pulled out two just to recheck the red/green markings. Fortunately, I didn't find overlap, but I sure will think twice the next time I go thru this process.

 

[poolboy]

Here's how Hap Waldrop dealt with the Grant rings

I always love taking these photos, TR6 engine rebuild (Page 2) : TR6 Tech Forum : The Triumph Experience

Let the assembly start Hap Waldrop Acme Speed Shop 864-370-3000 www.acmespeedshop.com

www.triumphexp.com

 

[CJD]

No Steve. With a tight motor you will only notice higher oil pressure, as the bearings will not sling as much oil internally. But the engine will run exactly the same, and will last longer, since you have more bearing material to wear before you notice any oil pressure loss.

The reason a high performance motor needs more clearance is for several reasons. First, at high RPM the steel will actually deform from the loads. Having a little extra clearance will protect the bearings from scraping the oil film when they distort. Then, it increases oil flow through the bearings, so it helps cool them. The engine will wear faster, but that is normally not a concern on a performance motor. Actually..."wearing faster" is a misnomer. It will wear just as fast, but starts with more clearance to begin with, so there is less to wear available before you notice a drop in oil pressure.

Oh...one down side I can think of with tight bearings is they will not tolerate overheating as well. At least for the first few thousand miles it would be prudent to take care from letting the engine over heat.