Drycoating Bearings

[Porshaah]

Hi All

Is drycoating new bearings an acceptable method to compensate a crankshaft close to the wear limit?

[HawgRyder]

If you are thinking about Moly coating or something similar...it wears away very quickly with oil pressure.
I have seen (long time ago) a trick of putting a shim behind the bearing to set the clearance to 1/2 of what the next size bearing would be.
It was used for an old flat head Ford....and short time only...those old racers would do anything to keep a car on the track...LOL.
Bob

[Steve@Rennsport]

Originally Posted by Porshaah

Hi All

Is drycoating new bearings an acceptable method to compensate a crankshaft close to the wear limit?

Nossir,....dry, anti-friction coatings are used simply to reduce friction.

If you have a bearing clearance problem, you need to either have the crank properly ground undersize and use the appropriate bearings, or source a crank thats not worn.

[AlfonsoR]

It's always worth it to check and double check and triple check clearances. Make sure you are getting repeatable numbers. Make sure everything is supper clean and spotless. Make sure all surfaces that should be flat are flat..check with a precision flat edge.... not a stanley square from Home Depot. Sometimes threaded holes can build a ridge that will throw off your clearances for example. So again, check and double check everything until you are very confident of what you have.

[aws]

Calico coatings has been doing my bearing coatings for years and they have a "heavy" versus a "light" coating that will take up.0003 to .0004 (inch) on the clearances. This coating wears---but not prematurely. ---When refreshing these engines after 20-40 hours the bearings clearances will open up slightly, but no more than non-coated bearings. The coating is a friction coating but it is really not there to reduce friction in regular use. I have always used it to reduce the chance of bearing failure if there is a problem. Certainly over the years I have seen more than a few rod bearings that I feel would have failed if they had not been coated. So consider it a small insurance policy. I would not hesitate to use the Calico coating to tighten up clearances on a minimum spec crank.

aws

[j911brick]

What motor is this? before you go off the deep end with this, have you actually checked the clearance of the bearing (with new shells)? What are the rods sized to? the reason I ask is because the rod bearings I have been getting from Glyco have been on the tight side and I actually have to open up the clearances. I Always get my bearings coated.

[Porshaah]

The engine is a 964 3.6L. The rods are/were std but have now worn about a 1/2 thou away from the wear limit. How do you open up the clearances?

[j911brick]

Originally Posted by Porshaah

The engine is a 964 3.6L. The rods are/were std but have now worn about a 1/2 thou away from the wear limit. How do you open up the clearances?

STD doesn't say anything. What are the actual measurements vs. Specs? I increased the clearance by opening up the rods to the max spec. Still didn't buy me enough in one case but the customer didn't want to pay to resize the crank. The clearance is within spec now, but on the tight side of where i want it.

[Porshaah]

It's my understanding that a crankshaft is considered STD/STD if the main journals measure between an upper limit of 59.990mm and a lower limit of 59.971mm and rod journals measure between an upper limit 54.990mm and a lower limit of 54.971mm. There's also a wear limit of 59.960mm and 54.960mm respectively and I'm not sure, if a journal which measures inbetween the lower and wear limit is still considered STD? My journals measure between the lower and wear limit. I havn't plastiguaged yet, but are you saying that the clearance may still be within spec as the main bearing saddles and rod big ends affect the clearance? This may explain why my oil pressure was fine before I tore it down. What is the clearance spec by the way?

[j911brick]

yes, you can adjust clearance by playing with with the rod bore size, as well as how tight the bearings crush. Since you crank is within spec I would leave it and see what happens when you get the bearings in there.

[aws]

Factory dimensions for rod bearing clearances is .001 to .003 (inch). Rod big end bore housing dimensions definitely affect crush and rod bearing clearances as a result ( although Carrillo states otherwise). Different engine builders prefer different clearances---Mike Bruns and Brian Pauter both recommend larger clearances on race engines and you might ask them for that number. As clearances go up, so does oil flow and cooling---but load carrying capacity goes down, so it is a two edged sword. Main bearing clearances are not near as critical as rod clearances---when the case heats up the clearances open up considerably anyway. So to answer your question, on a street engine I would want to keep my rod bearing clearances less than .0025 (inch) and mains bearings at .0035 (inch)

aws

[Porshaah]

Is there a reliable standard thickness measurment for the bearings? Rather than buying new bearings only to find out after plastiguaging them and discovering they don't fit I'd like to calculate whether they will be OK. I'll plastiguage my old berings first. That way I avoid having to return them.

[j911brick]

Originally Posted by Porshaah

Is there a reliable standard thickness measurment for the bearings? Rather than buying new bearings only to find out after plastiguaging them and discovering they don't fit I'd like to calculate whether they will be OK. I'll plastiguage my old berings first. That way I avoid having to return them.

Its not quite that easy. The length of the bearing also play into it since that effects the crush, hence the final clearance.

[brads911sc]

If it was fine when you took it apart, machine shop says all the journals, rods, crank are in spec... you are more than likely fine...

[lindy 911]

Also, Plasti-guage is junk; mediocre in accuracy at best. The cases and rods need to be assembled with clams in place and the bore measured at multiple locations with good equipment. The journals of the crank then need to be measured and the difference is the clearance. This is a MUCH better way to measure your clearance rather than mashing plastic in between the two.

[j911brick]

Originally Posted by lindy 911

Also, Plasti-guage is junk; mediocre in accuracy at best. The cases and rods need to be assembled with clams in place and the bore measured at multiple locations with good equipment. The journals of the crank then need to be measured and the difference is the clearance. This is a MUCH better way to measure your clearance rather than mashing plastic in between the two.

What's wrong with plasti-gauge?

[lindy 911]

I think I said it above; it's accuracy is not very good. I've read where some were getting .002 variance from hard measurements. If your ideal clearance is .004, that's a bunch to be off.

[j911brick]

Originally Posted by lindy 911

I think I said it above; it's accuracy is not very good. I've read where some were getting .002 variance from hard measurements. If your ideal clearance is .004, that's a bunch to be off.

I've been using plastigauge for years and use it on every motor; dozens of motors without incident. For those people who have trouble with it I would think they are either doing it wrong or using low quality gauge. ..and for those people using it incorrectly I would suggest they have just as good a chance to use instruments incorrectly as well. I'll admit it is not good for accuracy in the ten thousandths range, but instruments can also have error in that range as well. I use it primarily to verify my measurements, and I use the measurements to verify the plastigauge. Just one more step in trying to get it right.

[lindy 911]

I used to use it all the time too during motorcycle engine builds. I had trouble getting it to measure twice the same. Then I started reading and found many who said they would stay away from it if given the chance; many of those are the "experts" here. I'm sure you have plenty of skill to make it work for you and that's great. But there are a few of us who don't have the same experience level as you might and can't get it to work right.

[prschmn]

Cant help but comment. have you considered what clearance is appropriate for your application? Factory spec varies by300%. All depends on the application.
And I agree that plastigauge is unreliable.