# rear main problem



## army6741 (Jul 3, 2014)

I have a 1969 428 4 bolt main at engine builder for the last 12 months. Everything in motor has been replaced. All forged. heads are KRE. motor was stroked. It finally went on dyno and rear main from tin Indian leaked. Took it apart and a BOP one pc. was used. Went back on dyno and it leaked. Now he said he has ordered a Best gasket rope seal. it needs to come apart for the third time. Besides having 10K in the build its still leaking oil. I'm not convinced the rope seal is going to do it. crank, block, assembler??


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## PontiacJim (Dec 29, 2012)

army6741 said:


> I have a 1969 428 4 bolt main at engine builder for the last 12 months. Everything in motor has been replaced. All forged. heads are KRE. motor was stroked. It finally went on dyno and rear main from tin Indian leaked. Took it apart and a BOP one pc. was used. Went back on dyno and it leaked. Now he said he has ordered a Best gasket rope seal. it needs to come apart for the third time. Besides having 10K in the build its still leaking oil. I'm not convinced the rope seal is going to do it. crank, block, assembler??


Could be all of the above. Sometimes those BOP seals just don't seem to do the job. From what I have read, it can be related to the serrations found on the crank that the seal rides on, the recess in the seal is not correct, align boring the block without checking the main seal groove. Read this info about this on this forum with pictures: https://firstgenfirebird.org/forums/ubbthreads.php?ubb=showflat&Number=299538


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## geeteeohguy (Feb 2, 2008)

What Jim said. I'll add, that the 421-428-455 engines with the large journal crank have more sealing issues due to imperfections at the rear main. The Best Gasket square rope type seal is the preferred seal for these engines as it tolerates looser tolerances. Good luck.


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## BearGFR (Aug 25, 2008)

There's more to properly fitting one of the BOP 1-piece seals than meets the eye, and unfortunately the instructions that come with them don't cover it all yet either. I made a video about how to do it.






Bear


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## army6741 (Jul 3, 2014)

third seal failed. He thinks seal cavity out of round and out of alignment with centerline of crank??


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## PontiacJim (Dec 29, 2012)

army6741 said:


> third seal failed. He thinks seal cavity out of round and out of alignment with centerline of crank??


Did you read the link I posted and did you let your machinist read it?


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## army6741 (Jul 3, 2014)

Yes I did Jim. that was his message today. I will be going there Monday to discuss the problem. He has been doing this for thirty years. will discuss the article with him. thanks to Bear also.


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## PontiacJim (Dec 29, 2012)

army6741 said:


> Yes I did Jim. that was his message today. I will be going there Monday to discuss the problem. He has been doing this for thirty years. will discuss the article with him. thanks to Bear also.


OK, good. Wanted to make sure because I thought there was some info that might shed some light on your situation. 

I also know the main cap crush is important and this may also effect the seal. Here is what I found on the web: "The main bores must be inspected after the block is cleaned. You check for straightness and diameter size. The straightness is brought in by properly clipping the main caps, and then a skilled hone operator must hone the main bores carefully. We find that the housing bore size should be kept to the middle or to the minimum to maintain good bearing crush. NEVER PAST THE MIDDLE. Loose housing bores are the main culprit behind spun bearings. On a 3.250 main bore, (421/428/455) the factory says; 3.4380" - 3.4390". We use 3.4380" - 3.4385"."

Again, am sure your machinist knows this and probably has done this, but check out the BOP seal and installation on page 56 of this article: http://www.nitemareperformance.com/imgs/PPFeb18_Blueprint1.pdf

And this might be a long shot, but if the rear main is moving around at all, it could be off just enough to create a problem. Check out page 53 with regards to the longer dowel pins used to locate the main caps more securely: http://www.nitemareperformance.com/imgs/PPFeb18_Blueprint1.pdf

Eventually your machinist will get it. I'm sure he is pretty frustrated trying to figure it out. :thumbsup:


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