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3.8 Rebuild Setback

rkelly

Member
Joined
21 May 2015
Messages
15
Hi All,

I recently took the decision to rebuild the engine on my 964. The main factor for this was oil leaks rather than lack of hp.

After much deliberation I decided to go with the Mahle 3.8 pistons and barrels offered by FVD. (Part No. 100 103 038 02: Mahle 3.8 L Piston&Cylinder Set - 964/993 (11.3:1) 107mm, narrow rod).

In addition, I have gone with their recommendation of using Carrillo rods (Part No. FVD 103 975 03: Carrillo 3.8 RSR connection rod set 23mm pin).

These rods are standard 964 big end size. Unfortunately, we have now encountered a small problem when preparing to fit these rods to my crankshaft.

The new bearing shells do not seem to fit properly with the Carrillo rods. To cover the basics; we doubled checked the part numbers with the manufacturers and both shells and rods are correct. In addition we have taken some measurements of the new rods and they seem to be in tolerance. When the shells are seated and the cap end fitted there is a misalignment on the outer edge of the shells. There is a visible discontinuity where the two shells meet (outer edges not flush). This then gives rise to a unacceptable tightness when the rod is placed on the crankshaft.

When the same shells are placed in the original rods they sit perfect; so this may suggest there may be a problem with the new rods rather than the shells? But we have not been able to confirm this yet. Possibly with the locating lugs? Does anyone happen to have a technical drawing for Carrillo rods?

We have been liaising with FVD and Carrillo before Christmas. They have been very reasonable and say they have never had this problem reported before. They have suggested that we send the rods back and they will send me out a replacement.

I am wondering if anyone has encountered a similar problem with either shells or rods. NB The shells are standard size Porsche items.

Any suggestions are greatly appreciated.

Cheers,
Ryan
 
Thanks for the feedback. The shells are standard size; I understand there are two oversized options. However, my crank hasn't been reground
 
It sounds as if the tang grooves have been machined incorrectly in the rods. Try the rod and cap individually on the crank, hold the rod/cap square and find out if the shells are binding on the crank fillet radii.

Not Porsche specifically, but I have encountered this on other engines with incorrectly made rods. Fortunately, we have a machine shop, so in the past when we had no other option left, we made a jig (the same ID as the rod) and machined the edge of the shells in question, re-chamfering them afterwards. This is not the preferred route! Correctly machined rods are order of the day here, but when push comes to shove......

One other thing to check is that the tang groove in the rod is deep enough to allow the one on the shell to fully seat. If it isn't, it will pinch on the crank.

One other little gem for the local pub quiz :what: , is that the inside of the bearing shells are not round but lemon shaped. They are bigger across the split line axis than vertically.
 
Hi Mike,

Thanks for the detailed reply and pub quiz trivia :)

I will try to measure any discrepancy between the position and depth of the tang grooves between the new and old rods. This may explain why it is pinching/binding on the crank.

Hopefully this will be resolved with the replacement set of rods. I will let you know how it goes.

Cheers,
Ryan
 
Sounds an exciting project. Who is doing the build or are you doing it yourself?
I would ask the same question on rennlist as quite a few guys there have gone for the 3.8 upgrade and your'll probably get a much quicker response.
 
Sounds like a manufacturing issue but I cannot understand why you are replacing the rods - the standard ones in that are good for up to 700hp - you are never going to exceed that in any 964 build unless it is some wild turbo build. Just unnecessary. Replace the studs by all means as that is a weakness but not the rods.

Ken
 
996ttalot said:
Sounds like a manufacturing issue but I cannot understand why you are replacing the rods - the standard ones in that are good for up to 700hp - you are never going to exceed that in any 964 build unless it is some wild turbo build. Just unnecessary. Replace the studs by all means as that is a weakness but not the rods.

Ken
I think I'm correct in saying that the standard rods are good for around 7200 rpm whereas the Carrillo rods (combined with other expensive mods) are good for 9000 rpm

With regards to the problem you're having with the shells there's an engine rebuilding forum on the pelican parts forum, and there's some very knowledgable folk on there, also as mentioned try renlist
Edited to mention, is it Glyco bearings you're using from an Opc?
There's a bad batch which say on the packaging made in South Africa, I read it on a forum recently but can't remember where, these bearings were incorrectly machined and caused binding.
The good ones are made from the same company but made in Germany is written on the box .... Worth checking
 
Hi Guys,

Thanks again for all the feedback. My friend George McMillan of McMillan Porsche is rebuilding my engine.

The 3.8 mahle kit from FVD has 23 mm gudgeon pins. As a consequence, I would have to re bush the small end on the original rods if using them (FVD quoted me 400 euro for the bushing kit). More importantly, I would like the option to have a higher revving engine further down the line (subject to further modifications). That is the reason I decided to go with the Carrillo items.

The shells fit the original rods without binding so we are reasonably confident that the problem lies with the new rods. However, I will double check the batch of shells as suggested. As DynoMike pointed out this seems to be related to the tang grooves. We will take some further measurements when the workshop opens. The timing of this discovery was unfortunate just before everyone closed for Christmas.

I am sending the rods back to FVD next week. Hopefully we will find a resolution soon.

Cheers,
Ryan
 
Well good luck in getting the issue resolved, it does sound as though given that that the shells are fine with the standard rods that the problem may be with the Carrillo rods.
 
Hi Ryan
when you get the rebuild sorted, we would love to see the finished product in the flesh.
we are getting a good few 964s now in PCGB R27.
21647920583_eb01d95a12_z.jpg
 
Hi,

I will certainly bring it along to some of the R27 events this year. I would be keen to hear your car with the G-Pipe and de-cat. I'm also hoping to get a few sessions at Nutts Corner and Kirkistown as soon as the engine is run in.

Ryan
 
Anytime Ryan, I think it must be the loudest car in the club at the minute 😁. First club drive is in March. There is also several of us that go on regular drives around the country. We were up in portrush yesterday.
 
AVI_8 said:
996ttalot said:
Sounds like a manufacturing issue but I cannot understand why you are replacing the rods - the standard ones in that are good for up to 700hp - you are never going to exceed that in any 964 build unless it is some wild turbo build. Just unnecessary. Replace the studs by all means as that is a weakness but not the rods.

Ken
I think I'm correct in saying that the standard rods are good for around 7200 rpm whereas the Carrillo rods (combined with other expensive mods) are good for 9000 rpm

It is completely different for higher rpm on a 964. The rods are absolutely fine - it is the fastners that are the issue so you just replace those with better solution. The rods themselves will get you the rpm with correct fastners.

And if you are going higher rpm above 7200rpm (frankly anything over 8500rpm is a waste as it won't make any more power) you need to be doing other stuff now as number 2 and number 5 rod bearings go on these because of the crankshaft oiling - something which later crankshafts don't have an issue with.

Just trying to help here because if you truly want 8,500 rpm etc, you need to be making these changes now while the crankcase is open.

Ken
 
Hi Ken,

Thanks for the input, it is appreciated.

I am giving serious consideration to using my original rods. What is your view on re bushing my original rods to accommodate the 23 mm pin in my new mahle pistons. Do you know were I could source suitable bushings at reasonable cost?

Cheers,
Ryan
 

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