What is the opinion on reusing rod bolts, crank studs( MK3) ,a used cam with different followers?

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jimbo

jimbo

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I am rebuilding a MK3 and was wondering what is the running opinion on reusing rod bolts, crank studs( MK3) ,a used cam with different followers? I will buy new nuts and the rod bolts/studs are not original, they were replaced at the first rebuild years ago. I read somewhere that you could measure the used rod bolt length for stretch , but where do you find the comparison measurement numbers? In the old days. When I was working and had money to burn I would replace everything with new.But now throwing away seemly good parts when the replacements are so expensive I need to reconsider.
 
I wouldn't scrimp on the rod and crank fasteners, but a good used cam should be ok. I have (recently) seen cams in Japanese 4-cylinders with spalled lobes, and they run like that with no issues.
 
New nuts definitely, bolts and studs then old known good ones are better than unknown new ones, NOC Commando Notes concur. I had some crank studs that when I put a thread gauge across the 26 tpi there were 4 thread peaks nearer 25 tpi, these studs are in the bin and replaced with new studs, others showed good thread engagement on all peaks so not stretched and new nuts spun on easily and these are reused.
 
If the bolts and studs have previously been torqued properly then they have already been plastically deformed (read - started the failure process) includes the nuts though probably not as severe.
If they were not torqued properly (too loose) and were in a running engine for a while they are most likely some way down the path to fatigue failure.
Then again you might also enjoy a game of Russian Roulette!
 
robs ss said:
If the bolts and studs have previously been torqued properly then they have already been plastically deformed
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Plastic deformation does not take place at the correct torque on these fixings as the torque is set at a level to stay within the elastic limit and not enter the plastic range. There are some single use head bolts on Audi's and VW's that are first torqued to the edge of the elastic limit and then given extra 1/4 turns to bring them into the plastic limit, as this is within the plastic limit a torque wrench is not used as the torque drops or stays constant with the stretch of the bolt beyond the elastic limit hence the 1/4 turns.
 
These engines - and all conventional engines from that period - were typically rebuilt many times without replacing any fittings. There is no need to do it. As K noted, tightening to a certain point then adding another additional fraction of a turn to achieve a certain "stretch" is a relatively recent thing. It did not apply to any engine of this era and none of the fittings are designed for that. Inspect them and if there is no obvious damage, reuse them for as many rebuilds as you want! :)

Old cam/new followers are OK. The opposite is not!
 
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I suppose it is 75 USD for the rod bolts and nuts. Hard hard do you run it? I know expenses add up but when you are done how much of
your total investment is it?
 
thanks for the responses, I always thought proven used hardware does have its worth. I have a good used cam lying around and can find some good used followers too. But they were not together and might have a less than agreeable wear pattern when mated up to the used cam from another engine.
 
Do not, Do Not, DO NOT install a used cam with used lifters UNLESS those lifters were used with THAT cam and you put them back on the same cam lobes. If they were not originally together, the odds of cam failure is very high. I have seen cams destroyed in a few hundred miles doing that. The lifters/cam establish their wear profiles in the first 20-30 minutes of break in, which is why cam break-in is so important. Trying to establish a "new" profile on a used cam or lifters, which already have a wear profile, just doesn't work. The result is usually wiped cam lobes and a lot of metal particles .

Admittedly, one can argue that IF the lifters still have a good, convex profile on their faces - as new ones do - there is a fair chance that it will work but I would recommend new lifters regardless to avoid the higher odds of failure.
 
MexicoMike said:
Do not, Do Not, DO NOT install a used cam with used lifters UNLESS those lifters were used with THAT cam and you put them back on the same cam lobes. If they were not originally together, the odds of cam failure is very high. I have seen cams destroyed in a few hundred miles doing that. The lifters/cam establish their wear profiles in the first 20-30 minutes of break in, which is why cam break-in is so important. Trying to establish a "new" profile on a used cam or lifters, which already have a wear profile, just doesn't work. The result is usually wiped cam lobes and a lot of metal particles .

Admittedly, one can argue that IF the lifters still have a good, convex profile on their faces - as new ones do - there is a fair chance that it will work but I would recommend new lifters regardless to avoid the higher odds of failure.
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If you can find them,new lifters are expensive! Can I get a set of used lifters reground, to work on a used cam?
 
In operation, ie the interface between the cam and the follower, I fail to see any difference between a reground follower and a new one.
 
I think I will take apart a pre MK3 850 motor I have and see if the condition of the cam and followers is good . Keep all the pieces in the correct spots, transfer them to my cylinder and case and I should be good to go.
 
If you have good measuring equipment, you can check the cam lobes with a micrometer and dial indicator, and also check the cam followers for consistency. If they are a few ten-thousands off dead flat, and you mix them up wrong, you are inviting excessive/rapid wear and potential failure. If they are very nicely flat, I'd re-use them AS LONG AS THE FOLLOWER BORES ARE CLEAN, and the followers are unscarred.
 
kommando said:
Plastic deformation does not take place at the correct torque on these fixings as the torque is set at a level to stay within the elastic limit and not enter the plastic range. There are some single use head bolts on Audi's and VW's that are first torqued to the edge of the elastic limit and then given extra 1/4 turns to bring them into the plastic limit, as this is within the plastic limit a torque wrench is not used as the torque drops or stays constant with the stretch of the bolt beyond the elastic limit hence the 1/4 turns.
Click to expand...
I worked on Porsche air cooled engines for years and always replaced the long studs on the 6 cyl engines. I'd torque the head nuts to 20 ft lbs and use a torque angle gauge for the final stretch. I've seen a number of broken studs because of the heat cycles they go through. I think its always a good idea to replace stressed fasteners on a rebuild. Especially rods and fasteners that are 40+ years old.
 
I asked a similar question recently. I went with new crankshaft hardware and think the old stuff might have been better.

I just used new nuts on the conrods. They are only 25 ft/lb so I don’t think that is going to stretch anything. But do use locktite.
 
Cleaned up the crankshaft, and hardware, Everything, threads etc seems to be fine with no distortion or visible stretch.


What is the opinion on reusing rod bolts, crank studs( MK3) ,a used cam with different followers?
What is the opinion on reusing rod bolts, crank studs( MK3) ,a used cam with different followers?
What is the opinion on reusing rod bolts, crank studs( MK3) ,a used cam with different followers?
What is the opinion on reusing rod bolts, crank studs( MK3) ,a used cam with different followers?
 
The manual states 30 ft lb for the studs but even with a crows foot I don't think I could torque them up .
 
Just get two good spanner’s (sorry, wrenches) and practice getting the feel for a nut and bolt in the vice torqued up with a torque wrench, that’ll give you a good feel to tighten the crank fasteners.

In his excellent video, Mick Hemmings centre punches the fasteners that don’t have tab washers.
 
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