RB20 fitment issues.

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10 inch Terror
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Wed Nov 13, 2013 8:43 am

Hi all, I'm currently building an RB20 motor and have found that the cylinder seems very tight on the studs and won't slide down fully without a gentle tap to aid it down. Will this put any undue stress on the studs or around the alloy supporting them. Also I've noticed that the inlet manifold is very very tight to the the engine mount "arm". Will it do any harm to grind a very small amount off the manifold to gain some clearance? Thanks in advance Pete.

Minority
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Wed Nov 13, 2013 10:27 am

Fitting offset cones would help the manifold clearance issue if they're not already fitted.

10 inch Terror
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Wed Nov 13, 2013 11:22 am

Minority wrote:Fitting offset cones would help the manifold clearance issue if they're not already fitted.
I haven't even fitted or built the engine yet. I've only "dry" fitted the barrel to the case to check the transfers which I've matched. I will fit offset cones as I always do but it won't help as the manifold is actually touching the engine case.

Minority
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Wed Nov 13, 2013 11:41 am

10 inch Terror wrote:
Minority wrote:Fitting offset cones would help the manifold clearance issue if they're not already fitted.
I haven't even fitted or built the engine yet. I've only "dry" fitted the barrel to the case to check the transfers which I've matched. I will fit offset cones as I always do but it won't help as the manifold is actually touching the engine case.
Ah! I see what you mean. Must remember to put brain in gear before engaging mouth (or in this case finger) :oops:

Adam_Winstone
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Wed Nov 13, 2013 6:36 pm

If you are fitting 'fat' studs then that is probably your first thing to look at. Original fitment 'rolled' studs are thinner along the shaft section than the common replacement studs that have the threads cut off the larger diameter shaft... leaving you with a CRAP 'fat' stud that often causes alignment issues! Those that supply 'fat' studs defend their product by stating the error is in manufacture of the barrel and the incorrect placement of the stud holes, however, I have had alignment problems even when fitting mix and match factory barrel and casing combos with these studs.

WARNING - If you simply tighten down the barrel and head then you place a strong side-loading force on the barrel's spigot. I was once asked by a dealer, when visiting their shop, if I had any ideas why a customer's Rapido had broken the spigot off (along the step line of the spigot) during normal running conditions. Checking the casing on the bench revealed 'fat' studs fitted and that this was the cause of the problem (noting that it is the combination of parts that can lead to this issue, not the studs alone).

NB: The spigot should align the barrel to the casing mouth, not the studs. If the studs are trying to force the barrel into a different alignment than the spigot then you may well damage the casing or the kit. You have been warned!

This is one of the main reasons why many people fit original 'rolled' P2 studs to their motors, not just because the P2 studs are longer and allow more configuration options. FYI - Ron Moss can supply decent rolled studs (as supplied with his Avanti TT3 kits), as can many of the other dealers.

Adam

PS - Please note that this is not aimed at any single supplier of 'fat' studs. It should be noted that the dealer that sold me 'fat' studs is the same supplier of kits that often have alignment issues because of these studs :roll: ... go figure!

10 inch Terror
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Wed Nov 13, 2013 8:24 pm

Hi Adam, thanks for taking the time to post such a detailed reply. I'm using the MB long studs, as I always do. They work very well on my Imola and TS motors, with no "tightness" issues. I seem to remember seeing someone saying RB barrels are always a tight fit. I can't remember where I saw it, it could of been MB's review of them when the RB 1st came out. What would you recommend I do next? Thanks again for your time Pete.

sydduckett
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Wed Nov 13, 2013 8:38 pm

I have an rb20 and have had the same issues. I have tried most studs including the Vespa and they are all on the tight side, needing a little help from a toffee hammer or similar. not ideal I know but to be honest the only way it was going to happen was to persuade it....I think this is often the case with the 20 kit. hope this helps.

sef

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Wed Nov 13, 2013 8:57 pm

Process of elimination:
1. Check an original lamby cylinder goes on. If it does the fault lies with the RB barrell.

2. Assuming the lamby barrel did go on. Check the RB stud holes are as big as the lamby barrel.

3. Assuming that they are try the RB barrel on upside down, over the studs, to & see if it goes on easy. If not the RB barrel spacings are wrong. Send it back.

4. If it went on easy then the fault lies either with the spigot (too big on diameter) or the stud holes have been drilled out of position in relation to the spigot. So........

5. If you can get the studs out just try the spigot in the case. If it tight its the spigot diameter is too big. If it goes in easy then its the positions of the stud holes relative to the spigot that are incorrect. Either way its made wrong. Send it back.

SF

hydra
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Wed Nov 13, 2013 9:38 pm

I contacted AF about this problem some time ago, and their reply was to ream the first inch or so of the stud holes out from the spigot end. I know it doesn't seem right, but it works. Have done several this way since with no problems.

Adam_Winstone
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Wed Nov 13, 2013 11:07 pm

So you are using 'fat' studs and you have a fitting (probably an alignment) issue, which is exactly what I've described above.

First thing to do is to take all of the studs out and fit the barrel to the casing (this is mentioned above). If the barrel goes into the casing without issue then the spigot is fine and it is a problem caused by the studs and/or stud holes.

Note that this isn't me stating that the studs are too big individually, as putting the loose studs down the barrel's stud holes should be easy enough, HOWEVER, by fitting fatter studs you lessen the clearance and make any misalignment of the holes in the barrel more obvious. Should you blame the barrel manufacturer IF you have chosen to reduce clearance by fitting oversize (which is what these are) studs?

If you decide to wear thick socks with tight shoes, what is at fault... the socks or the shoes, or you for not considering the appropriate socks for the shoes that you intended to wear. Fitting 'fat' studs is like choosing thick socks, then blaming the shoe manufacturer!

This is an EXTREMELY common problem, which even with rolled studs causes some barrels (even standard barrels!) to slide most of the way home and then go tight when the barrel gets down to the stud threads that protrude beyond the gasket face of the casing. This tightness is the combination of barrel hole position variation and the holes 'fighting' against the exposed threads, which then increases the side loadings if you choose to just tighten it down and not address the cause. I regularly find that I have exactly this problem and that it is almost certain to happen with some kits (I can immediately think of 2 kits from 2 different manufacturers). When the spigot then goes as deep as the step in the spigot then the resistance and side loadings increase massively as you tighten the barrel further home (if you want to crack a thin spigot or damage the casing then just keep going!).

I first got around this issue by having a machinist pal oversize the holes but then figured out that the fat studs were the actual cause on this particular 'standard' 200 rebuild... so no kit manufacturer to blame in this case, I chose to fit fat studs and these were the cause! You choose to fit fat studs then you have chosen to increase your chances of alignment problems.

With other kits, I have found that it is often only 1 or 2 of the studs that causes issue. You can easily determine which studs/stud holes are causing issue by fitting the barrel to the casing mouth and then adding the studs 1 by 1, seeing which are poorly aligned and addressing these ones only... you can oversize the hole or reduce the stud diameter... the choice is yours.

However, I have yet to have an alignment issue that offers any resistance (or applies side loading to the spigot!) if I fit rolled studs to the casing (I often run a tap down into the casing to ensure that the stud hole is tapped to its full drilled depth) and then mark all threads that extend beyond the base gasket face, then file or Dremel off these threads so that they are the same diameter as the shaft of the rolled stud. This removes only the exposed threads that are causing the alignment issue and side loadings and allows the barrel to fit without issue, being correctly aligned by the spigot, rather than the studs/stud holes. When the studs are refitted to the casing then the barrel should then seat correctly into the casing with no resistance whatsoever.

I can't understand why anyone would chose to fit 'fat' studs as it introduces a restriction/limit that can seriously damage your kit (as per snapped Rapido spigot example I mentioned in previous post) or at the very least will limit the kits that you can fit without modification to one or t'other.

If you chose to play basketball with a beach ball then don't be surprised if it is too big to go through the hoop!

Adam

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