Primary Chain Tensioner
There are several options for the primary chain tensioner, from the original spec to various levels of replacement.
This is an attempt to lay out all the options in one place so that you can tell what you have, what the weaknesses are, what you might wish to change to, and how to do so.
Regardless of which is selected (except the DS tensioner, which has an integrated chain oiler), be sure to consider a chain oil spray as described in Primary Chain Oil Spray.
Also, fit an Air Conditioner Cutout, and if possible a 'Trinary' switch.
- Original Chain Limiter
- * Flat guide
- * Curved Fixed Limiter** Strongly recommended to be replaced with the Renold later design version as shown.
- Citroen factory approved recommendation - REMOVES STANDING WAVES FROM CHAIN
- Small curved pivoted shoe held against the chain by a scissor type ratcheting tensioner
- Two fitment types of tensioner and moving shoe. Renold modification suggests a third
- (Obviously) no modifications necessary to install or replace
- Once Curved Fixed Limiter is installed, this is sufficient to allow any combination of OE parts to be used on the tensioner side if they will fit, as stresses are so much lower.
- Rubbing surface is Nitrile, this does eventually go hard and may break up. Correct automotive chain is much gentler on this surface, so will extend life. There are on exchange refinished items where the lining is Nylatron, which will likely last a very long time.
- Leading shoe configuration increases load on chain as chain stretches.
- The Merak Tensioner uses a timing chain tensioner lifted from e.g. the Maserati 4.9 Liter V8 (Bora, Ghibli, Mexico). This is a conventional spring-loaded tensioner with ratchet and oil hole at the end of the shoe, similar to the Brampton-type tensioners used on the Citroen DS. (Incidentally, in the Maserati V8 it is not a hydraulic tensioner. The metering orifice at the body inlet is 0.5mm while the hole at the end of the shoe is 1.0mm (4 times the area) therefore there is only an infinitesimal amount of oil pressure in the piston. The purpose of having oil in the tensioner is often misunderstood. It is not for hydraulic pressure but simply so that the shoe dribbles oil on the chain, and also to provide some vibration damping.)
- In the Merak application, instead of the tensioner shoe pressing directly on the chain as in the Maserati V8 engine, the tensioner shoe presses on one end of a lever arm, which carries at the other end a sprocket wheel that rides on the chain. Because the tensioner shoe presses against the lever arm instead of directly on the chain, the oiling hole in the shoe does not lubricate the chain.
- Can be used with Curved fixed tension side limiter without modification
- The rubber shoe at the end of the ram has an oil drip hole that presses against a bearing surface on the lever arm. As the shoe compresses, the oil drip hole in the ram can become obstructed at its interface with the lever arm. This can cause oil pressure to build up. This will cause the arm to over-tension the chain leading to excessive wear on the chain. A pressure relief valve such as shown above is recommended to prevent this. Mk1 arm is subject to cracking because of this unintended stress and needs modifying to Mk2 status. Very expensive, requires engine modification to use on SM block, requires care when setting cam timing, does not prevent chain vibration on its own, does not prevent straight limiter on tension side being knocked off by vibrating chain. Oil drip hole does not lubricate chain - oil spray hole modification should be performed
- available from: http://shop.bergmann-motorentechnik.com/maserati-performance-parts-1/6-zylinder-v-motor/hydraulischer-kettenspanner-modifikation-merak.html?___store=shop_en&___from_store=shop_de
- This modification was pioneered by the German SM Club. It uses the chain tensioner from 1966-75 Citroen DS/Peugeot 304.
- Spring tensioner with chain oiling through shoe
- Provides precisely metered chain oiling.
- All steel design is more robust than OE plastic ratchet. Eliminates "leading shoe" chain tensioner for more consistent tensioning as chain wears.
- No need to drill precision 0.5mm oil-spray hole in block (unless preferred over integral chain oiling).
- Proven modification with over 80,000 miles on engine shown in photo below as well as countless cars in Germany.
- Requires drilling/tapping mounting holes in block as shown here. Media:DS_SM_Tensioner_drawing_ABX.pdf
- (Also requires drilling oil feed hole from main bearing journal if chain oiling feature is desired.)
- The JBM tensioner was a well-intentioned albeit misguided attempt build an improved chain tensioner that would tension the chain even tighter than the Maserati tensioner, which, as discussed elsewhere, already over-tensions the chain under certain circumstances. Moreover, the JBM tensioner has even more throw than the Maserati tensioner which, if left unattended, will allow the chain to saw through the oil transfer passage from the timing cover to the engine block. The JBM tensioner is a true hydraulic tensioner with a piston that is pressurized by oil pressure from a passage drilled into the #1 main bearing saddle. It has a very robust (and coarse) ratchet mechanism to keep the piston engaged when there is no oil pressure. Unfortunately, the SM is well known to have oil pressure excursions to over 80 psi. This causes the JBM tensioner to over-tension the chain and then, because of the very coarse ratchet, the over-tensioned setting remains. Installation of the tensioner involved drilling and tapping the block for the support plate and drilling an oil feed passage into the #1 main bearing saddle.
- If you have a JBM tensioner installed, it is advisable to have it removed and to tap and plug the oil feed hole. This operation may involve removing the engine, especially if the support for the original tensioner has been removed.