Stroked Suzuki RGV250 Options

© Vincent Crabtree

20^th March 2005

Last update 23^rd Aug 2009

The VJ21 and VJ22 RGV250 uses 56 bore by 50.6mm stroke with 105mm centre to centre con-rods.  More modern race replica 250 engine tend to uses 54 by 54 square and seem to deliver better midrange drive, ideal for exiting corners.  Converting the RGV to 54mm stroke could deliver more torque, while providing a useful capacity increase when retaining 56mm pistons.  Even larger pistons could allow 350cc or bigger RGV engines with even more torque.

This crank stroking work has become tied up with the TS200R big bore conversion for the RGV, but a stroked crank is beneficial regardless.

Analysis

The general outline of a crank is shown below – this is useful when comparing against others.

$ provided by Alan/Tasswipe, * Provided by George TP, Toronto, Ca,

# provided by CraigM – http://www.triplemmmracing.co.uk/  % provided by Peter Shires, Australia

Falicon Cranks can perform welded stroking, balancing or design and build from scratch if desired.  However, the prices are outside my budget.  As I understand, Crank Stroking can be performed in one of three ways:

Option 1 and 2 are very expensive and outside my capabilities at home.

To do this on a tight budget, I started investigating options 3 and 4 – top hat bushes and new big end pins.

However, it became clear that option 5 was viable, and this option didn’t appear originally.

Remachine or Top Hat Bush?

	I first saw this modification on Jarmo’s TL125 Thumper page. A Bush is pressed into an existing crank web, but on different centres to achieve the desired stroke. A hole is bored into the web to clean up the new hole, just touching the outside of the existing big end pin.   Photo courtesy Mark Worsefold
	To do this on an RGV, we need to examine the RGV crank in more details. The RGV crank uses a One piece forging on one side of the inner web. The Big end pin is cast into the web, to reduce the likelihood of twisting/cracking by too many press fit joints. The Outer crank webs are more conventional, and only one big floating end pin is used. The picture above left shows the two crank ends of the RGV crank. The big end pin hole is 22mm, and the distance from the edge of the big end pin to the outer edge of the flywheel is around 10.5mm.
	When using the alternative approach of machining new big end hole opposite the existing one would produce a crank severely out of balance. The ‘old’ big end hole would need to be filled with something like Lead to restore correct balance. Obviously separate pressed in big end pins would be required.   The picture above shows VJ21 crank centre pieces – VJ22 pieces are very similar. The left hand web has a flat machined in place, so the distance from edge of flat to edge of big end pin is only 7.3mm. The web with the integral big end pin is problematic, since the reverse side of the web has a void due to forging. Machining away the integral pin and boring a hole would require a bush welding in place on the reverse side. The gives problems since a larger wall thickness bush would be needed, and then machined in situ.

 Because of the extensive machining required, modifying an existing crank was deemed to difficult/expensive.  Less extensive machining is required, so a crank has to be much closer in terms of fit in orde to stay in budget. 

Crank from a Different Bike

One option could be to machine down a different crank to fit the RGV cases.

The RGV outer webs for alternator and drive would then use pressed in top hat bushes. However, the RGV crank uses forged crank with recesses where the big end pins are sited, as shown in the picture above.  It is probably easier to use RD350LC 54mm crank ends with new tapers machined in place.

I obtained cranks from the RD350LC, RD250DX, RD250e and RD250 b. The diameter on the Yamaha cranks are all too large to fit std RGV cases, so would need machining down.  The RD250a/b centres have balance holes too close to the outside edge, either led filled or open.  The RZ350 and RD350LC have a rebuildable crank, and also have integral big end pins. The RD400 cranks, shown left, shows the bore holes are very close to the crank edge, and would have the same problem. Thanks again to Writebike for the picture: However, the RD250e crank uses two small balance holes which allows the crank to be machined down, but the alternator end is very short.  It may also be possible to replace the standard RGV 6305 variant centre bearing, currently 17mm wide, with a 19mm wide roller version for increased load, since the single centre bearing gets a hard time, hence fed from the oil pump.

On reflection, the amount of machining was too great for this application.  However, teh RD crank could be a useful next logical progression after developing a simpler, but possibly less rugged, stroked crank below.

Suzuki RG250 GJ21 Crank

I can't believe no one has looked at this before! 

The RG250 GJ21 has the same crank wheel diameter, same diameter bearings, same big end pin diameters, same primary gear Nut and accepts the RGV250 primary gear. 

It also accepts the RGV250 alternator but I am not sure yet if the alternator shaft length is correct.

A part number and parts list is useful:-

Courtesy of www.CMSNL.com, The RG250 crank parts fich is on the left, and RGV on the right. 

Compare the RG250 item 14 with the RGV inner crank half in the picture above.  The RG250 seals shoulders are 33mm, whereas RGV spacer 23 is 25.5 x 33 x 12.  Same situation with spacer 28 on the RGV - this is part of the web on the RG (Part 14).  Suzuki must have learned this is a better option since crank seal wear on the RG250 can scrap the crank, whereas on the RGV the worn spacer is renewed.

I hope the seal shoulder on the RG250 part 13 can be machined off (faced) to make the web the same width as part 14 on the RGV.  Then part 14 on the RG250 needs the press fit shaft shortening to set the distance between inner webs right, and the big end pin on part 13 shortened if required.

I dont see why the crank cannot be reassembled at 360 degree. 

Then the crank needs to be rebuilt with RGV rods and bearings, and the single RGV big end pin used.

Pictures below - this was £1 from ebay (plus £17 postage) so only half is any good - the non-rebuildable half!

Conrods

The VJ21/22 Conrods have been implicated in some crank failures. However, many racers who take care of their engines properly have no problems so long as they rebuild every year. If we are looking at rebuilding cranks, then we can investigate different rods, especially since the 105mm rod length may not be adequate.

The RGV uses shims on the small end to position the Rod – other manufacturers use shims either side on the big end pin. The inside of a std RGV piston is 20.1mm where the small end slots in, but most data books don’t give this information.

The table below shows some conrods that may be usable.

A common tuning mod for Banshee and RD350 engines is to fit longer 115mm rods from the RD400. This changes the dwell of the ports, and changes the rod / stroke ratio, permitting torque to be extracted from the piston down stroke for longer. With short rods, the rod tries to push the big end out the side of the cases sooner than compared to a longer rod. I think a longer rod allows the cylinder spacer volume to hold mixture in the transfer ports, ready for pushing into the cylinder.

Porting and Compression

Increasing the stroke by 3.4mm will have significant effects on port timing and compression. One of the reasons for investigating this project was that the squish clearance is very large on the RGV, since the deck height can be upto 1mm, and the head gasket is around 1.2mm thick. This means that a small stroke increase could be accommodated at the top in with the existing gaskets and deck height.

After thinking about this, I think the barrel could be positioned in one of three possible positions.

1. Same port heights as stock. This would probably require the bottom of the cylinder skimming by 1.7mm, since the piston will also be 1.7mm lower at BDC. The stroke increase would mean the piston would extend ~3.4mm further ‘out the top’ than before, so coupled with the 1.7mm head gasket and deck height would require a cylinder head with a recessed combustion chamber or thick head gaskets.  If the rings pop out the cylinder this will not work. 
2. Std Base Gasket, no cylinder base machining, effectively bolting the cylinder up as normal. Since the piston is 1.7mm lower at BDC that stock, this would make the ports 1.7mm taller for free. The piston would ‘take up’ the extra height offered by the head gasket and cylinder deck, so a custom thickness copper head gasket (1.7+0.8)=2.5mm – woudl set squish at 0.8mm (2.2mm in reality due to deck height). Lowering the port floor with a lower TDC would increase port time areas. The table below shows a 2.2mm base gasket but in reality this is because of the extra 1.7mm the piston descends.
3. 3.4mm base gasket, skimmed cylinder deck. This approach is could be impractical since the ports would be very tall.

The numerical effects of these modifications are shown in the table below (Yellow cells are calculated, typical values entered for CC volume, squish, ex port height, transfer port height. Squish in stroked engine is setup for 0.8, which may involve machining or spacers etc). Note that differing cylinder heads are fitted on the stroked crank.

The cylinder deck height is 0.5mm, but this can vary between engines, typically from 0 when skimmed, 0.3 to 0,7 or more depending.

Note 8cc K heads are stated for the stock engine, and 10cc VJ22 heads on the stroked, but of course these can all be changed. This also shows the port timings obtained when fitting 1.4mm Kit gaskets, to compare against when using different cylinder spacing for the stroked crank.

Option 2, the No Barrel spacing position, looks best, since:-

1. since it is easy to do
2. offers port timing very similar to std
3. option to increase port time area by lowering the bottom port edge
4. similar compression to standard
5. easily able to set squish.

Options 1, skimming the cylinder base, would reduce the port timings for an engine with more drive. However, it requires a large, ~3.5mm, head gasket to accommodate the increased stroke. Since this is the most highly stressed part of the engine this would be difficult to do properly.

Option 3, large base spacer, is practically easy, but would give the ports very advanced timings, making a very high-speed engine. This option could be useful for a race only bike, but the idea of the modification was to increase midrange drive, not increase top end. The extra transfer volume could increase cylinder scavenge efficiency, however.

If tuned RGV cylinders were to be fitted, then the level of tuning and port open duration would affect the ideal cylinder position. In a perfect world, all three standard cylinder positions would be tried (at least two, without requiring cylinder base skimming) on a dyno.

Performance Tuners might want to look at the following links for ideas on 54mm stroke RGV port maps, based on HRC RS125, assuming the crank will stay together at 13,500rpm!!

http://www.bonavolta.ch/hobby/en/moto/rs125r.htm